Three Lines of Evidence for Innate Sex Differences
An excerpt from my forthcoming book, A Billion Years of Sex Differences
This is the twelfth excerpt from my forthcoming book, A Billion Years of Sex Differences. You can access the full collection here, and preorder the book here.
In this installment and the next, we’ll explore six lines of evidence that can help us decide whether a given sex difference has an innate basis. We’ll cover three in this post and three in the next.
Note that the question is not whether learning, socialization, and culture play a role; clearly they do. The question is whether these factors operate alone or interact with deeper, innate mechanisms that nudge male and female development in different directions.
To get the ball rolling, we’ll look at the following lines of evidence for the latter option:
Developmental trajectory: When do the differences first appear?
Stability over time: Do the differences persist across generations and through significant social changes?
Resistance to social pressure: Do the differences endure despite attempts to override them?
On their own, none of these lines of evidence is decisive. Taken together, however, they form the basis of the case for a non-trivial innate contribution to any given human sex difference.
Three Lines of Evidence for Innate Sex Differences
The standard sociocultural explanations for sex differences are flawed in various ways. This doesn’t mean, however, that the differences necessarily have an innate basis. It could just be that we need better sociocultural theories. How, then, can we know whether nature plays a role?
A common reaction to this question is to say that we can’t know, because there’s never been a society in which the sexes have been treated identically. The implicit assumption is that treating the sexes identically is the only way to establish whether there’s an innate contribution. Not true! At least six lines of evidence can help us settle the issue in the absence of identical treatment.
Before getting to that, though, I should highlight something that many will find surprising, namely that brain evidence isn’t one of the six. People often assume that tracing a psychological sex difference to a brain difference constitutes unambiguous evidence for its innateness. It doesn’t. The reason is simple: The brain isn’t shaped only by nature; it’s shaped by nurture as well. That means that, as a general rule, sex differences in the brain are silent witnesses to their own causes, telling us nothing about the nature–nurture issue. Certainly, some differences appear before birth, ruling out socialization as a cause. But even then, the implications are far from straightforward. That’s because differences in the brain don’t necessarily imply differences in the mind or behaviour. Intelligence, for example, is instantiated in somewhat different ways in men’s brains than in women’s, and yet men’s and women’s average intelligence is the same. For both these reasons, leaping from ‘brain sex difference’ to ‘innate psychological sex difference’ is a risky game. And it’s not one we need to play. There’s plenty of other evidence.
1. Developmental Trajectory
The first line of evidence concerns the developmental trajectory of the sex difference under investigation. To start with, many female–male differences appear in a rudimentary form remarkably early in life. Examples include not only sex differences in physical traits like size, strength, and body fat percentage, but also differences in behavioural traits such as activity levels, aggression, and play patterns. In all such cases, the differences appear long before children are exposed to TV, movies, or media, and long before they even know their own sex.