#69: Sex – Penis Size, BDSM, Missionary Position | Robert Sapolsky Father-Offspring Interviews

0:00 This is Robert Seapolski and you're listening to Father Offspring Interviews. [Music] Hello there. Welcome back to Father Offspring Interviews, episode 69. It is time for another themed episode. So, today is all about sex. All right. You ready, Pops? Uh, sure. So, we are going to dive into it with a question from Sam from New York who asks, "Is there any evolutionary research to explain the dramatic variance in the size of penises? Is sexual selection meant to explain much larger penises?" H well, it's come to this, my offspring. Well, first off, Sam, what makes you so

0:46 sure that human variation is so dramatic? Are we comparing to any other known primate species? Um but regardless of variation between species talking about variation within as I think you ultimately are uh the question is where does variation come from? What does evolutionary selection have to do with it? Okay. Basic building blocks of evolution natural selection Darwin ever since all the things we learned are not true. It is not true that animals behave for the good of the species. It is not true that evolution is about survival of the fittest. It's about leaving as many copies of your genes as possible in the next generation. So natural selection is

1:32 selective forces for Darwinian fitness for you and your descendants surviving long enough to reproduce. So all along that's been the backbone of the whole business. But even ever since Darwin, uh it's been clear, as Sam alluded to, that there's a second branch of selection, sexual selection. And this is very different from natural selection. Natural selection, how good are you at running away from predators? How good are you at digesting food? How good are you at surviving climate change? All that sexual selection. Forget anything about how well adapted your body is. Are you attractive to the opposite sex? Do they select you to mate with? So, we

2:21 have the evolution of traits that are for being more adapted to life. And then we have the evolution of traits that are result of whatever it is the opposite sex is considering to be appealing. That is what is sexually selected for. Now in general natural selection and sexual selection um they can often work in opposition to each other. For example, sexual selection female choice has produced some species where the males have some ridiculous gigantic antlers as has been the case. So that's the upside in terms of sexual selection. The downside, your neck is sore. you're putting so many calories into growing

3:07 this gigantic thing. There's a downside. You have to have a tradeoff. And in general, male secondary sexual characteristics, horn, antlers, big manes, wild flamboyant coloration, those reflect sexual selection. For whatever reason, females are into that trait. And sexually selectively that could be advantageous. But at the same time, these very flamboyant secondary sexual characteristics make males more conspicuous to predators. So we have typically a trade-off between the forces of sexual selection and the forces of natural selection. Okay. So turning those now to the issue of penis size. Penis size and what's going on? Is it a

3:56 function of natural selection? Is a function of sexual selection? Now to get some insights into this we have to take a detour first from the penis into the testes. Testes testes size. What are testes good for? Predominantly for making sperm. And what you see is something very interesting across different species. Just primate species for example. How big are the testes in the males of a particular species? And testicular size is a good surrogate for how many sperm do they make, how many are in ejaculate. And you see something very interesting which is when you express testes size as a percentage of total body weight. There's some primate

4:42 species that have gigantic testes who make tons of sperm and dump a ton of it with each ejaculation. And then there's primate species that have relatively proportionately very small testes and very low sperm counts. What's that about? It turns out to make perfect sense. Low sperm count, small testes in species that are pair bonders because you're not having male male competition. You're having stable pair bonds. And in contrast, in species that have multi-male, multi-female systems where females are mating with multiple males over the course of a cycle, you get sperm competition. Your sperm are in the vaginal tract there and some other guys

5:29 is there as well. And the more sperm you dump there, the likelier you are to out compete them and get to the end zone first. So in tournament species, in species where you have females mating with multiple males over a cycle, you see selection for bigger sperm count and that requires bigger testes. Where you might ask out of curiosity, where do humans fit in the testes size sort of competition? And just like everything else with us as a species, when it comes to sexual systems, we're not a classic parabonded species in terms of tiny testes. We're not a classic torn one in terms of gigantic testes. We're about halfway in between the ways in which we

6:15 are classical versions of neither. We're a mixture. Okay. So with that in mind, now let's use the same logic and apply it to penis size. Is there any evidence of natural selection selecting for penis size which would be in the same way that we just saw that natural selection selects for big testes and tornament species small testes and pair bonders. Is there any naturally selective pressures going on with penis size? And in general, the answer is no. Except for one element that takes us back to this issue of sperm competition, postcopulatory sperm competition where a female has made it with more than one male and

7:00 there's sperm from both of them in there. And there's all sorts of strategies sperm have to try to poison the other guy's sperm to swim faster with you and the other sperm from your human. And in addition to this, there then winds up being a very simple spatial issue. The closer the sperm are to the egg when they are deposited, the longer the penis, uh, the more the likelihood of being able to fertilize. Okay, so that's one element of natural selection going into penis size. Sexual selection, however, is what it has mostly been about. And this opens us up to a whole literature of basically asking women what they look for. And

7:46 this field has been just revolutionized in recent years by digital technology so that you no longer have to have pictures of guys who volunteered and their eyes are blacked out for anonymity or just from the neck down or so. You can make digital versions. You can make avatars and modify the length, the width, all of that. So this has just been this is the golden age of penis size research and preference for penis size research. So one of the most sort of informative papers about this was published a few years ago in this journal very prestigious proceedings of the national academy of sciences. This is a very highbrow journal. was hard to get published in it and interestingly

8:33 proceedings of the National Academy of Science is PNAS and when somebody your competitor gets their paper into PNAS and you didn't you suffer from PNAS envy which is actually what people say okay so this paper in PNAS they got a bunch of women and basically said what are you looking for here and overall uh we get a Goldilocks in three bears result here and that there's a penis length and width that's just right and too short and too long are both not as appealing. We have an inverted U function and this paper made I suspect penis length research history by actually having to use quadratic

9:20 equations to figure out this inverted U pattern. They were not kidding around in this paper. And what came out of that is there's an optimal size, not too big, not too small. What was also noted was it's not that high of a priority for the women in these samples. How important is penis width to you? 30% of the women said it's important. How important is penis length to you? 25%. One interesting thing that came out of it though, which is penis preference. If women are thinking about having a hookup, a one night stand, they're paying more attention to penis parameters than if they're thinking about whoever it is they're going to get elderly with. And this fits with the

10:06 whole literature. When women are thinking about single sexual encounters, they are much much more attracted to male secondary sexual characteristics. jutting jaw and high forehead and biceps out the wazoo. And when you're looking for a long-term partner, there's less of an emphasis on that. Okay. One other thing that came through that study was amid 25% of women care about this, 30% care about that. 75% of them said that males are way too hung up on penis size. It's much more important to them than it is to women. And this makes sense in the context of body dysmorphic syndromes. Uh the most famous being anorexia where you were convinced you

10:53 are much heavier than you actually are. This has given rise to sort of a subvariety of a body dysmorphic problem of small penis syndrome. And somehow apppropo of that, the Kama Sutra advises men, if you want to have a larger penis, get it stung by wasps. That was medical advice back when. Now, mind you, there's two unrepresentative things going on in the science I'm describing to you. First off, it is overwhelmingly from heterosexual individuals. So, that's a whole other arena that's way underststudied. The other is that these studies have been overwhelmingly carried out on subjects who were weird. Weird weir d this has become a very trendy

11:41 concept in psychology. The notion that a huge percentage of what we know about human psyche and human behavior and such comes from undergraduates who have volunteered for a psych experiment in their psych 101 class. In other words, the literature is way disproportionately over represented by individuals who were white, who were educated, who are from an industrialized country, from a rich country, from a democratic country, WIRD. And what you have then is a tremendous move in all these fields now to do cross-cultural studies so that you don't inadvertently say the 5% of humans that fit in this category are in fact the

12:28 exemplars for our entire species. And when you look at the larger range of humans, one of the things you find out is there has not always been this emphasis on large peni. ancient Greek small penises were considered more attractive. In fact, saters who were these like ugly troll things were always characterized as having gigantic penises and the the god priapis which gave rise to the medical term priism which is when you have an erection 24/7 which is not pleasant and is an indication of something majorly wrong there with the plumbing. The god Priappis was always presented as being ugly, hideous, and with a gigantic erect penis. So that's

13:16 what's come out of some of the cross-cultural studies. But now studying contemporary cultures across the world. First off, counter to myths, snarky, stereotypical, porative, whatever, um there are no consistent differences in penis size as a function of race, as a function of ethnicity. Uh there's no correlation between hand size and penis size, foot size, and penis size. However, this was so surprising to me I had to actually get the name of this paper, the title of it. This was a 2021 paper in the journal basic and clinical andrology. And the title is nose size is highly related to

14:04 stretched penile length in Japanese male cadaavvers. And I'm very glad they specified that these were male kadaavvers. So at least in this population, that's some sort of correlate. But in general, lots of the urban legends about what things correlate with penis size in fact do not. One interesting thing is over the last three decades there has been what people in the business call a secular trend. On the average worldwide penis size has been getting gradually bigger over the decades. I don't know how this was figured out. I don't know who studied this over these 30 years, but that seems to be out there. Okay. So amid all of this there's one other confound in the field which is at least

14:51 at the beginning there was a big distortion brought in and this began with Alfred Kinsey who's the godfather of sexology research in the 1940s 1950s University of Indiana incredibly brave visionary pioneer of this stuff and he among other things figured out what average penis size is and The slight mistake he had was having way too much faith in his audience. He asked people to self-report it. And the best evidence is that your average person kind of exaggerated the size a little bit. And what averages out to about 5 in is what they reported to be 6 in. So be careful of inflation. Next, Eton from California

15:39 asks, "When and why did humans start using clothing as a means of sexual modesty?" Well, Eton, the answer is nobody has a clue, but it certainly is one of the odd things about humans. A couple of odd things about us and our genitals. Uh, the first one is that we are the only species that has private sex. That's culturally a universal among humans. No other primate. No other species cares about it. And that's a good thing because that's the way in which young juvenile baboons learn about the birds and the bees by observation. So there's that feature of us in our sexual lives. And the other, of course, is not just clothing, but some types of clothing

16:26 driven by a need for modesty. And clothing is a pretty distinctive thing about humans. Just to quote Mark Twain, uh, clothes make the man. Naked people have had virtually no impact on society. So the question becomes, when did clothes first get invented? And all we have at this point is the oldest bit of clothing made out of who knows what, goat hide. Um, but it's only about 5,000 years old. And clearly clothing was around way lot longer than that. So, we're not exactly sure when people started wearing clothing for modesty because it's hard to even tell when people started wearing clothing except for one very cool finding. Okay, you go

17:14 back to human remnants from a 100,000 years ago and skulls and stuff and what do you see? There were lots of head lice. Head lice all over the place. And what we now know is a 100,000 years ago, you were some head lice that got into your head that you wanted to see what was down south and you migrated downward. And what would happen is you would shrivel up like a raisin in the sun because you were far from an optimal ecosystem for you. And about 70,000 years ago, a new type of head lice appeared. headlouse, a new species of it, one that was adapted for clothing, not for hair. There was a speciation event around that time. So, that's as

18:02 good of a hint as we've got as to when clothes were first becoming widespread. When they were becoming widespread for reasons of uh sexual modesty, nobody knows. So, now we have a few related questions. Um first is the question of whether we understand the biology of non-consensual sexual sadism. Then Kaman from Iran asks is there any neuroscience basis for consensual sexual sadistic and masochistic tendencies for example the confusion or links between fear and arousal? And Marcio from Brazil asks why do people sometimes feel pleasure in having pain either physical or psychological like the folks into BDSM? Well, first off, uh, daughter, I think

18:48 you need to leave the room right now. Um, second, as seen in the range of these three questions, we're talking about two very different categories of human behavior. Consensual and non-consensual, nonconsentual, sexual violence, sexual sadism, nightmare stuff. Uh, there's a handful of studies about these sorts of folks. And big surprise, they're kind of a mess. Big surprise, they have the usual sort of suspect list of things in their background. A very high frequency of childhood abuse, a very high frequency of instability at home. Interestingly, a high rate of parental infidelity and the

19:34 histories of these individuals, substance abuse, high rates of suicide attempts. what's going on biologically with these folks. I was only able to find one study that did any sort of neuroiming and it was a pretty uh dissatisfying result in that it essentially reported that in the majority of these sexual satists incarcerated there were quote neurological abnormalities. We are not told anything beyond that except for one interesting factoid which is the paper notes that a lot of these abnormalities were centered in the area bordering the temporal lobe and the

20:19 parietal lobe two parts of the brain. What's interesting about that? What's at the juncture of the temporal lobe and the parietal lobe? Shockingly enough something called the temporal parietal juncture. And what does the TPJ do? It is very involved in theory of mind, perspective taking, seeing the world from somebody else's viewpoint. It seems to be an important mediator of the cognitive aspects of empathy, trying to see the world through somebody else's eyes. And if that part of your brain is not working so well and you are having a lot of trouble conceiving the possibility that anybody else has feelings, anybody else has any needs,

21:05 that's kind of a meaningful finding. However, in that context, it's part of just this huge undifferentiated hodgepodge of a literature that people who do wildly damaging things of all sorts, serial murderers, abusive, all they tend to just have a whole board of developmental problems. So, there's nothing that really stands out here. Okay, shifting now to consensual sexual behavior involving bondage, domination, sadism, massochism. What's up with that? And for one thing, what the uh what the surveys show is interest in it, experimentation with it is more prevalent than one might have guessed. I

21:52 don't know 20 30% of people depending on the study. Um, but looking as we are want to do here for what's going on in the brain with that, there's a really interesting piece of the story that might explain some of this having to do with the amygdala. Oh, we have heard of the amydala so many times by now. What's the amygdala about? It's about aggression. It's about aggression and it's central to it and people have known this forever. And we heard a bunch of episodes back a weird additional feature of the amygdala, aggression, fear, anxiety. An additional feature is that in people who are clinically depressed, you show them a picture of an angry face

22:39 and the amydala doesn't activate the way it does in most people, you show them a picture of a sad face and it activates. What's the interpretation of that? If you were depressed, the scariest thing on earth is a stimulus that's going to make you feel sadder and thus it is very frightening and threatening. Okay, so that as part of the portfolio of the amydala, but then it turns out amydalas in males do something in addition. Part of what they also mediate is sexual motivation. Okay, motivation. time to talk about sex the way the the rat professionals do. They broadly break it into two categories of behaviors. Motivated behaviors and performative behaviors.

23:27 Motivation and performance. You were a male lab rat and let's look at your motivation. How many times are you willing to press a lever in order to get access to this female rat in the next cage? Once you get there, what do you do with it? What is performance? What's your latency until you do this or that? How much interest do you show? So, two dissociated measures of sexual behavior, sexual motivation and sexual performance. Now, in both females and males, sexual performance is anchored in the hypothalamus. In females, sexual motivation is anchored in the hypothalamus. In males, it's anchored in the amygdala.

24:14 Whoa, wait a second. So, a part of the brain that is most famously involved in fear and aggression. Aggression, it also deals with sexual motivation. What this begins to suggest is maybe what stuff like BDSM is about is people where the circuitry in the amygdala separating sexual arousal from aggressive arousal gets a little bit intermixed. And that seems like the best we have for trying to explain that. That doesn't explain everything because remember female sexual motivation is not based in the amydala. Even though overall rates of interest in things like BDSM are lower

24:59 among women than among men, it's not that much lower. So that's a bit of a complication in this picture. But overall, what we're seeing with this possibility that the amygdala does aggression, the amygdala does male sexual motivation, that they can get confused. When you're looking at functions like this, you can see how challenging it is for the brain and how readily it could wind up there being confusion as to what is being mediated, whether you are mediating a text and a subtext of aggression or sexual arousal. When I saw these questions, it reminded me of one of my favorite studies from a social psychology course in college having to do with the two-factor theory

25:45 of emotion, which was the theory by Stanley Shar and Jerome Singer that when we feel an emotion, what's really happening is some psychological arousal occurs and then we use context and cues to determine what emotion that arousal must be, potentially opening the door for misattributions. So the study which has come to be known as the misattribution of arousal was done by Donald Dutton and Arthur Aaron in 1974. In the initial version, men were stopped by a female interviewer while crossing either a sturdy bridge, the control group, or a scary suspension bridge, the experimental group, and the female interviewer asked them to fill out a short questionnaire, and then offered them her phone number in case they had any follow-up questions. Men in the experimental group, who were in the

26:30 middle of crossing the fearinducing suspension bridge, were significantly more likely to both accept her phone number and to contact her later than those in the control group. The study concluded that subjects were mistaking the physiological arousal of fear for the arousal of attraction and blurring the lines between them, similar to how the physiology of aggression and of arousal can be confused. the other side of that and viewing all of this from the perspective of say your sympathetic nervous system. What it tells you is you look at a very activated heart that's pumping like crazy. It is very difficult to decide if it is in that state because of fear, because of aggression or because of

27:16 sexual arousal. They are very overlapping physiological states. We will wrap up with a question from Ros from Denmark who says, "Is there an evolutionary reason that human beings usually have intercourse by facing each other, ventro vententral intercourse, instead of by rear entry, dorsal vententral like most other mammals? Does it have to do something with gender dominance or social change?" Uh, first off, saluting your scientific jargon, use of dorsal ventil. Yes, humans have mostly been the pioneers of face-to-face sex. Um, dogma used to be we're the only ones and we're not naturally. Those bonobos have every

28:02 type of sex imaginable, every combination and permutation including face-toface sex. So, we're not the only ones in that regard. Moreover, there's now been reports of the same in other ape species. recent report, first photographic evidence of western gorillas having face-to-face sex, likewise reported for orangutans, for gibbons. So, we're not the only primates. Snakes, snakes have face-toface sex because I guess snakes don't really have rear ends. What they do, however, just as an aside, is male snakes have hemi pen. They have two penises and they have like these barbed curved velcro things. So, they're weird

28:48 all around. So, this is this is not unique to humans, but it's pretty much only us and other apes and still way proportionately more in us than in the other apes. However, I found a 1996 paper in the ornithology journal Ibis talking about the heird. The he bird, also known as the stitch bird, um is found in New Zealand and apparently it's been documented that they will have face-to-face sex also. So, we've got to include them in our small exclusive club here. So when we're talking about humans having very high rates of face-to-face sex, more so than any other species, uh

29:35 typically what comes to mind is we're talking about missionary position. Uh something worth mentioning is different languages have different terms for that sexual position. In Arabic, for example, it's called in the manner of serpents, which takes us back to the snake information we just had. In Brazil, apparently in Brazilian Portuguese, it's referred to as the mommy daddy position. But uh for most of the west, it is referred to as missionary position. And of course, one wants to know where did that come from? Is that true? That missionaries used to trump out all over the corners of the world trying to destroy indigenous cultures by among other things saying this is the only

30:20 allowable type of sex. And that turns out to be an urban legend. Turns out it's mostly due to this British anthropologist Bronoslov Melanowski who in the 1920s was studying people in the Trojan Islands. And he was noticing the frequency or lack of frequency of face-to-face sex. And he apparently made like just some sort of snide comment to the effect of, "Oh, this is exactly the kind the missionaries would be telling people here is the only thing they should be doing." And somehow it caught on and spread and Kinsey apparently played a role in that. Okay. So, we now have the issue of why is this a human specialty? Not a human uniqueness as in

31:07 the only type of sex we have. not a human uniqueness in terms of we the only species but why is it so predominant in us and there's two theories one of them is heartwarming and I think insupportable as follows which is all sorts of people speculating that face-to-face more intimate contact this is meant to stabilize the pair bond monogous bonding and all you could conclude is that these scientists were gibbons who were suggesting this because again going back to earlier humans are not particularly a pair bonding species, not particularly a tornament species. So that speculation seems a little bit iffy to me. The other speculation makes more sense and it's an anatomical one. Okay,

31:55 take your average non-human primate, sit them down and ask them what they think of human vaginas and their answer will be they're really strange. They're really strange according to them for two reasons. First off, compared to other primates, the vagina is very big in cross-section. What's that about? Human fetuses, big brained humans, big fetal head, and not only has the challenge of getting through the birth canal, the pelvic arch, but then getting down through the vaginal tract and need it winds up being bigger. So proportionately, human vaginas and cross-section are bigger than in other primates. And that probably has something to do with why on the average

32:42 human penises are wider than in other primate species. Okay. The other thing this non-human primate would think is weird is the angle of the vaginal canal. It's at a weird angle compared to other primates. And the general interpretation is humans have come up with what has been the norm in hominins for a long long time which is to give birth standing up and the vaginal canal has had to evolve sort of a bend there which facilitates going through labor standing up. The notion is that humans have a distinctive angle to the vaginal canal. And the notion thus is that face-to-face sex is the one that most facilitates fertilization by the penis there,

33:29 accommodating this relatively unique angle that you find at humans. Okay, so that's the theorizing about where human sexual preference for face-to-face sex comes from. And it's probably right around now to call it a day. And that is episode 69. I'm Offspring Sher Seapolski and thanks for your continued support of science and the beard.