The role of, and research into hormones and genes in aggression.
High levels of testosterone are associated with increased aggression. Chang et al. (2012) found that that the more testosterone a fish had in its blood the more aggressive it was so showing the that aggression and testosterone are linked.
Exposure to higher levels of testosterone in the womb affects the developing brain and can lead to an increase in aggression.
Low levels of oxytocin are thought to decrease trustworthiness and so increase levels of aggression. Lane et al. (2014) found more oxytocin led to an increase in trustworthiness.
People with low levels of cortisol have an under aroused autonomic nervous system (ANS) so to increase arousal they become aggressive. Burnett et al. (2000) found that boys who had lower levels of cortisol in their saliva were three times more aggressive than boys with higher levels of cortisol, so cortisol does seem to play a role in aggressive behaviour.
Chang et al. (2012) found that when aggression, exploring and boldness increased in fish so did the amount of testosterone in their blood. In general animal studies have shown that increased testosterone leads to aggression/castration leads to lowered aggression. Animal studies may not be generalisable to humans as aggression in humans is can be social whereas in animals this is rarely the case.
Dabbs et al. (1987) found that the level of testosterone was higher in the males who had committed violent crimes compared to those who had committed non-violent crimes. Dabbs et al. (1987) had 89 participants which could be considered generalisable due to the large sample size.
Studies on humans may be correlations that find a relationship between hormones and aggression. Correlations cannot tell us if an increase in hormones causes aggression or vice versa. For example, it may be that aggression increases the levels of testosterone, rather than testosterone increasing aggression so testosterone does not play a role in causing aggression.
Whilst Lane et al. (2014) found more oxytocin did lead to an increase in trustworthiness, they did not study whether lack of the hormone caused aggression so it may not be a factor in aggressive behaviour.
Psychological research into the influence of hormones on aggression does not consider other explanations social learning theory would suggest that aggression is the result of imitating aggressive role model and not a failure of hormone regulation.
Psychologists have used selective breeding in animals and have found that aggression is a trait that can be transmitted from parent to offspring, thus supporting the genetic explanation.
Research with human subjects has focused on twin studies that have looked at the incidence of aggression displayed by monozygotic (MZ or identical) and by dizygotic (DZ or non-identical) twins. Differences in rates (concordance) of aggression between these sets of twins have indicated that aggression has a genetic element.
With the advances in genetic testing in the last fifty years, specific genes have been identified which have been shown to carry the aggression trait down to individuals. One such gene is the MAOA gene, and one variant has been named the ‘warrior gene’. The MAOA gene is responsible for the production of the protein monoamine oxidase. This protein allows the metabolising of noradrenaline, serotonin and dopamine. A dysfunction in this gene can result in these neurotransmitters not being broken down in the body. If adrenaline isn’t metabolised, then we end up with too much adrenaline. This can cause hypersensitivity in the fight or flight response and individuals may overreact to an external stimulus and perceive a threat where one does not exist. Furthermore, if dopamine is not broken down, increased or excessive levels of dopamine are also linked to aggressive behaviour. Serotonin has a calming influence, and low levels have been implicated in a reduction of control over impulsive behaviour.
Brendgen et al. (2005) studied physical aggression and found that the concordance rate for monozygotic twins was nearly double the concordance rate for dizygotic twins. Brendgen et al. (2005) used both teacher ratings and peer ratings of aggression, both scores were similar suggesting the data is reliably measuring the participants’ aggression.
McGuffin and Gottesman (1985) found a concordance rate of 87% for monozygotic twins and aggression and 72% for dizygotic twins, suggesting genes influence aggression. As the concordance rate was 87%, and not 100% in McGuffin and Gottesman (1985) it suggests there must be other factors apart from genes that influence aggression.
Twin studies are often used to study the genetic influence on aggression by comparing the concordance rates of monozygotic and dizygotic twins. Twin studies may not be valid, as it could be that monozygotic twins have a more similar environment than other siblings and this could be a reason for the higher concordance rates in aggression.
Other research into aggression has bred animals, such as mice, to investigate whether aggressive animals will have aggressive offspring or not. As animals are kept under controlled conditions researchers can assume the aggression in any offspring is due to the breeding of two aggressive parents, and not to environmental factors.
Stuart et al. (2014) studied 97 men who had been involved in severe domestic abuse to their partners. They found the most violent men had the faulty MAOA gene. These men engaged in the highest level of physical and psychological aggression and inflicted the worst injuries on their partners.
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