As I look around the room, I can see, I think it's about half of you have been exposed to high levels of a powerful chemical, and it is circulating in your blood as we speak. It's flowing freely through almost all of your cells, including neurons, where it affects whether they live or die and how they grow and function. So this one chemical has profound and lasting effects -- not just on your body, but also on your brain and behavior.
This is testosterone. Both sexes have it, but men have much more than women, about 15 to 20 times more. And that is what explains why half of you are bigger and hairier than the other half, on average, like all sex differences.
For the last 20 years, I've been teaching about behavioral endocrinology, and this is a field that focuses on how hormones affect behavior and vice versa. And I'm particularly drawn to testosterone because it explains so much about why the sexes are different.
But this area of science has become deeply entangled in the culture wars. And now, simply saying things that are obvious to most biologists, like that there are two reproductive classes, male and female, can land you in a heap of trouble. And I know because I've had some personal experience with this. And I also know that for some trans people, their allies and others, that this language about the biology of sex can feel painful and that others are trying to weaponize the science.
And here I do not have the answers, but I can tell you that my students have really enjoyed learning about this science, partly because it helps them understand more about their own bodies and feelings. And they come away with a sense of more compassion for other people who are different from themselves.
So I'm going to be talking about sex differences in play to try to convince you that this science is fascinating, and it can help us all be better off.
So first, I want to share the results from a joint project I started 15 years ago, in which I grew two small organs, that secreted testosterone, in my uterus.
(Laughter)
Those organs were attached to the new human I was growing in there. And now ...
(Laughter)
(Applause)
And now you can observe the behavior of that human in this video of two boys wrestling each other. It's from a few years ago, and my son is the one with the short hair.
This kind of rough-and-tumble play is more common in boys than girls everywhere in the world. And when my son was growing inside of me, his tiny little testes were cranking out the testosterone. And at that early stage, it has some very big and important jobs to do. It has to masculinize the genitalia, and it has to ensure that everything is set up for sperm to be produced and delivered later on. And it is also acting in the brain to motivate later behaviors, like play fighting, that are particularly beneficial to many male mammals.
I never gave much thought to testosterone until I spent the better part of a year in western Uganda studying wild chimpanzees. And as I watched them living their lives -- eating, playing, sleeping, fighting, having sex -- I was really struck by our shared patterns of behavior. And particularly relevant to this talk was the fact that the little males did much more play fighting than the little females. And these connections to us are so striking because we don't share any culture with chimpanzees. But we do share almost all of our genes and our hormones, including higher levels of testosterone in the males.
And another pattern of behavior we share is that relative to females, male chimps spend much more time and energy competing for social dominance. And in chimps, this involves lots of fighting and also lots of threats and having a sense about when to challenge other males but also when to submit and when to flee. And these males aren't fighting each other because they know that this is a great way to get more sex with fertile females. But males who behave this way do tend to pass on more of their genes into future generations, and then their sons inherit similar propensities.
So simply surviving without passing on one's genes is an evolutionary dead end. So in addition to survival skills, young animals like chimps also need to learn reproductive skills, and they do that through play. And our ancestors also practiced these skills through play. And that legacy is reflected in our own kids.
Now not all boys have any desire to tackle their friends, and they would rather play house or dress up, and they should go for it. There are no hard rules in nature about how the sexes should play. When I was little, I used to wrestle with my three older brothers, and I played Little League baseball. But when I played with my girlfriends, like my best friend Annie, she's the dominant one there in the stripes, and she's sitting right here.
(Laughter)
Our play just didn't involve that kind of roughhousing that my son and his friends were into. Annie and I would do stuff like have tea and run class for our stuffed animals -- sorry, I'm just getting a little emotional because she's here -- the smallest of whom lived in a doll house. And for some reason, we loved playing office. And -- I know, crazy. And we developed a filing system and --
(Laughter)
And we spent a lot of time filling out forms. OK, I'm not kidding. Filling out forms, and I'm saying something publicly I've never said before, and I'm sorry, Annie. You know what I'm going to say. Filling out forms from junk mail, which we might have stolen from neighborhood mailboxes.
(Laughter)
OK, so wait, wait, am I really trying to tell you that knowing how to set up tea for five or subscribe to "National Geographic" was supposed to make us better moms or whatever? OK, not exactly. The specifics of play are always influenced by culture, but social play in general helps to develop skills that both sexes need, like how to resolve conflict, how to take turns, and even to figure out just what you can get away with. So ... But the sex-specialization part, with more nurturing play in girls and more fighting play in boys, did likely evolve because these are skills that each sex needs to learn how to reproduce.
OK, that's a little evolutionary background. And now we can ask why scientists think that testosterone drives any of this. So our strongest evidence comes from experiments in other animals. And here's a couple of headlines. So in females, in rats and monkeys anyway, jacking up testosterone during that early developmental period causes rough play to increase dramatically. And in males, the reverse is true. Blocking testosterone during that same early period causes rough play to plummet.
And in humans, of course, we can't go messing around with the kind of hormones that fetuses are exposed to, so we have to rely on less direct evidence. And we do have a wealth of studies on the play styles of girls who were exposed to unusually high testosterone levels in the womb. And there, study after study shows that these girls do have an increased preference for rough play.
So we still have so much to learn about how genes, hormones and culture all interact to produce sex differences in behavior. But all of the evidence we do have points in the same direction. And that is that differences in testosterone and evolutionary pressures explain why boys more than girls, think it's a great idea to tackle their friends.
OK, so let's just assume that there is something natural about all this. It's also natural to hook up with your ex.
(Laughter)
OK? But that does not mean it's a good thing to do.
(Laughter)
And according to one mom, rough play in boys is also something we should probably reconsider. And she expressed this view in a parenting magazine, quote, "Letting my boys wrestle with each other only reinforces toxic masculinity, and I want no part of it."
OK, so perhaps we could create more peaceful men by nipping it in the bud and just preventing them from practicing aggression when they were little. OK, that's one idea. Except the available evidence just doesn’t support it.
So, for instance, depriving male rats of the kind of rough play they want leads to adult male rats who never learned how to manage their aggressive impulses. They actually end up more rather than less aggressive. They fail to cooperate, they fail to respond appropriately to social cues, and ultimately they fail to find mates. And having males like these around doesn't help anybody.
(Laughter)
So we are not rats, and we are not chimpanzees. For one thing, if you men were chimpanzees, you would be ripping each other to shreds. If you were meeting for the first time, that is. It would be mayhem. And we humans have something no other animal has. And that is the ability to reflect on, talk about and, together, to determine how we can control some of our more harmful impulses.
And culture here makes all the difference. For example, men here in Canada are much less violent than our male neighbors over the southern border. Apart from hockey, apparently.
(Laughter)
And that is not because Canadian men have lost their testicles or ...
(Laughter)
Or because Canadian boys don't wrestle each other. Instead, it's because of differences in the Canadian culture, perhaps in the gun laws or in the health care system or in levels of socioeconomic inequality.
So as a society, we lose a lot if we leave the science of sex out of the conversation or distort the facts. And that science strongly suggests that male typical play is not budding toxic masculinity. Instead, it is a healthy behavior that we should not discourage.
(Applause)
So exaggerating only slightly, testosterone from those tiny -- Sorry, tiny testes I grew 15 years ago, made my son who he is today. And he is still a great kid, even with his testosterone reaching record levels.
(Laughter)
And I want him and all of you to be captivated by the science of sex and to feel comfortable talking about it. And last, I hope that all our kids can just play more in real rather than virtual life, and preferably outside.
Thank you.
(Applause and cheers)
