Hi there.
I could be wrong, but I think this talk may have the distinction of being the one talk in this series that ends with orgasm.
(Laughter) But let's not get ahead of ourselves.
(Laughter) Have you ever thought about the fact that you're here, alive on this planet because every one of your ancestors reproduced?
Every one, in an unbroken chain, all the way back to the first life on this planet, over three and a half billion years ago.
That's a lot of reproducing.
And for the past billion years, your ancestors reproduced sexually.
So sex is a pretty big deal.
But you probably knew that.
But let's talk about human mating.
Why does human mating take the forms that it does?
Why are we attracted to certain people?
Why do we sometimes form long-term romantic relationships?
Why do we sometimes cheat?
Now I don't mean why consciously do we do these things.
I don't mean what happens in the brain to cause it.
I mean, why did we evolve these feelings and these behaviors?
In other words, how did the underlying brain structures and brain chemistry contribute to our ancestors' reproductive success so that those traits got passed on into the present generation while others didn't.
Answering evolutionary questions like this is like being a crime scene investigator, we're left with the evidence, and we have to try to establish what happened.
So let's go back six or seven million years ago to our early ancestors.
This is right after the split between our lineage and the lineage that would eventually give rise to chimpanzees.
Now these were small brained apes, they walked on two legs, and males probably fought each other for mating opportunities.
We know this because males fight for mates in all of our closest living relatives, chimpanzees, orangutans, gorillas.
And because males are larger than females when they fight for mates.
And the fossil record indicates that our male ancestors were larger than females.
So males tend to be larger, more muscular, stronger, more physically aggressive, when they fight for mates.
Our species has all the hallmarks of a species that's experienced an evolutionary history of male fighting for mates.
For example, men have, on average, 60 percent more muscle mass, and 75 percent more upper body muscle mass, and those differences in musculature translate into large sex differences in strength.
The average man is stronger than 99.9% of women.
These are data on hand strength, which is a good predictor of overall upper body strength, on over 600 men and women.
And as you can see, there's a large sex difference.
And in fact, not one of almost 400 women had as strong of a hand strength as the average man.
So, men can open jars.
(Laughter) And move furniture or at least two things that we're good for.
Who cares, right?
The answer is that men care.
Men, especially young men, seem really concerned about figuring out who's the toughest or strongest, or the most physically formidable, and sometimes they devise elaborate ways for determining this.
From early development, boys and men are more physically aggressive than girls and women all over the world, and this aggression sometimes results in violence.
Men have a virtual monopoly on same-sex homicides.
In other words, men are vastly more likely to kill each other than women are to kill each other.
These are data from every society from every time period in history for which data were available when the authors compiled them, on proportion of same-sex homicides that are male killing male.
And as you can see, the percentage is always close to 100%.
On average, 95% of same-sex homicides are committed by males, and importantly, these don't include war killings, which would bring the percentages even closer to 100%.
And from what evidence that we have, a dominance among men translates into mating and reproductive opportunities.
So we're a species that's experienced an evolutionary history in which our male ancestors won mating opportunities through the use or threat of force.
In that regard, our apple has not fallen far from the evolutionary tree.
But in other ways, human mating and reproduction are profoundly different from what we see in our close relatives, and they've changed a lot since our early ancestors.
For example, males in chimpanzees, orangutans and gorillas, spend time and effort competing for mates, but don't spend much time with individual females and don't provide resources.
They don't provide food for their offspring.
So that's a big change.
Although most human societies allow polygamous marriage, that is one man married to more than one woman, even within polygamous societies, most marriages are monogamous.
And in the average hunter-gatherer society, almost 80% of married women are monogamous, so that's different.
And, importantly, men provide resources for their mates and offspring.
So how did we get there?
Well, in species where males fight each other for mates, dominant males indicated by the larger, darker male symbols here, tend to have more mating opportunities, and hence more offspring.
And subordinate males tend to have fewer mating opportunities and are more likely to fail to reproduce.
So this sets up an interesting situation, because for subordinate males, it would be advantageous to attempt monogamy rather than winning lots of mating opportunities.
One mate is better than none.
The problem is that in general, subordinate males cannot defend females from dominant males, and besides, females tend to prefer mating with dominant males for the genetic benefits, producing stronger, healthier offspring.
So what changed all of this was probably several transitions happening together around the same time.
By about two and a half million years ago, we had started to incorporate more meat into our diet.
We know this from various lines of evidence, including - this is cool - stone tool cut marks on animal bones dated to 2.5 million years ago.
That's cool.
I love this stuff!
And then by about 2 million years ago, brain size really started to increase, and with that came a lengthening of the juvenile period, so now kids became both really costly and costly for a long period of time.
And this made male provisioning both possible and necessary.
Possible because it's much easier to bring back calories, protein, fats, in the form of meat than trying to do that by transporting plant foods, and necessary because kids became so energetically costly that individual females would have had trouble providing resources for themselves and their offspring.
And when we look at modern hunter-gatherer societies, that's what we see.
These are aggregate data across several hunter-gatherer societies on net daily calories.
Are you bringing in more calories than you consume, through foraging, or are you consuming more than you bring in?
And the green bars are net daily caloric surplus, in other words, bringing in more than you consume.
And the red bars are a deficit, so you're consuming more than you bring in.
And you'll notice that men from about 20 years of age to 60 years of age are operating at a daily caloric surplus.
They bring in, generally through hunting, more calories than they can consume, and these calories are distributed.
If it's large game, it's generally distributed equally to everybody in the village or camp.
Smaller items can be brought back to individual family, but this contrasts with what's going on with women, in their reproductive years, they're operating at a daily caloric deficit.
Gestation, lactation, carrying babies, are extremely costly energetically and limit one's ability to forage efficiently.
So male provisioning, both possible through hunting, and necessary.
And this change had profound impacts on human mating and reproduction.
In a sense, it tipped the balance for females.
So now it was sometimes worth mating with a subordinate male, even if he may not possess the best genes, if he provided resources.
And this is baboon pornography.
(Laughter) I probably should have warned you there'd be monkey porn.
This is from PlayBaboon Magazine.
Alright, I'm going to stop with the jokes.
This is a female baboon in estrus, so her genitals are swollen, and this happens in a lot of primate species.
Females' appearance changes over the cycle, and becomes more attractive and this incites male competition for females during the fertile part of the cycle, with dominant males tending to monopolize copulations, closer to ovulation.
Well.
We don't look like this.
And you knew that.
But what you might not know is that women's attractiveness does change over the cycle.
My lab, and others, have shown that women's faces, voices, even odors, are more attractive to men during the fertile part of the cycle.
But these changes are extremely subtle.
And compared to other primates, the evidence indicates that we've evolved to suppress cues to ovulation.
That in a sense, ovulation is concealed in humans.
But think about what impact this would have.
This would mean that dominant males would not be able to monopolize copulations near ovulation.
It would protect the pair bond from invasion by a dominant male.
So that a male in a pair would have more confidence that he was the father of the offspring.
The couple is having sex throughout the cycle.
And this is unique to human mating, we don't see it in many other primates.
We have sex throughout the cycle.
And so this would essentially increase a male's confidence in paternity, because a dominant or some other male wouldn't be able to target the female and bully their way in at the fertile point in the cycle.
And this would have important implications for parental investment, in particular, males providing resources for their offspring.
Because across species, when males provide resources for offspring, they target those resources toward their own biological offspring, and they avoid investing in the offspring of unrelated males.
And so the evolution of male care for offspring and investing in resources and offspring, pair bonding, and concealed ovulation, went very much hand-in-hand over our evolution.
We have also evolved a specialized psychology for forming long-term romantic relationships with the possibility of investing in offspring together.
We fall in love.
All around the world, people prefer mates who are kind and generous and capable and willing to care for mates and offspring.
In one of the largest cross-cultural studies of human mate preferences ever conducted, covering 33 countries shown in red here, the single most important mate choice criterion to both men and women, was mutual love and attraction.
But as you also know, people are not always perfectly faithful to their mates.
And in particular, women sometimes face a tradeoff between good genes and investment.
Women sometimes find themselves in relationships with men who may be caring providers, but may not possess the best quality genes for offspring making them strong and healthy.
And several features of women's mating psychology seem to have evolved, in part, to resolve this trade off.
And I mean, recruiting genes, if you will, from outside of the long-term relationship.
For example, women have more sexual fantasies about men other than their long-term partner, during the fertile part of the cycle, and that's particularly true if the long-term partner has physical signs of being lower in genetic quality, like he's less physically attractive.
I think that's interesting.
(Laughter) That's why I'm talking about it, I hope you do too.
And women's mate preferences similarly change over the cycle so that they prefer more dominant, more masculine males during the fertile part of the cycle.
These are results of a study that I conducted on women's preferences for men's voices.
And I used computer software to manipulate recordings of men's voices to make them sound either more masculine or more dominant, or more subordinate, more feminine.
And I had women rate them on how attractive would this man be for a short-term, purely sexual relationship, and for a long-term committed relationship?
And I also got information about where women were in their cycles.
Were they in the fertile or non-fertile part of the cycle?
These were all women not taking hormonal contraception.
And what I found was that women preferred a more masculine, dominant-sounding voice, specifically in the fertile point of the cycle, and only for a sexual relationship versus a long-term committed relationship.
Now, this sounds like science fiction, but it's science, fact.
Because this result has been shown lots of times across a variety of domains from women's preferences for men's voices, that this result was replicated by another lab.
Women's preferences for men's faces, bodies, odors, and even behavior.
Well, I said that we would get to orgasm.
(Laughter) And we're there.
I just want to start by saying I'm for it.
(Laughter) I'm pro-orgasm.
I think more people should have more orgasms.
But from a scientific perspective, women's orgasm is especially fascinating because there's evidence indicating that it increases the probability that conception will result from an act of sex.
There's evidence that it brings sperm up through the female reproductive tract and toward the egg.
And think about what the implications here could be.
If women were more likely to have orgasms with some men than others, then this could be a mechanism by which they choose, not consciously, to be fertilized by some males and not others.
And wouldn't you predict that women would be more likely to have orgasms with males of high genetic quality?
And in fact, a study by my lab published just a couple of years ago found that women reported more orgasms, earlier timed orgasms, that is, they were easier to achieve, they achieved them more quickly, when they were having sex when their mate was more masculine and more dominant, and what's interesting is that this was true only for their orgasms from sexual intercourse, but not from other partnered sexual behaviors.
I'll let you use your imagination what those might be.
So we've seen that thinking like an evolutionist can enable us to predict things about ourselves that we did not already know, and would not likely have guessed for a long time.
We didn't know that women's mate preferences changed over the cycle.
Until evolutionary thinking led us to that discovery.
So that's one point that I want to make.
But we've also seen how evolutionary thinking can clarify and unite diverse parts of the human experience, and help us understand the best and the worst of ourselves, from violence and aggression and infidelity to men's care for their children, sexual attraction, sexual pleasure, and even the strength and fragility of romantic love.
Thank you.
(Applause)