Why Evolutionary Psychology Needs Behavior Genetics, and Vice Versa
Twenty claims about heritability and human nature
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Evolutionary psychology and behavior genetics are two of my favorite fields in science. Between them, they’ve shed more light on how nature and nurture shape us than the rest of the social sciences combined - and more than thousands of years of philosophical musings on the topic.
Until recently, the two areas have developed largely in parallel, with each pursuing its own unique agenda. Over the past decade or so, however, it’s become increasingly clear that this separation is neither necessary nor desirable. Properly integrated, evolutionary psychology and behavior genetics can powerfully inform each other. Most notably, evolutionary psychology can furnish hypotheses about why heritable individual differences exist and persist, while behavior genetics can test, refine, and sometimes overturn evolutionary psychological hypotheses.
In this post, I lay out twenty key claims about evolutionary psychology, behavior genetics, and what we stand to gain by bringing these fields into closer contact.
Evolutionary psychology and behavior genetics both tackle the nature-nurture problem, but they do so in very different ways.
Evolutionary psychology focuses primarily on the things we all have in common, aiming to explain universal aspects of mind in terms of selection pressures acting in our ancestral past. A good example is fear. All normally developing human beings have the capacity for this emotion. Evolutionary psychology asks why. Why did fear evolve? (The answer, roughly speaking, is that it motivates us to avoid or escape threats.)
Behavior genetics, in contrast, focuses on individual differences. For example, rather than asking why everyone has the emotion of fear, it asks why some people are more prone to fear than others. To what extent are the differences due to genes and to what extent to non-genetic causes? Behavior geneticists address questions like these via twin studies, adoption studies, and newer methods linking specific gene variants to observable traits. When individual differences in a trait are correlated with individual differences in genes, we say the trait is heritable. According to the First Law of Behavior Genetics, all traits - including fear - are heritable to some degree.
For the most part, evolutionary psychology and behavior genetics have developed in isolation. This makes some sense, given that they focus on such different questions.
There are, however, various ways in which the two fields can productively inform each other - or even be integrated into a hybrid field that Brendan Zietsch and colleagues call evolutionary behavior genetics.
To see how this might work, let’s first dispel some confusions. One common confusion is that if a trait is heritable, it must therefore be an adaptation crafted by natural selection. This view is doubly wrong. First, traits shaped strongly by selection often have relatively low heritability levels. This is true, for instance, of the number of legs people have. Almost all of us have the same genes for leg number, and those genes give almost all of us two legs. As a result, differences in leg number are due largely to environmental causes such as accidents, rather than to genes. That’s another way of saying that leg number has low heritability. Having two legs is innate, of course; we’re not socialized to have two rather than 22. But that doesn’t mean having two legs is especially heritable. It’s not.
Conversely, many traits that are clearly not adaptations have surprisingly high heritability levels. Mobile phone use is my go-to example. Geoffrey Miller and colleagues showed that individual differences in how people use their phones are significantly heritable: Identical twins are more similar in phone-use patterns than non-identical twins. Needless to say, we don’t have genes specifically dedicated to mobile phones; the technology is far too new. Instead, we have genes that affect broader psychological dispositions - personality traits, for example - which in turn help shape our mobile phone habits.
Heritability is a prerequisite for natural selection: Selection can’t operate unless there are heritable individual differences from which it can “choose” the most useful variants. According to the breeder’s equation, the more heritable a trait is, the faster it can evolve under selection. By favoring some gene variants and removing others, selection can reduce heritability.