ENDURING ISSUES IN THE BIOLOGICAL BASIS OF BEHAVIOR
In this chapter you will encounter all five "Enduring Issues" introduced in Chapter 1. To what extent is behavior caused by internal processes, as opposed to environmental factors (Person-Situation)? The core of this chapter - the notion that biological processes affect thoughts, emotions, and behavior - directly addresses this question. The chapter also sheds light on the connection between what we experience and our biological processes (Mind-Body) and on the extent to which heredity affects behavior (Nature-Nurture). In addition, you may be surprised to learn that the nervous system changes permanently as a result of experience (Stability-Change). Finally, you will learn that there are significant differences between men and women in the way that the brain works (Diversity-Universality).
NEURONS: THE MESSENGERS
The brain of an average human being contains as many as 100 billion nerve cells, or neurons. Billions more neurons are found in other parts of the nervous system. Neurons vary widely in size and shape, but they are all specialized to receive and transmit information. Like other cells, the neuron's cell body is made up of a nucleus, which contains a complete set of chromosomes and genes; cytoplasm, which keeps the cell alive; and a cell membrane, which encloses the whole cell. What makes a neuron different from other cells is the tiny fibers that extend out from the cell body, called dendrites. Their role is to pick up incoming messages from other neurons and transmit them to the cell body. The single long fiber extending from the cell body is an axon. The axon's job is to carry outgoing messages to neighboring neurons or to a muscle or gland. Axons vary in length from 1 or 2 millimeters to 3 feet. Although a neuron has only one axon, near its end the axon splits into many terminal branches. When we talk about a nerve (or tract), we are referring to a group of axons bundled together like wires in an electrical cable. Terminal buttons at the end chemical of each axon release chemical substances called neurotransmitters.
The axon of many neurons is surrounded by a white, fatty covering called a myelin sheath. The myelin sheath is "pinched" at intervals, making the axon resemble a string of microscopic sausages. Not all axons have this covering, but myelinated axons are found in all parts of the body. (Because of this white covering, tissues made up primarily of myelinated axons are called "gray matter.") The myelin sheath has three principal functions: First, it provides insulation, so that signals from adjacent neurons do not interfere with each other; second, it increases the speed at which signals are transmitted; and third, it protects neurons from disease.
Neurons that collect messages from sense organs and carry those messages to the spinal cord or the brain are called sensory (or afferent) neurons. Neurons that carry messages from the spinal cord or the brain to the muscles and glands are called motor (or efferent) neurons. And neurons that carry messages from one neuron to another are called interneurons (or association neurons).
A recently identified group of specialized cells called mirror neurons are involved in mimicking the behavior of others. First discovered in monkeys in the 1990s, mirror neurons fire not only when an action is performed, but also when a similar action is witnessed (Fadiga, Fogassi, Pavesi, & Rizzolatti, 1995). Found in the brains of humans and other primates, mirror neurons appear to play a key role in how a primate's brain is wired to mimic the sensations and feelings experienced by other related animals and, thus, to identify and empathize with them (Ehrenfeld, 2011; Rizzolatti, Fogassi, & Gallese, 2008). Mirror neurons may also play a key role in how humans represent events cognitively, in processes we refer to as thinking and understanding (Cross, Torrisi, Losin, & Iacoboni, 2013; Fogassi, 2011).
The nervous system also contains a vast number of glial cells (or glia; the word glia means "glue"). Glial cells hold the neurons in place, provide nourishment, remove waste products, prevent harmful substances from passing from the bloodstream into the brain, and form the myelin sheath that insulates and protects neurons. Recent evidence suggests that glial cells and astrocytes (a type of star-shaped glial cell) also may play an important role in neuron regeneration, learning and memory, and enhancing communication between neurons (Haas & Fischer, 2013: Navarrete et al., 2012).
The Neural Impulse
Neurons speak in a language that all cells in the body understand: simple "yes-no," "on-off" electrochemical impulses. When a neuron is at rest, the membrane surrounding the cell forms a partial barrier between the fluids that are inside and outside the neuron. Both solutions contain electrically charged particles, or ions. Because there are more negative ions inside the neuron that outside, there is a small electrical charge (called the resting potential) across the cell membrane. Thus, the resting neuron is said to be in a state of polarization. A resting, or polarized, neuron is like a spring that has been compressed but not released. All that is needed to generate a neuron's signal is the release of this tension.
When a small area on the cell membrane is adequately stimulated by an incoming message, pores (or channels) in the membrane at the stimulated area open, allowing a sudden inflow of positively charged sodium ions. This process is called depolarization; now the inside of the neuron is positively charged relative to the outside. Depolarization sets off a chain reaction. When the membrane allows sodium to enter the neuron at one point, the next point on the membrane opens. More sodium ions flow into the neuron at the second spot and depolarize this part of the neuron, and so on, along the entire length of the neuron. As a result, an electrical charge, called a neural impulse (or action potential), travels down the axon, much like a fuse burning from one end to the other. When this happens, we say that the neuron has been "fired." The speed at which neurons carry impulses varies widely, from as fast as nearly 400 feet per second on largely myelinated axons to as slow as about 3 feet per second on those with no myelin.
A single neuron may have many hundreds of dendrites and its axon may branch out in numerous directions, so that it is in touch with hundreds or thousands of other cells at both its input end (dendrites) and its output end (axon). At any given moment a neuron may be receiving messages from other neurons, some of which are primarily excitatory (telling it to "fire"), and from others, primarily inhibitory (telling it to "rest"). The constant interplay of excitation and inhibition determines whether the neuron is likely to fire or not.
As a rule, single impulses received from neighboring neurons do not make a neuron fire. The incoming message causes a small, temporary shift in the electrical charge, called a graded potential, which is transmitted along the cell membrane and may simply fade away, leaving the neuron in its normal polarized state. For a neuron to fire, graded potentials caused by impulses from many neighboring neurons - or from one neuron firing repeatedly - must exceed a certain minimum threshold of excitation. Just as a light switch requires a minimum amount of pressure to be turned on, an incoming message must be above the minimum threshold to make a neuron fire.
Every firing of a particular neuron produces an impulse of the same strength. This is called the all-or-none law. However, the neuron is likely to fire more often when stimulated by a strong signal. Immediately after firing, the neuron goes through an absolute refractory period: For about a thousandth of a second, the neuron will not fire again, no matter how strong the incoming messages may be. Following that is a relative refractory period, when the cell is returning to the resting state. During this period, the neuron will fire, but only if the incoming message is considerably stronger than is normally necessary to make it fire. Finally, the neuron returns to its resting state, ready to fire again.
The Synapse
Neurons are not directly connected like links in a chain. Rather, they are separated by a tiny gap, called a synaptic space, or synaptic cleft, where the axon terminals of one neuron almost touch the dendrites or cell body of other neurons. The entire area composed of the axon terminals of one neuron, the synaptic space, and the dendrites and cell body of the next neuron is called the synapse.
For the neural impulse to move onto the next neuron, it must somehow cross the synaptic space. The transfer is made by chemicals.
When a neuron fires, an impulse travels down the axon, out through the axon terminals, into a tiny dwelling called a terminal button, or synaptic knob. Most terminal buttons contain a number of tiny oval sacs called synaptic vesicles. When the neural impulse reaches the end of the terminals, it causes these vesicles to release varying amounts of chemicals called neurotransmitters into the synaptic space. Each neurotransmitter has specific matching receptor sites on the other side of the synaptic space. Neurotransmitters fit into their corresponding receptor sites just as a key fits into a lock. This lock-and-key system ensures that neurotransmitters do not randomly stimulate other neurons, but follow orderly pathways.
Once their job is complete, neurotransmitters detach from the receptor site. IN most cases, they are either reabsorbed into the axon terminals to be used again, broken down and recycled to make new neurotransmitters, or disposed of by the body as waste. The synapse is cleared and returned to its normal state.
Neurotransmitters
In recent decades, neuroscientists have identified hundreds of neurotransmitters; their exact functions are still being studied. However, a few brain chemicals are well known.
Acetylcholine (ACh) acts where neurons meet skeletal muscles. It also appears to play a critical role in arousal, attention, memory, and motivation. Alzheimer's disease, which involves loss of memory and severe language problems, has been linked to degeneration of the brain cells that produce and respond to ACh (Kihara & Shimohama, 2004).
Dopamine generally affects neurons associated with voluntary movement, learning, memory, and emotions. As with most neurotransmitters, the precise location in the brain where dopamine is released determines the effect it will have on behavior.
Serotonin is popularly known as "the mood molecule" because it is often involved in emotional experiences. Serotonin is an example of a neurotransmitter that has widespread effects (Lutz, 2013). Serotonin is like a master key that opens many locks - that is, it attaches to as many as a dozen receptor sites (N. Thierry et al., 2004).
Other brain chemicals regulate the sensitivity of large numbers of synapses, in effect "turning up" or "turning down" the activity level of whole portions of the nervous system. Glutamate, for example, is principally an excitatory chemical that speeds up synaptic transmission through the central nervous system. It is involved in enhancing learning and memory by strengthening synaptic connections between neurons (Balschun et al., 2010; Potter, 2009). Conversely, endorphins appear to reduce pain by inhibiting, or "turning down," the neurons that transmit pain messages in the brain. Morphine and other narcotics lock into the receptors for endorphins and have the same painkilling effects. Research on endorphins has provided clues to why people become addicted to morphine, heroin, cocaine, and, in some cases, alcohol (Charbogne, Kieffer, & Befort, 2014). When a person takes one of these drugs repeatedly, the body's production of natural painkillers slows down. As a result, an addict needs more of the artificial drug to feel "normal."
Imbalances in neurotransmitters appear to contribute to many types of mental illness. Schizophrenia, for example, has been associated with an overabundance of, or hypersensitivity to, dopamine (H. Silver, Goodman, Isakov, Knoll, & Modai, 2005; Seeman, 2013). An undersupply of serotonin and norepinephrine has been linked to depression. Imbalances in GABA (Gamma aminobutyric acid) have been associated with sleep and eating disorders as well as with increased levels of anxiety (Murphy, Ihekoronze, & Wideman, 2011).
Neural Plasticity and Neurogenesis
In a classic series of experiments, M. R. Rosenzweig (1984) demonstrated the importance of experience to neural development and the establishment of neural networks. In the laboratory, Rosenzweig assigned baby rats into two groups. Members of one group were raised in an impoverished environment, isolated in barren cages. Members of the second group were raised in an enriched environment; they lived in cages with other rats and a variety of toys that offered opportunities for exploration, manipulation, and social interaction. Rosenzweig found that the rats raised in enriched environments had larger neurons with more synaptic connections than those raised in impoverished environments. More recent experiments, by Rosenzweig (1996) and others (Ruifang & Danling, 2005), have shown that similar changes occur in rats of any age. Other researchers have found that rats raised in stimulating environments perform better on a variety of problem-solving tests and develop more synapses when required to perform complex tasks (Kleim, Vij, Ballard, & Greenough, 1997). These combined results suggest that the brain changes in response to the organism's experiences, a principle called neural plasticity. Furthermore, they demonstrate that neural plasticity is a feedback loop: Experience leads to changes in the brain, which, in turn, facilitate new learning, which leads to further neural change, and so on (Seppanen, Hamalainen, Pesonen, & Tervaniemi, 2012; Singer et al., 2011).
Reorganization of the brain as a result of experience is not limited to rats (Kolb, Gibb, & Robinson, 2003). For example, violinists, cellists, and other string musicians spend years developing precise left-hand sensitivity and dexterity. Researchers have found that the area of the musicians' brains associated with left-hand sensation is larger than the area that represents the right hand, and larger than the left-hand area in nonmusicians (Stewart, 2008). Plasticity in the brain is not just limited to changes that affect motor behaviors. The brains of female mammals apparently change in response to hormonal changes that occur during pregnancy (Kinsley & Lambert, 2006). Plasticity also permits changes in the way our nervous system responds to sensation. For example, in blind people, the portion of the brain normally responsible for vision reorganizes to respond to touch and hearing (Amedi, Merabet, Bermpohl, & Pascual-Leone, 2005).
Experience also causes changes in the strength of communication across synapses. For example, when neurons in the hippocampus (a brain structure involved in forming memories in humans and other animals) are stimulated by an electrical pulse, the initial response in nearby neurons is very weak. But repeated stimulation of the same pathway causes the nearby neurons to respond vigorously, an effect that lasts weeks after the stimulation was stopped (Taufiq et al., 2005). Long-term potentiation (LTP), as this is called, appears to help the brain learn and store new information (Bliss, Collingridge, & Morris, 2004).
The effects of neural plasticity are made more profound because neurons are functionally connected to one another forming circuits or neural networks that mature and develop in response to experience. These complex neural networks, made up of thousands of individual cells, serve as the foundation for all psychological processes, including thoughts, emotions, and consciousness. These uniquely different neural networks are what underlie individual differences in thinking and behaving, and also appear to play an important role in producing cultural differences. For example, because people from the same culture share similar experiences, their neural networks would tend to be more similar to one another than they would be to people from a different culture. Conversely, people from different cultures often have very different experiences, leading to the development of very different neural networks, thus causing them to think, perceive, and behave differently from one another (Park and Huang, 2010).
Finally, there is evidence that experience can also produce new neurons. For many years, psychologists believed that organisms are born with all the brain cells they will ever have. New research, however, has overturned this traditional view by showing that adult brains are capable of neurogenesis, the production of new brain cells (Yang, Bi, & Feng, 2011; Yirmiya & Goshen, 2011).
The discovery of adult neurogenesis raises new possibilities in the treatment of neurological disorders and brain and spinal cord injuries (Hayashi, Ohta, Kawakami, & Toda, 2009; Luo, 2011). Scientists have long known that embryos contain large numbers of stem cells: undifferentiated, precursor cells or "precells" that, under the right conditions, can give rise to any specialized cell in the body - liver, kidney, blood, heart, or neurons. Remarkably, in tests with animals, stem cells transplanted into a brain or spinal cord spontaneously migrated to damaged areas and began to generate specialized neurons for replacement (Kokaia & Lindvall, 2003; Nowakowski & Hayes, 2004). In clinical trials with patients suffering from Parkinson's disease, fetal nerve cell transplants have improved motor control for periods of 5 to 10 years (Barinaga, 2000; Newman & Bakay, 2008).
However, the supply of fetal tissue is limited, and its harvest and use raise ethical questions (Kuflik, 2008). Fortunately, scientists have recently found ways to coax mature cells to behave like stem cells, eliminating the need to use fetal cells to stimulate neurogenesis. One study, for example, demonstrated that the skin cells of mice could be transformed into neurons by using viruses to alter their genetic code (Vierbuchen, Ostermeier, Pang, Kokubu, Sudhof, & Wernig, 2010). Research like this has enormous promise in helping researchers overcome the ethical issues associated with using stem cells to promote neurogenesis.
Another potential use of new research findings is to stimulate the brain's own stem cells to provide "self-repair." For instance, research aimed at identifying specific chemicals that stimulate neurogenesis in the brain and spinal cord has shown promising results (Telerman, Lapter, Sharabi, Zinger, & Mozes, 2011). Other research has demonstrated that exercise may stimulate neurogenesis, resulting in improved learning and memory (Kerr & Swain, 2011). To translate these discoveries into treatments, scientists need to learn more, but people suffering from such neurological disorders as Parkinson's and Alzheimer's diseases, as well as victims of spinal cord injuries and stroke, now have hope (Gage, 2000; Newman & Bakay, 2008).
THE CENTRAL NERVOUS SYSTEM
The Organization of the Nervous System
Every part of the nervous system is connected to every other part. To understand its anatomy and functions, however, it is useful to analyze the nervous system in terms of its divisions and subdivisions. The central nervous system includes the brain and spinal cord, which together contain more than 90% of the body's neurons. The peripheral nervous system consists of nerves that connect the brain and spinal cord to every other part of the body, carrying messages back and forth between the central nervous system and the sense organs, muscles, and glands.
The Brain
The human brain is the product of millions of years of evolution through which our brains have increased in size and synaptic complexity. As new, more complex structures were added, older structures were retained. One way to understand the brain is to look at three layers that evolved in different stages of evolution: (1) the primitive central core; (2) the limbic system, which evolved later; and (3) the cerebral hemispheres, which are in charge of higher mental processes such as problem solving and language. We will use these three basic divisions to describe the parts of the brain, what they do, and how they interact to influence our behavior.
The Central Core
At the point where the spinal cord enters the skull, it becomes hindbrain. Because the hindbrain is found in even the most primitive vertebrates, it is believed to have been the earliest part of the brain to evolve. The part of the hindbrain nearest to the spinal cord is the medulla, a narrow structure about 1.5 inches long. The medulla controls such bodily functions as breathing, heart rate, and blood pressure. The medulla is also the point at which many of the nerves from the body cross over on their way to and from the higher brain centers; nerves from the left part of the body cross to the right side of the brain and vice versa. Near the medulla lies the pons, which produces chemicals that help maintain our sleep-wake cycle. Both the medulla and the pons transmit messages to the upper areas of the brain.
The part of the hindbrain at the top and back of the brain stem is the cerebellum, sometimes called the "little brain." Though the cerebellum takes up only a small space, its surface area is almost two-thirds that of the much larger cerebrum. It also contains more neurons than the rest of the brain. Traditionally, it has been thought that the cerebellum is responsible for our sense of balance and for coordinating the body's actions to ensure that movements go together in efficient sequences. Damage to the cerebellum is also involved in psychological processes, including emotional control, attention, memory, and coordinating sensory information (Della Sala, 2011).
Above the cerebellum, the brain widens to form the midbrain. The midbrain is especially important for hearing and sight. It is also one of several places in the brain where pain is registered.
Almost directly over the brain stem are the two egg-shaped structures that make up the thalamus. The thalamus is often described as a relay station: Almost all sensory information passes through the thalamus on the way to higher levels of the brain, where it is translated and routed to the appropriate brain location. Directly below the thalamus is the smaller hypothalamus, which exerts an enormous influence on many kinds of motivation. Portions of the hypothalamus govern hunger, thirst, sexual drive, and body temperature and are directly involved in emotional behavior such as experiencing rage, terror, or pleasure.
The reticular formation (RF) is a netlike system of neurons that weaves through all of these structures. Its main job is to send "Alert!" signals to the higher parts of the brain in response to incoming messages. The RF can be subdued, however; during sleep, the RF is turned down. Anesthetics work largely by temporarily shutting this system off, and permanent damage to the RF can even induce a coma.
The Cerebrum
Ballooning out over and around the central core, virtually hiding it, is the cerebrum. The cerebrum is divided into two hemispheres. This is what most people think of first when they talk about "the brain"; it is the part of the brain that processes thought, vision, language, memory, and emotions. The cerebrum is the most recently evolved part of the nervous system and is more highly developed in humans than in any other animal. The human cerebrum takes up most of the room inside the skull, accounting for about 80% of the weight of the human brain. It contains about 70% of the neurons in the entire central nervous system.
The surface of the cerebrum is a thin layer of gray matter (unmyelinated cells) called the cerebral cortex. Spread out, the human cortex would cover 2 to 3 square feet and be about as thick as the letter "T." To fit inside the skull, in humans the cerebral cortex has developed intricate folds called convolutions. In each person, these convolutions form a pattern that is as unique as a fingerprint.
A number of landmarks on the cerebral cortex allow us to identify distinct areas, each with different functions. The first is a deep cleft, running from front to back, that divides the brain into right and left hemispheres. Each of these hemispheres can be divided into four lobes, which are separated from one another by crevices, or fissures, such as the central fissure. In addition, there are large areas on the cortex of all four lobes called association areas that integrate information from diverse parts of the cortex and are involved in mental processes such as learning, thinking, and remembering.
The different lobes of the cerebral hemispheres are specialized for different functions. The frontal lobe, located just behind the forehead, accounts for about half the volume of the human cerebrum. It receives and coordinates messages from the other three lobes of the cerebrum and seems to keep track of previous and future movements of the body. This ability to monitor and integrate the complex tasks that are going on in the rest of the brain has led some investigators to hypothesize that the frontal lobe serves as an "executive control center" for the brain (E. Goldberg, 2009). The area on the surface of the frontal lobe known as the primary motor cortex plays a key role in voluntary action. The frontal lobe also plays a pivotal role in the behaviors we associate with personality, including motivation, persistence, emotional responses, character, and even moral decision making (Jackson et al., 2003; Umeda, 2013).
Until recently, our knowledge of the frontal lobes was based on research with nonhuman animals, whose frontal lobes are relatively undeveloped, and on studies of rare cases of people with frontal lobe damage. Perhaps the most famous case involved a bizarre accident reported in 1848. Phineas Gage, the foreman of a railroad construction gang, made a mistake while using some blasting powder. The explosion blew a nearly 4-foot-long tamping iron more than an inch thick into his cheek and all the way through the top of his head, severely damaging his frontal lobes. To the amazement of those who witnessed the accident, Gage remained conscious, walked part of the way to a doctor, and suffered few physical aftereffects. He did, however, suffer lasting psychological changes, including difficulty reasoning and making decisions, as well as difficulty controlling his emotions. These changes were so radical that, in the view of his friends, he was no longer the same man. A century later, most neuroscientists agree that personality change - especially loss of motivation and ability to concentrate - is the major outcome of frontal lobe damage.
The forward-most surface of the frontal lobe, known as the prefrontal cortex, plays a crucial role in goal-directed behavior, the ability to control impulses, judgment, and metacognition - which involves awareness and control of our thoughts (Modirrousta & Fellows, 2008). Psychologists are only beginning to understand how this small but complex area of the brain contributes to such a wide range of subtle and important mental activities (Bzdok, 2013).
The occipital lobe, located at the very back of the cerebral hemispheres, receives and processes visual information. Damage to the occipital lobe can produce blindness or visual hallucinations.
The parietal lobe occupies the top back half of each hemisphere. This lobe receives sensory information from all over the body - from sense receptors in the skin, muscles, joints, internal organs, and taste buds. Messages from these sense receptors are registered in the primary somatosensory cortex. The parietal lobe also seems to oversee spatial abilities, such as the ability to follow a map (Silver & Kastner, 2009). This lobe is also typically involved in eye-hand coordination. In one study of young men in their twenties who were asked to perform complex visual-motor tasks, those who rarely played video games relied primarily on the parietal lobe of the brain. Interestingly, those men who played video games at least four hours a week relied primarily on an entirely different portion of the cerebrum: the prefrontal cortex (Granek, Gorbet, & Sergio, 2010). You might recognize this as yet another example of neuroplasticity.
The temporal lobe, located roughly behind the temples, plays an important role in complex visual tasks such as recognizing faces and interpreting the facial emotions of others (Martens, Leuthold, & Schweinberger, 2010). The temporal lobe also receives and processes information from the ears, contributes to balance and equilibrium, and regulates emotions and motivations such as anxiety, pleasure, and anger. The ability to understand and comprehend language is concentrated primarily in the rear portion of the temporal lobes, though some language comprehension may also occur in the parietal and frontal lobes (Crinion, Lambon-Ralph, & Warburton, 2003; Hutsler, 2003).
Beneath the temporal lobe, hidden deep within the lateral fissure, which separates the parietal and temporal lobes, is an area known as the insula. Involved in the conscious expression of emotion and desire, the insula has been shown to play an important role in addiction by controlling the conscious urge to seek drugs and assess risk (Naqvi & Bechara, 2009). For example, cigarette smokers who have sustained damage to this area stop smoking quickly and resist the urge to take up the habit again (Naqvi, Rudrauf, Damasio, & Bechara, 2007). Other studies have demonstrated a link between the insula and addiction to amphetamines, cocaine, and alcohol (Contreras, Ceric, & Torrealba, 2007).
The Limbic System
The limbic system is a ring of loosely connected structures located between the central core and the cerebral hemispheres. In evolutionary terms, the limbic system is more recent than the central core and is fully developed only in mammals.
The limbic system plays a central role in times of stress, coordinating and integrating the activity of the nervous system. One part of the limbic system, the hippocampus, plays an essential role in the formation of new memories. People with severe damage to this area can still remember names, faces, and events that they recorded in memory before they were injured, but they cannot remember anything new. Another structure, the amygdala (working together with the hippocampus) is involved in governing and regulating emotions and in establishing emotional memories (R. J. Davidson, Jackson, & Kalin, 2000; LaBar & Cabeza, 2006), particularly those related to fear and self-preservation (Donley, Schulkin, & Rosen, 2005). In one case, a woman whose amygdala was destroyed by disease in her childhood reported that she never felt fear, even when threatened by a knife or a gun (Feinstein, Adolphs, Damasio, & Tranel, 2010). The amygdala also plays a role in the experience of pleasure (Salzman & Fusi, 2010), as do other limbic structures (Burgdorf & Panksepp, 2006). Even our ability to read the facial expressions of emotion in other people (such as smiling, frowning, or surprise) is registered in the limbic system (Guyer et al., 2008; Vrticka, Lordier, Bediou, & Sander, 2013). For example, people with a rare genetic disorder known as Williams syndrome, which involves amygdala damage, are often characterized by an inability to properly interpret the facial expressions of anger or worry in other people. As a result, individuals with Williams syndrome are socially awkward and lack social fear (Jarvinen-Pasley et al., 2008; Sarpal et al., 2008).
Hemispheric Specialization
The cerebrum, as noted earlier, consists of two separate cerebral hemispheres. Quite literally, humans have a "right-half brain" and a "left-half brain." The primary connection between the left and the right hemispheres is a thick, ribbonlike band of nerve fibers under the cortex called the corpus callosum.
Under normal conditions, the left and right cerebral hemispheres are in close communication through the corpus callosum and work together as a coordinated unit (Funnell, 2010). But research suggests that the cerebral hemispheres are not really equivalent.
The most dramatic evidence comes from "split-brain" patients. In some cases of severe epilepsy, surgeons cut the corpus callosum to stop the spread of epileptic seizures from one hemisphere to the other. In general, this procedure is successful: The patients' seizures are reduced and sometimes eliminated. But their two hemispheres are functionally isolated; in effect, their right brain doesn't know what their left brain is doing (and vice versa). Since sensory information typically is sent to both hemispheres, in everyday life, split-brain patients function quite normally. However, a series of ingenious experiments revealed what happens when the two hemispheres cannot communicate (Gazzaniga, 2005; Sperry, 1964).
In one such experiment, split-brain patients were asked to stare at a spot on a projection screen. When pictures of various objects were projected to the left of that spot, they could pick them out of a group of hidden objects by feeling them with their left hands but they couldn't say what the objects were! In fact, when asked what objects they saw on the left side of the screen, split-brain patients usually said "nothing." When asked to pick out the objects with their right hands, they couldn't do so even though they were able to name the objects.
The explanation for these unusual results is found in the way each hemisphere of the brain operates. When the corpus callosum is cut, the right hemisphere receives information only from the left side of the visual field and the left side of the body. As a result, it can match an object shown in the left visual field with information received by touch from the left hand, but it cannot verbally identify those objects. Conversely, the left hemisphere receives information only from the right side of the body and the right half of the visual field. Consequently, it cannot match an object shown in the left visual field using only the right hand, but it can verbally identify any objects touched with the right hand.
But why can't the right hemisphere verbally identify an object that is shown in the left visual field? The answer is that for the great majority of people (even for most left-handers), in the process of learning to read, language ability becomes concentrated primarily in the left hemisphere. As a result, when an object is in the left visual field, the nonverbal right hemisphere can see the object, but can't name it. The verbal left hemisphere, in contrast, can't see an object in this location, so when asked what it sees, it answers that nothing is on the screen.
Does the left hemisphere specialize in any other tasks besides language? Some researchers think that it may also operate more analytically, logically, rationally, and sequentially than the right hemisphere does (Kingstone, Enns, Mangun, & Gazzaniga, 1995; Martins, Caeiro, & Ferro, 2007). In contrast, the right hemisphere excels at visual and spatial tasks - nonverbal imagery, including music, face recognition, and the perception of emotions and color (K. J. Barnett, 2008; Buklina, 2005; Steinke, 2003). Put another way, the left hemisphere specializes in holistic processing and in solving problems that require insight or creative solutions (Shamay-Tsoory, Adler, Aharon-Peretz, Perry, & Mayseless, 2011).
Although such research is fascinating and fun to speculate about, be cautious in interpreting it. First, not everyone shows the same pattern of differences between the left and right hemispheres. In particular, the differences between the hemispheres may be greater in men than in women (Mucci et al., 2005). Second, it is easy to oversimplify and exaggerate differences between the two sides of the brain. Split-brain research has given rise to several popular but misguided books that classify people as "right-brain" or "left-brain" thinkers. It is important to remember that under normal conditions, the right and left hemispheres are in close communication through the corpus callosum and so work together in a coordinated, integrated way (Gazzaniga, 2005, 2008). Furthermore, the plasticity of the brain means that both hemispheres have the potential to perform a range of tasks.
Language
The notion that human language is controlled primarily by the left cerebral hemisphere was first set forth in the 1860s by a French physician named Paul Broca. Broca's ideas were modified a decade later by the scientist Karl Wernicke. Thus, it should come as no surprise that the two major language areas in the brain have traditionally been called Broca's area and Wernicke's area.
Wernicke's area lies toward the back of the temporal lobe. This area is crucial in processing and understanding what others are saying. In contrast, Broca's area, found in the frontal lobe, is considered to be essential to our ability to talk. Support for these distinctions comes from patients who have suffered left-hemisphere strokes and resulting brain damage. Such strokes often produce predictable language problems, called aphasias. If the brain damage primarily affects Broca's area, the aphasia tends to be "expressive." That is, the patients' language difficulties lie predominantly in sequencing and producing language (talking). If the damage primarily affects Wernicke's area, the aphasia tends to be "receptive," and patients generally have profound difficulties understanding language (listening).
Interestingly, the areas of the brain involved in processing speech and language may not be as uniquely human as once thought (Belin, 2008). Investigators have identified a similar region in the brain of macaque monkeys, which is also involved in voice recognition for members of their own species (Petkov et al., 2008).
Handedness
A common misconception is that hemispheric specialization is related to handedness. The fact is that speech is most often localized in the left hemisphere for both right- and left-handed people. However, in a small percentage of left-handed individuals, language functions are concentrated in the right hemisphere (Knecht et al., 2000).
Nevertheless, psychologists have uncovered a number of interesting facts about handedness. Approximately 90% of humans are right-handed, with slightly more males than females showing a tendency toward left-handedness. Though research has not fully explained why some people are left-handed and others are right-handed, the tendency toward right-handedness appears to be a trait humans have possessed for quite some time, as anthropological studies of prehistoric cave drawing, tools, and human skeletons have shown (James Steele, 2000). Most chimpanzees, bonobos, and gorillas also favor their right hands, but most orangutans favor their left hands (Hopkins et al., 2011).
Tools for Studying the Brain
For centuries, our understanding of the brain depended entirely on observing patients who had suffered brain injury or from examining the brains of cadavers. Another approach - that is still in use - is to remove or damage the brains of nonhuman animals and study the effects. But the human brain is far more complicated than that of any other animal. How can scientists study the living, fully functioning human brain? Contemporary neuroscientists have four basic techniques - microelectrodes, macroelectrodes, structural imaging, and functional imaging.
Microelectrode Techniques
Microelectrode recording techniques are used to study the functions of single neurons. A microelectrode is a tiny glass or quartz pipette or tube (smaller in diameter than a human hair) that is filled with conducting liquid. When technicians place the tip of this electrode inside a neuron, they can study changes in the electrical conditions of that neuron. Microelectrode techniques have been used to understand action potentials, the effects of drugs or toxins on neurons, and even processes that occur in the neural membrane.
Macroelectrode Techniques
Macroelectrode recording techniques are used to obtain an overall picture of the activity in particular regions of the brain, which may contain millions of neurons. The first such device - the electroencephalograph (EEG) - is still in use today. Flat electrodes, taped to the scalp, are linked by wires to a device that translates electrical activity into lines on a moving roll of paper (or, more recently, images on a computer screen). This graph of so-called brain waves provides an index of both the strength and the rhythm of neural activity. This technique has given researchers valuable insights into changes in brain waves during sleep and dreaming.
Structural Imaging
When researchers want to map the structures in a living human brain, they turn to two newer techniques. Computerized axial tomography (CAT or CT) scanning allows scientists to create three-dimensional images of a human brain without performing surgery. To produce a CAT scan, a X-ray photography unit rotates around the person, moving from the top of the head to the bottom; a computer then combines the resulting images. Magnetic resonance imaging (MRI) is even more successful at producing pictures of the inner regions of the brain, with its ridges, folds, and fissures. With MRI, the person's head is surrounded by a magnetic field and the brain is exposed to radio waves, which causes hydrogen atoms in the brain to release energy. The energy released by different structures in the brain generates an image that appears on a computer screen.
Advances in MRI technology enable scientists to compare precise three-dimensional images obtained over extended periods. this permits tracking of progressive structural changes in the brain that accompany slow neurodegenerative disorders like Alzheimer's disease (Bruen, McGeown, Shanks, & Venneri, 2008).
Functional Imaging
In many cases, researchers are interested in more than structure; they want to look at the brain's activity as it actually reacts to sensory stimuli, such as pain, tones, and words. Such is the goal of several functional imaging methods. EEG imaging measures brain activity millisecond-by-millisecond. In this technique, more than two dozen electrodes are placed at important locations on the scalp. These electrodes record brain activities, which are then converted by a computer into colored images on a television screen. The technique has been extremely useful in detecting abnormal cortical activity such as that observed during epileptic seizures.
Two related techniques, called magnetoencephalography (MEG) and magnetic source imaging (MSI), take the procedure a step further. In standard EEG, electrical signals are distorted as they pass through the skull; and their exact source is difficult to determine. However, those same electrical signals create magnetic fields that are unaffected by bone. Both MEG and MSI measure the strength of the magnetic field and identify its source with considerable accuracy. By using these procedures, neuroscientists have begun to determine exactly which parts of the brain do most of the work in such psychological processes as memory (Campo et al., 2005), language processing (Ressel, Wilke, Lidzba, Lutzenberger, & Krageloh-Mann, 2008), and reading, and shed light on mental disorders such as schizophrenia (Haenschel & Linden, 2011).
Another family of functional imaging techniques - including positron emission tomography (PET) scanning - uses radioactive energy to map brain activity. In these techniques, a person first receives an injection of a radioactive substance. Brain structures that are especially active immediately after the injection absorb most of the substance. When the substance starts to decay, it releases subatomic particles. By studying where most of the particles come from, researchers can determine exactly which portions of the brain are most active. PET has been used to investigate how our memory for words and images is stored in the brain (Cabeza & Nyberg, 2000; Craik et al., 1999), and locate damage resulting from Alzheimer's disease (Zetterberg, 2008).
One of the newest and most powerful techniques for recording activity in the brain is called functional magnetic resonance imaging (fMRI). Functional MRI measures the movement of blood molecules (which is related to neuron activity) in the brain, permitting neuroscientists to pinpoint specific sites and details of neuronal activity. For example, in recent years fMRI has contributed to our understanding of the neural networks that underlie language (Blumstein & Amso, 2013) and memory (Cabeza & Moscovitch, 2013). It has also provided evidence to help us understand the decline in memory and cognition that often accompanies aging, and lead us to interventions to slow these declines (Reuter-Lorenz, 2013). By comparing brain activity in normal learners with brain activity in children with learning problems, researchers have begun to identify the neurological mechanisms associated with attention deficit hyperactivity disorder (ADHD) (Schneider et al., 2010); dyslexia (T. L. Richards & Berninger, 2008); Huntington's disease (Bohanna, Georgiou-Karistianis, Hannan, & Egan, 2008); and disorders that involve difficulties in controlling emotions (Heinzel et al., 2005). With fMRI it is even possible to determine with some accuracy what a person is thinking about, may decide to do, and whether he or she is lying (Ganis, Rosenfeld, Mneixner, Kievit, & Schendan, 2011). FMRI studies have even debunked the myth that we use only 10% of our brain since studies using fMRI have not shown any area of the brain that is perpetually inactive (Lilienfeld & Arkowitz, 2008). Because fMRI enables us to collect extremely precise images rapidly and is noninvasive in that it does not require the injection of radioactive chemicals, it is especially promising as a research tool (Mather, Cacioppo, & Kanwisher, 2013).
The Spinal Cord
We talk of the brain and the spinal cord as two distinct structures, but in fact, there is no clear boundary between them; at its upper end, the spinal cord enlarges into the brain stem.
The spinal cord is our communications superhighway, connecting the brain to most of the rest of the body. When the spinal cord is severed, parts of the body are literally disconnected from the brain. Thus, people who suffer damage to the spinal cord lose all sensations from the parts of the body that no longer send information to higher brain areas, as well as control over the movements of those body parts.
The spinal cord is made up of soft, jellylike bundles of long axons, wrapped in insulating myelin (white matter) and surrounded and protected by the bones in the spine. There are two major neural pathways in the spinal cord. One consists of motor neurons, descending from the brain, that control internal organs and muscles and help to regulate the autonomic nervous system. The other consists of ascending, sensory neurons that carry information from the extremities and internal organs to the brain. In addition, the spinal cord contains neural circuits that produce reflex movements (and control some aspects of walking). These circuits do not require input from the brain.
To understand how the spinal cord works, consider the simple act of burning your finger on a candle flame.
Following this reflex, your body goes on "emergency alert": You breathe faster, your heart pounds, your entire body (including the endocrine system) mobilizes itself against the wound. Meanwhile, your brain is interpreting the messages it receives: You feel pain, you look at the burn, and you run cold water over your hand. In summary, this simple, small burn triggered a complex, coordinated sequence of activities initiated in the spinal cord.
THE PERIPHERAL NERVOUS SYSTEM
The origins of the quick response your body makes to touching a hot pan are in your peripheral nervous system. The peripheral nervous system (PNS) links the brain and spinal cord to the rest of the body, including the sensory receptors, glands, internal organs, and skeletal muscles. It consists of both afferent neurons, which carry messages to the central nervous system (CNS), and efferent neurons, which carry messages from the CNS. All the things that register through your senses - sights, sounds, smells, temperature, pressure, and so on - travel to your brain via afferent neurons. The efferent neurons carry signals from the brain to the body's muscles and glands.
Some neurons belong to a part of the PNS called the somatic nervous system. Neurons in this system are involved in making voluntary movements of the skeletal muscles. Other neurons belong to a part of the PNS called the autonomic nervous system. Neurons in the autonomic nervous system govern involuntary activities of your internal organs, from the beating of your heart to the hormone secretions of your glands.
The autonomic nervous system is of special interest to psychologists because it is involved not only in vital body functions, such as breathing and blood flow, but also in important emotions as well. To understand the workings of the autonomic nervous system, you must know about the system's two parts: the sympathetic and the parasympathetic divisions.
The nerve fibers of the sympathetic division are busiest when you are intensely aroused, such as being enraged or very frightened. For example, if you were hiking through the woods and suddenly encountered a large, growling bear, your sympathetic division would be instantaneously triggered. In response to messages from it, your heart would begin to pound, your bronchi would dilate and breathing quicken, your pupils would enlarge and your digestion would stop. All these changes would help direct your energy and attention to the emergency you faced, giving you the keen senses, stamina, and strength needed to flee from the danger or to stand and fight it. Your sympathetic division would also tell your glands to start pumping hormones into your blood to further strengthen your body's reactions. Sympathetic nerve fibers connect to every internal organ - a fact that explains why the body's response to sudden danger is so widespread.
Although sympathetic reactions are often sustained even after danger is passed, eventually even the most intense sympathetic division reaction fades and the body calms down, returning to normal. The heart then goes back to beating at its regular rate, the stomach muscles relax, digestion resumes, bronchi constrict slowing breathing, and the pupils contract. This calming effect is promoted by the parasympathetic division of the autonomic nervous system. Parasympathetic nerve fibers connect to the same organs as sympathetic nerve fibers do, but they cause the opposite reaction.
Traditionally, the autonomic nervous system was regarded as the "automatic" part of the body's response mechanism (hence its name). You could not, it was believed, tell your own autonomic nervous system when to speed up or slow down your heartbeat or when to stop or start your digestive processes. However, more recent studies have shown that human (and animals) have some control over the autonomic nervous system. For example, people can learn to moderate the severity of high blood pressure (Reineke, 2008), heart rate variability (Sakakibara, Hayano, Oikawa, Katsamanis, & Lehrer, 2013), migraine headaches (Nestoriuc, Martin, Rief, & Andrasik, 2008), and even treat hyperactive attention deficit disorder (Maurizio et al, 2013) through biofeedback.
THE ENDOCRINE SYSTEM
The nervous system is not the only mechanism that regulates the functioning of our bodies. The endocrine system plays a key role in helping to coordinate and integrate complex psychological reactions. In fact, the nervous system and the endocrine system work together in a constant chemical conversation. The endocrine glands release chemical substances called hormones that are carried throughout your body by the bloodstream. Hormones serve a similar function to neurotransmitters: They carry messages. Indeed, the same substance - for example, norepinephrine - may serve both as a neurotransmitter and as a hormone. A main difference between the nervous and the endocrine systems is speed. A nerve impulse may travel through the body in a few hundredths of a second, but hormones may take seconds, even minutes, to reach their target.
Hormones interest psychologists for two reasons. First, at certain stages of development, hormones organize the nervous system and body tissues. At puberty, for example, hormone surges trigger the development of secondary sex characteristics, including breasts in females and a deeper voice in males. Second, hormones activate behaviors. They affect such things as alertness or sleepiness, excitability, sexual behavior, ability to concentrate, aggressiveness, reactions to stress, even desire for companionship. Hormones can also have dramatic effects on mood, emotional reactivity, ability to learn, and ability to resist disease. Radical changes in some hormones may also contribute to serious psychological disorders, such as depression.
The pituitary gland, which is located on the underside of the brain, is connected to the hypothalamus. The pituitary produces the largest number of different hormones and thus has the widest range of effects on the body's functions. In fact, it is often called the "master gland" because of its influential role in regulating other endocrine glands. The pituitary influences blood pressure, thirst, sexual behavior, body growth, as well as other functions.
The pea-sized pineal gland is located in the middle of the brain. It secretes the hormone melatonin, which helps to regulate sleep-wake cycles.
The thyroid gland is located just below the larynx, or voice box. It produces one primary hormone, thyroxin, which regulates the body's rate of metabolism and, thus, how alert and energetic people are and how fat or thin they tend to be. An overactive thyroid can produce a variety of symptoms, including: overexcitability, insomnia, and reduced attention span. Too little thyroxin leads to the other extreme: constantly feeling tired and wanting to sleep.
Embedded in the thyroid gland are the parathyroids - four tiny organs that control and balance the levels of calcium and phosphate in the body, which in turn influence levels of excitability.
The pancreas lies in the curve between the stomach and the small intestine. The pancreas controls the level of sugar in the blood by secreting two regulating hormones: insulin and glucagon. These two hormones work against each other to keep the blood-sugar level properly balanced. Underproduction of insulin leads to diabetes mellitus, a chronic disorder characterized by too much sugar in the blood and urine; oversecretion of insulin leads to hypoglycemia, a condition in which there is too little sugar in the blood.
The two adrenal glands are located just above the kidneys. Each adrenal gland has two parts: an inner core, called the adrenal medulla, and an outer layer, called the adrenal cortex. Both the adrenal cortex and the adrenal medulla affect the body's reaction to stress. Stimulated by the autonomic nervous system, the adrenal cortex pours several hormones into the bloodstream. One, epinephrine, activates the sympathetic nervous system. Another hormone, norepinephrine (also a neurotransmitter), not only raises blood pressure by causing blood vessels to become constricted, but also is carried by the bloodstream to the anterior pituitary, where it triggers the release of still more hormones, thus prolonging the response to stress. This process is why it takes time for the body to return to normal after extreme emotional excitement.
The gonads - the testes in males and the ovaries in females - secrete hormones that have traditionally been classified as masculine (the androgens) and feminine (the estrogens). (Both sexes produce both types of hormone, but androgens predominate in males, whereas estrogens predominate in females.) These hormones play a number of important organizing roles in human development. For example, human and animal studies have shown that if the hormone testosterone is present during critical periods of prenatal development, the offspring (regardless of its sex) will develop a variety of male characteristics such as increased aggressiveness and male sex-type play. On the other hand, the absence of testosterone during this period promotes female behaviors such as nesting and female sex-type play (Auyeung, 2009; Kalat, 2001; Knickmeyer et al., 2005).
Testosterone has long been linked to aggressive behavior, perhaps explaining why violence is greatest among males between the ages of 15 and 25, the years when testosterone levels are highest. Male prisoners with high levels of testosterone are likely to have committed more violent crimes, at an earlier age, than other prisoners (Dabbs, Carr, Frady, & Riad, 1995). In addition, high levels of testosterone in normal men are associated with competitive aggression (Carre Gilchrist, Morrissey, & McCormick, 2010), increased risk taking (Goudriaan et al., 2010), and increased tendency toward spousal abuse (Romero-Martinez, Gonzalez-Bono, Lila, & Moya-Albiol, 2013). Even in elderly men with dementia, high levels of testosterone are related to an increase in aggressive behavior (Orengo, Kunik, Molinari, Wristers, & Yudofsky, 2002).
Interestingly, testosterone levels also appear to differ between married and unmarried men, and between married men who have children and those who do not. Research has shown that married men have lower testosterone levels than unmarried men, and this difference is even larger for married men with children (Gray, Kahlenberg, Barrett, Lipson, & Ellison, 2002). Evolutionary psychologists suggest these variations in testosterone level may be associated with a physical response of the male body that increases the nurturing capacity of men who become fathers and husbands.
GENES, EVOLUTION, AND BEHAVIOR
Our brain, nervous system, and endocrine system keep us aware of what is happening outside (and inside) our bodies; enable us to use language, think, and solve problems; affect our emotions; and thus guide our behavior. To understand why they function as they do, we need to look at our genetic heritage, as individuals and as members of the human species.
Genetics
Genetics is the study of how living things pass on traits from one generation to the next. Offspring are not carbon copies or "clones" of their parents, yet some traits reappear from generation to generation in predictable patterns. Around the beginning of the 20th century, scientists named the basic units of inheritance genes. To understand more about these blueprints for development, we must look at some cellular components.
The nucleus of each cell contains chromosomes, tiny threadlike bodies that carry genes - the basic units of heredity. Each chromosome contains hundreds or thousands of genes in fixed locations. Chromosomes vary in size and shape, and usually come in pairs. Each species has a constant number: mice have 20 pairs, monkeys have 27, and peas have 7. Human beings have 23 pairs of chromosomes in every normal cell, except the sex cells (eggs and sperm), which have only half a set of chromosomes.
At fertilization, the chromosomes from the father's sperm link to the chromosomes from the mother's egg, creating a new cell called a zygote. That single cell and all of the billions of body cells that develop from it (except sperm and eggs) contain 46 chromosomes, arranged as 23 pairs.
Genes are composed primarily of deoxyribonucleic acid (DNA), a complex organic molecule that looks like two chains twisted around each other in a double-helix pattern. Amazingly, a six-foot strand of DNA is crammed into the nucleus of every cell in your body. DNA is the only known molecule that can replicate or reproduce itself, which happens each time a cell divides.
Genes, like chromosomes, occur in pairs. In some cases, such as eye color, one may be a dominant gene (B for brown eyes) and the other a recessive gene (b for blue eyes). A child who inherits the gene for blue eyes from both parents (bb) will have blue eyes. A sibling who inherits the gene for brown eyes from both parents (BB) will have brown eyes. And, because the brown-eye gene dominates, so will a sibling who inherits the gene for brown eyes from one parent and the gene for blue eyes from the other (Bb or bB).
Polygenic Inheritance
Thus far, we have been talking about single-gene inheritance: We have said that a gene is a small segment of DNA that carries directions for a particular trait or group of traits. Examples of a single gene that controls a single trait are rare, however. In polygenic inheritance, multiple genes contribute to a particular trait. Weight, height, skin pigmentation, intelligence, and countless other characteristics are polygenic.
Your own unique genetic "blueprint" is internally coded on your 46 matched chromosomes and is called your genotype. Except for reproductive cells, your genotype is contained in every cell in your body. But heredity need not be immediately or fully apparent. Even identical twins, who have the same genotype, differ in small ways that allow family members to tell them apart. In some cases expression of a trait is delayed until later in life. For example, many men inherit "male-pattern baldness" that does not show up until middle age. Moreover, genes may predispose a person to developing a particular trait, but full expression of the characteristic depends on environmental factors. Given the same environment, for example, a person who inherits "tall" genes will be tall, and a person who inherits "short" genes, short. But if the first person is malnourished in childhood and the second person is well nourished as a child, they may be the same height as adults. Since an individual's genotype does not always obviously correspond directly to what is expressed, we use the term phenotype when referring to the outward expression of a trait. For example, people with an inherited tendency to gain weight (genotype) may or may not become obese (phenotype), depending on their diet, exercise program, and overall health.
The Human Genome
The term genome refers to the full complement of an organism's genetic material (all the genes and all the chromosomes). Thus, the genome for any particular organism contains a complete blueprint for building all the structures and directing all the living processes for the lifetime of that organism. The human genome, the sum total of all the genes necessary to build a human being, is approximately 20,000 to 25,000 genes, located on the 23 pairs of chromosomes that make up human DNA. At first that seems like a surprisingly small number for our species since these genes, contained within every cell of our body, distinguish us from other forms of life. However, only very small variations in the genetic code distinguish humans from other organisms. For instance, humans share 98.7% of their genes with chimpanzees (Olson & Varki, 2003). Even smaller variations in the human genome are responsible for the individual differences we see in the world's 7 billion people. Experts believe that the average variation in the human genetic code for any two different people is much less than 1%.
In June 2000, researchers working on the Human Genome Project announced the first rough map of the entire human genome. The results of this project have since led researchers to identify genes on specific chromosomes that are associated with Alzheimer's disease (Eriksson et al., 2011), alcoholism (Cao et al., 2011), schizophrenia and bipolar disorder (Georgieva et al., 2008; Lipina et al., 2011), cognitive functioning (Szekely et al., 2011), intelligence (Pan, Wang, & Aragam, 2011), mathematical ability (Docherty et al., 2010), and even happiness in females (H. Chen et al., 2013). By using these genetic markers, researchers expect not only to understand better the role of heredity in complex behaviors, but also to develop individualized genetic treatments for a wide variety of disorders (Collins, 2010).
These are the mechanisms of heredity and the dramatic advances in genetics that may someday lead to functional improvements in a variety of medical arenas. For the most part, we have used physical characteristics as examples. Behavior geneticists apply the same principles to psychological characteristics.
Behavior Genetics
Behavior geneticists study the topics that interest all psychologists - perception, learning and memory, motivation and emotions, personality, and psychological disorders - but from a genetic perspective. Their goal is to identify what genes contribute to intelligence, temperament, talents, and other characteristics, as well as genetic predispositions to psychological and neurological disorders (Willner, Bergman, & Sanger, 2008). Of course, genes do not directly cause behavior. Rather, they affect both the development and operation of the nervous system and the endocrine system, which, in turn, influence the likelihood that a certain behavior will occur under certain circumstances (Vinkhuyzen, van der Sluis, & Posthuma, 2010).
Animal Behavior Genetics
Much of what we know about behavior genetics comes from studies of nonhuman animals. Mice are favorite subjects because they breed quickly and have relatively complex behavior patterns. In strain studies, close relatives, such as siblings, are intensively inbred over many generations to create strains of animals that are genetically similar to one another, but different from other strains. When animals from different strains are raised together in the same environment, differences between them largely reflect genetic differences in the strains. This method has shown that performance on learning tasks, as well as sense of smell and susceptibility to seizures, are affected by heredity.
Selection studies are another way to assess heritability, the degree to which a trait is inherited. If a trait is closely regulated by genes, when animals with the trait are interbred, more of their offspring should have the trait than one would find in the general population. Humans have practiced selective breeding for thousands of years to create breeds of dogs and other domesticated animals that have desirable traits - both physical and psychological.
Human Behavior Genetics
For obvious reasons, scientists cannot conduct strain or selection studies with human beings. But there are a number of ways to study behavioral techniques indirectly.
Family studies are based on the assumption that if genes influence a trait, close relatives should share that trait more often than distant relatives, because close relatives have more genes in common. For example, overall, schizophrenia occurs in only 1% to 2% of the general population (N. L. Nixon & Doody, 2005; L. N. Robins & Regier, 1991). Siblings of people with schizophrenia, however, are about 8 times more likely (and children of schizophrenic parents about 10 times more likely) to develop the disorder than someone chosen randomly from the general population. Unfortunately, because family members share not only some genes but also similar environments, family studies alone cannot clearly distinguish the effects of heredity and environment.
To obtain a clearer picture of the influences of heredity and environment, psychologists often use twin studies. Identical twins develop from a single fertilized ovum and are therefore identical in genetic makeup at conception. Any differences between them must be due to their experiences. Fraternal twins, however, develop from two separate fertilized egg cells and are no more similar genetically than are other brothers and sisters. If twin pairs grow up in similar environments and if identical twins are no more alike in a particular characteristic than fraternal twins, then heredity cannot be very important for that trait.
Twin studies suggest that heredity plays a crucial role in schizophrenia. When one identical twin develops schizophrenia, the chances that the other twin will develop the disorder are about 50%. For fraternal twins, the chances are about 15% (Gottesman, 1991). Such studies have also provided evidence for the heritability of a wide variety of other behaviors, including cognitive ability (Briley & Tucker-Drob, 2013), verbal skills (Viding et al., 2004), mild intellectual impairment (Spinath, Harlaar, Ronald, & Plomin, 2004), aggressiveness (Niv, Tuvblad, Raine, & Baker, 2013), compulsive gambling (Shah, Eisen, Xian, & Potenza, 2005), specific phobias (Van Houtem et al., 2013), depression, anxiety, and eating disorders (Eley, Stevenson, 1999; O'Connor, McGuire, Reiss, Hetherington, & Plomin, 1998; Silberg & Bulik, 2005).
Similarities between twins, even identical twins, cannot automatically be attributed to genes, however; twins nearly always grow up together. Parents and others may treat them alike - or try to emphasize their differences, so that they grow up as separate individuals. In either case the data for heritability may be biased. To avoid this problem, researchers attempt to locate identical twins who were separated at birth or in very early childhood and then raised in different homes. A University of Minnesota team led by Thomas Bouchard followed separated twins for more than 10 years (Bouchard, 1984, 1996; Bouchard et al., 1990; W. Johnson, Bouchard, Segal, & Samuel, 2005). They confirmed that genetics plays a major role in mental retardation, schizophrenia, depression, reading skill, and intelligence. Bouchard and his colleagues have also found that complex personality traits, interests, and talents, and even the structure of brain waves, are guided by genetics.
Studies of twins separated shortly after birth have also drawn criticism. For example, the environment in the uterus may be more traumatic for one twin than the other (J. A. Phelps, Davis, & Schartz, 1997; Ross, Krauss, & Perlman, 2012). Also, since adoption agencies usually try to place twins in similar families, their environments may not be much different (Joseph, 2001). Finally, the number of twin pairs separated at birth is fairly small. For these reasons, scientists sometimes rely on other types of studies to investigate the influence of heredity.
Adoption studies focus on children who were adopted at birth and brought up by parents not genetically related to them. Adoption studies provide additional evidence for the heritability of intelligence and some forms of mental illness (Insel & Wang, 2010; Jacobs, van Os, Derom, & Thiery, 2008) and the role of genetics in behavior previously thought to be solely determined by environmental influences like smoking (Boardman, Blalock, & Pampel, 2010) and the risk for drug abuse (Kendler et al., 2012).
By combining the results of twin, adoption, and family studies, psychologists have obtained a clearer picture of the role of heredity in many human characteristics, including schizophrenia. The average risk of schizophrenia steadily increases in direct relation to the closeness of one's biological relationship to an individual with the disorder.
Social Implications
Science is not simply a process that takes place in a laboratory; it can also have widespread effects on society at large. To the extent that we can trace individual differences in human behavior to chromosomes and genes, we have a potential, biologically, to control people's lives. This potential raises new ethical issues.
Contemporary techniques of prenatal screening now make it possible to detect many genetic defects even before a baby is born. Chorionic villus sampling and amniocentesis are two procedures for obtaining samples of cells from fetuses in order to analyze their genes. In the first, the cells are taken from membranes surrounding the fetus; in the second, the cells are harvested from the fluid in which the fetus grows. Using these procedures, genetic problems are often detectable during pregnancy. Do the parents have a right to abort the fetus? Should society protect all life, no matter how imperfect it is in the eyes of some? If not, which defects are so unacceptable that abortion is justified? Most of these questions have a long history, but recent progress in behavior genetics and medicine has given them a new urgency. We are reaching the point at which we will be able to intervene in a fetus's development by replacing some of its genes with others. For which traits might this procedure be considered justified, and who has the right to make those decisions? Such questions pose major ethical dilemmas, and it is important that we all think critically about them (Sherr, Michelson, Shevell, Moeschler, Gropman, & Ashwal, 2013).
So far, we have been talking about the environment as if it were something out there, something that happens to people, over which they have little control. But individuals also shape their environments. The genes and predispositions individuals inherit alter the environment in several ways (Plomin, DeFries, Craig, & McGuffin, 2003). For example, people tend to seek environments in which they feel comfortable. A shy child might prefer a quieter play group than would a child who is more outgoing. In addition, our own behavior causes others to respond in particular ways. For example, one study showed that children with a genetic predisposition toward fearfulness were treated differently by their caregivers, which in turn reshaped their neurocircuitry through plasticity, thus exacerbating their fearfulness (Fox, Hane, & Pine, 2007). Because genes and environments interact in so many intricate ways, trying to separate and isolate the effects of heredity and environment - nature and nurture - is artificial (Kline, 2008).
The study of behavior genetics and that of evolutionary psychology, which we will consider next, makes many people uneasy. Some fear that it may lead to the conclusion that who we are is written in some kind of permanent ink before we are born. Some people also fear that research in these fields could be used to undermine movements toward social equality. But far from finding human behavior to be genetically predetermined, recent work in behavior genetics shows just how important the environment is in determining which genetic predispositions come to be expressed and which do not. In other words, we may inherit predispositions, but we do not inherit destinies. The emerging picture confirms that both heredity and environment (nature and nurture) work together as allies to shape most significant behaviors and traits (Kline, 2008).
Evolutionary Psychology
Much as behavior geneticists try to explain the individual differences in human behavior, evolutionary psychologists often focus more on trying to explain the behavioral traits that people have in common. The key to these shared characteristics, according to evolutionary psychology, is the process of evolution by natural selection, first described by Charles Darwin in On the Origin of Species (1859).
According to the principle of natural selection, those organisms that are best adapted to their environments are most likely to survive and reproduce. If the traits that give them a survival advantage are genetically based, those same genetic characteristics are passed on to their offspring. Organisms that do not possess the adaptive traits tend to die off before they reproduce, and hence, the less-adaptive traits do not get passed along to future generations. Natural selection, therefore, promotes the survival and reproduction of individuals who are genetically well adapted to their particular environment.
Evolutionary psychologists study the origins of behaviors and mental processes, emphasizing the adaptive or survival value of such traits. Evolutionary psychologists look at the role that natural selection might have played in selecting for adaptive behaviors, especially during the long period that our ancestors lived as hunter-gatherers. They argue that, just as our hands and upright posture are products of natural selection, so are our brains. As a result, our brains are "prewired" to learn some things more easily than others, to analyze problems in certain ways, and to communicate in distinctively human ways.
Evolutionary psychologists cite language as a prime example (Pinker, 2007). All normal children acquire language without specific instruction; children in different cultures acquire language at about the same ages and in predictable stages; and the underlying structure of all human languages (nouns and verbs, subjects and objects, questions and conditional phrases, and so on) is basically the same. Taken as a whole, evolutionary psychologists argue, this evidence suggests that human brains have a built-in "program" for language. Evolutionary psychologists cite mate selection as another example. In choosing a partner, males and females tend to pursue different strategies. Why? Evolutionary psychologists answer this way: Human females usually have only one child at a time. Women also invest more in each child than men do - going through pregnancy, caretaking, and providing nourishment. It would seem to be most adaptive for females to look for males who will provide the best genes, resources, and long-term parental care. Males, on the other hand, are limited only by the number of prospective mates they can attract, because sperm are plentiful and quickly replaced. It may be most adaptive for males to seek to mate with as many females as they can and to compete with other males for access to females.
Evolutionary psychologists have also made important contributions to other areas of psychology such as learning and memory. For example, in one study of what is called "adaptive memory", it was found that words with "survival relevance" such as those referring to food, shelter, or predators, are more likely to be remembered in a surprise recall task than control words (Nairne & Pandeirada, 2008). Another study of adaptive memory showed that people have an increased likelihood of recalling the "location" of objects important to their survival, compared to objects with less survival value (Nairne, VanArsdall, Pandeirada, & Blunt, 2012).
Evolutionary psychology is not without its critics (Brinkmann, 2011). Some opponents argue that science is being used to justify perpetuating unjust social policies. These critics claim that simply by saying a trait is adaptive implies that it is both genetically determined and good. In the past, racists and fascists have misused biological theories to promote social injustices. In Nazi Germany, for example, Jews were considered genetically inferior, a view that was used to justify their extermination. Similarly, the evolutionary theory of male-female differences in mate selection could be seen as endorsing male promiscuity, since it is biologically adaptive. In response, evolutionary psychologists are quick to point out that their aim is not to shape social policy, but to advance knowledge in psychological science and provide insights into the origins of human behavior (Buss, 2013; Confer et al., 2010).