Factual fallacies result in unsound deductive arguments (even if the arguments are valid).
Factual fallacies broadly refer to deductive arguments whose premises are not true. It may seem unnecessary to term an argument where one of the premises is not true a "fallacy." After all, an important part of scientific progress is making arguments with premises (often assumptions) that scientists later discover to not be true. However, there are some common errors involving untrue premises that are often termed deductive "fallacies." Among the factual "fallacies" are:
1) Over-generalization: a premise is too broad. For example,
PREMISE: All mammals give birth to live young.
PREMISE: Echidnas do not give birth to live young, but lay eggs.
CONCLUSION: Therefore, echidnas are not mammals.
EXPLANATION: The goal of science is often to discover generalizations that we can make. However, true generalizations are extremely difficult to find (easy generalizations are often trivial). Therefore, it is important to be very careful when making generalizations, because the generalizations may not be true. Although it would be convenient to make the generalization about mammals that they all give birth to live young, the generalization is simply not true.
APPLICATION: People are naturally inclined to generalize and stereotype (about the world, other people, and even themselves; Steele and Aronson, 1995). Clearly, over-generalizations can have real and damaging consequences, such as biases against groups identifiable by race, gender, nationality, etc. Because non-trivial scientific generalizations are so rare, arguments involving premises that involve the words "All," or "Never" or "None," etc. are likely to be over-generalizations and therefore fallacies.
2) False Dichotomy: Ignoring the middle ground. For example,
PREMISE: Either we can stimulate the economy or we can combat climate change.
PREMISE: We need to stimulate the economy.
CONCLUSION: Therefore, we cannot combat climate change.
EXPLANATION: False dichotomies involve mis-representing a set of options by asserting that there are only two choices, or presenting a "false choice." In the case of the economy vs. addressing climate change, we do not have to choose between a strong economy and reducing human-caused climate change (BCG, 2018). There are many alternatives that potentially involve stimulating the economy while also combatting anthropogenic climate change (e.g. creating new industries, improving efficiency, etc.).
APPLICATION: False dichotomies are commonly used to try to constrain arguments to two options, where only one option is reasonable. For example, a politician may argue "you either support the war or you do not support our country," knowing that most people would like to support the country.
However, dichotomies can also be extremely useful for science! Dichotomies can help scientists structure research and generate cleanly-testable hypotheses. Therefore, it is critical for scientists to make sure that the dichotomies that they posit are not false: that the dichotomies involve two reasonable alternatives, and do not ignore other reasonable possibilities.
3) Weak Analogy: Falsely considering analogous systems to be the same. For example,
PREMISE: Complex things like wristwatches need designers.
PREMISE: Organisms are much more complex than watches.
CONCLUSION: Organisms have a designer.
EXPLANATION: Analogies can help us gain an intuitive understanding of unfamiliar concepts. However, for an analogy to be a valid representation of the system that it helps to explain, the analogy must embody the fundamental parts or processes of the system. Although it is true that wristwatches and organisms are both complex, wristwatches are built in a fundamentally different way than organisms are, and do NOT embody the developmental processes of organisms. Wristwatches are constructed from pre-fabricated parts that must be designed beforehand to fit together. Organisms, on the other hand, develop from continuous interactions among inherited genes, proteins, and the environment. The phenotype of an organism is not pre-determined like a wristwatch is. Although a wristwatch may seem hueristically"representative" of an organism, it is not. Therefore, a wristwatch is a false analogy for an organism.
APPLICATION: Weak analogies are commonly used to sell products or services that are not evidence-based or based on empirical principles (Kelso, 2018). Weak analogies appeal to false similarity, and take advantage of situations where audiences may not be well-informed about the nature of objects or processes (such as organismal development).