A Billion Years of Sex Differences
How evolution shaped the minds of men and women: A transcript of my recent professorial lecture
I recently gave a professorial lecture at my university, focused on some of my theoretical work on the evolution of human sex differences. The following is a lightly edited transcript of the lecture.
Abstract
According to an old joke, everyone knows that men and women are different - except social scientists. How do men and women differ, where do the differences come from, and how do they compare to those in other species? In this talk, I’ll argue that many human sex differences have evolutionary roots, and mirror those found in other animals. At the same time, however, humans are an unusual species: Our sex differences are relatively modest, and in many ways, we're more like the average pair-bonding bird than the average mammal. This peculiar arrangement ultimately traces back to an unexpected source: our big brains and extreme intelligence.
A Billion Years of Sex Differences
Thank you for that great introduction. Much appreciated. And thanks for coming everyone. My name is Steve, and I’m going to tell you a little about some of my research.
I’ve done research on various topics in the course of my career, but my favorite area - my real research passion - is a field called evolutionary psychology.
Evolutionary Psychology
Evolutionary psychology involves taking theories from evolutionary biology and using them to shed light on the mind and behavior. It starts from the premise that humans are animals and that, like all animals, from beatles to bonobos, we’ve evolved to pass on our genes.
To cut a very long story very short, organisms evolve to do this in three main ways: They evolve to survive, to have offspring, and to help their genetic relatives to survive and have offspring as well.
Importantly, this applies not only to the body but also to the mind. To see how, let’s first look at how we use evolutionary theory to explain anatomical features in other species.
Applying Evolution to the Body
So, here’s a question for you: Why do lions have sharp teeth? Why did sharp teeth evolve? Why are they useful for the lion?
The answer, of course, is that they use them to catch their prey then devour their prey. In other words, to obtain lunch.
Here’s a second question for you: Why do gazelles have fast legs? Why are fast legs useful for gazelles?
The answer is that they’re to escape the clutches of hungry lions and other predators. In other words, to avoid becoming lunch.
Applying Evolution to the Mind
What evolutionary psychology does is take this explanatory framework and apply it to the fundamental constituents of the mind.
I’ll give you some examples of how this might work. First, why do we have the capacity for fear? Why might fear have evolved? Why might it be useful?
Simple: Fear motivates us to escape or avoid danger and harm: to run away from the lion, just like a gazelle, or stay away from the edge of the cliff.
Second question: Why do we have sexual desire? And I thought I probably shouldn’t include an image for this slide…
I’ve also thought of a polite way to answer the question: Sexual desire motivates us to engage in certain activities that, throughout most of our evolutionary history - i.e., before we invented reliable birth control - typically resulted in babies.
Speaking of babies, why do we have parental love?
Simple again: To motivate us to look after our offspring so they survive to adulthood and can potentially start this whole process again.
Those three examples are, I think, pretty uncontroversial. Most psychologists would sign off on them, especially the first two. But what evolutionary psychology does is apply this reasoning to more complex aspects of mind and social behavior, and to areas that psychologists have traditionally explained largely in terms of learning, socialization, and culture.
I’ve done research on a few of these areas. I’ve done research, for instance, on altruistic behavior, and how it varies between genetic relatives and non-relatives (see here and here). But the area that I love most and that I always come back to is the evolution of sex differences. That’s what we’ll focus on for the rest of the talk.
Two Models of Sex Differences
In particular, I want to focus on some theoretical work that I did with my then-PhD student, now colleague, Andrew Thomas. We laid out two possible models of the evolution of human sex differences and human sexual psychology, which we called the MCFC model and the MMC model. MCFC stands for male-compete/females choose; MMC stands for mutual mate choice. Our aim was to figure out which model applies best to our species.