- What are the data on how females signal, let's just say, testosterone, estrogen, and other relevant hormones, and for males as well, what are the external signals or behavioral signals? - Yeah, so that's a really important point that you made because they both, those things go together. So it's been most controversial for females, but in my view, the data is pretty clear, and it aligns, I think, with our own intuitions just from daily life, which is, well, some things are apparently not consciously perceptible.

It's hard to report, but through studies where you just ask males for like, okay, how attractive is this woman, or et cetera, that there are changes in the face, for example, and that's been one argument is that, this is gonna sound funny, but that the signals that in non-human primates are in the rear are, because we're walking upright, you can't see that really, so now it's kind of in the face, and so these changes that happen, that the ovulatory cycle is reflected in the turgidity, how tight the skin is in the face, because it gets a little plumper and a little bit redder, and we may not be consciously aware of that, but that it's there, right, and it shows up in sort of preference data when you ask heterosexual males, how attractive is this woman, et cetera, so that seems to be the case.

And also behavioral, so sort of flirtatious behavior-- - Increases around the time of ovulation. - Yeah, yeah, yeah, yeah, there is a classic study that exotic dancers, strippers, would actually get bigger tips, more tips, when they were ovulating than when they're not ovulating. - Interesting. - So there may be-- - And it could be by virtue of their behavior, but it could be the way they dance, proximity to the, what I guess the observers, clients, whatever you call them.

- I don't recall that being quantified, but it suggests that there's a latent signal there. And that men are unconsciously processing this. They're not saying, "Oh, her cheeks are particularly "plump and red right now." But that if you measure their ratings or scores of attractiveness, when she's ovulating, it's these features that might be drawing out that response.

- Correct, we can take this back to the monkey porn studies, which was our first real foray into trying to quantify the value of various kinds of social information for guiding decisions. And we already came into this with a sense that like, yeah, things like status, physical prowess, mating status, are you, you look like a good mate, bad mate, are you in mating condition, et cetera.

And so when you think about that, how do you ask a monkey that question? You could ask them, they're not gonna tell you 'cause they can't talk, but you have to develop a behavioral way to elicit that. And so what we did, I think it was pretty clever, was to riff on the studies that I had already done looking at varying the expected value of two options.

So this was the work I did as a postdoc with Paul Glimcher, where we revealed economic signals in the brain, in the parietal cortex, an area between where visual signals come in and where you make a choice to make a behavioral response. And we varied, like in this case, monkeys don't work for money, though they work for juice.

Okay, it's been actually, it's really fun. You spend a lot of time figuring out what juice they really love best. And then economically, you would vary the size of the juice reward that each of the two offered, or its probability while maintaining size constant, that when you combine those, you multiply those together, you get expected value.

That's the first model of economic decision-making that was really ever developed, right? You compute the expected value, different options, you choose the one that has the highest value. It doesn't work all the time, but it's sort of a rough proxy and we showed that, yeah, neurons in the parietal cortex signal that.

Monkeys are good economists. They choose the one that has a higher expected value. Okay, so now take that experiment. I'm gonna have monkeys choosing between two options that vary in how much juice they pay out. But I'm also gonna pop up a picture when they choose one of them, okay?

And they don't know what picture's coming up, but the picture's gonna be, it could be a nothing burger, just like some gray square, it doesn't mean anything. Or it could be the perineum of a female, if it were males that we were studying. We did this with males, sorry, females making choices eventually as well.

Could be face of a dominant male, face of a subordinate male, face of female, et cetera. - What's the equivalent of the swollen taint of a female monkey for, if you reverse the experiment and it's the female monkey who's making a choice about male monkeys, what do they find really attractive in a male monkey?

- Yeah, so it's the taint of the male monkey 'cause it's providing a signal about how much- - Monkeys looking at taints about monkeys. - Yeah, how much testosterone is circular, that they've got on board basically, which is a good predictor of their status. It's a good predictor of their fighting ability, all that kind of stuff.

And if you're a female, that's a reasonable kind of choice to make 'cause if you have male offspring and females are predisposed to choose that, then your male offspring are gonna do pretty well. So that's what we did. And we varied how much juice. So sometimes monkeys would get paid, they'd have to give up juice to see the pictures.

Sometimes they get paid more to see the pictures. And what we did then is we construct a choice curve and we use the differential. If it's not 50/50, if it slides one way or the other, it tells us that monkeys are paying X amount to see certain kinds of pictures or you have to overpay them, right?

And so what did we find? It was really, I think, scientifically revealing, but it's pretty fun. People got it immediately. They will pay-- - Juice. - Juice. They will give up juice. They will pay it to see pictures of the perineum, the hind quarters of females. This was original study was in male monkeys.

They will pay to see the faces of dominant males. And you had to pay them to see the faces of subordinate males. Okay, so females will give up juice to see the taints of testosterone rich male monkeys and male monkeys will pay juice to see the swollen taints of female monkeys that are, because of the swelling, indicates a better reproductive competence.

- Yes, better, you know, douse the time. The time is ripe, okay, to mate. But it's just in general, it's a signal that is like, what we would say is it's important. It has value. - Monkey porn. - It's something you should track. And in fact, yeah, they're paying for it.

So, you know, it just blew up on the internet. Even back then, it was like suddenly million, every website was like, oh, you've proven monkey porn, blah, blah, blah. It was kind of a fun ride. It was a New York Times idea of the year in 2005, which was, again, kind of shocking.

You know, there's like, a little word on that. But people, it makes sense. And the thing I want to point out is that we ran this same experiment in people, not with unclothed humans. So we used, and we used only, well, no, it was, and we had to create our own stimulus set because all the stimulus sets that were out there for visual studies of humans were like a bunch of, you know, German people looking very dour.

They were very well controlled. And we wanted something that was more natural. So we downloaded thousands of photos from this website, hotornot.com. I don't know if you recall that, but it was a website where you could upload pictures and people would rate you. I mean, now that's like- - Probably wouldn't be allowed now.

I remember Rate My Pet. - Rate My Pet, Rate My Professor, I think, which is still around. - And we were saying rate. - Rate. - Rate. - Rate. - With a T, my pet. - Yeah. - But this was hotornot.com. So you get all these really natural looking.

And then we had, this was really funny though too. So we had a group of, separate groups of raters from the people who we actually tested in the experiment. So we had, you know, a group of males, heterosexual males rating the female photos and vice versa. And that was interesting in its own right.

So we were just trying to establish like, we're not saying why they're attracted or anything like that, just like, let's measure it, okay. And it was really fun because, you know, by the, and it took, it was hard work. You're having to do one every three seconds. And it took like an hour.

And the, you know, when the women were done rating, they're like, whew, okay, I'm glad that's over. The hour's over and our male raters were like, did you have any more? You know, can I, I'd be happy to sit here and rate more photographs for you. - Interesting. So women got sort of like, they got tired of rating males for attractiveness.

- Yes. - Males did not tire of rating females for attractiveness. - They did not at all, which is, that's anecdotal, but it's still, I think it's revealing. Then we ran the pay-per-view experiment, just like in monkeys, on humans. - Pay-per-view. - And we also ran a couple of other economic lead, you know, standard economic tasks.

One would be, how long are you willing to wait? So that's a delay discounting. Like, in general, you will wait longer for a bigger reward. A smaller reward. And also how hard would you work? And we, the work was like, you had to alternate pressing two keys on a keyboard.

It was really just menial, laborious, you know, et cetera. So, the two interesting, just sociologically it's interesting, what comes out of this. Our female subjects basically wouldn't give up money. They were working for money. They were hearing the sound of coins coming out of a slot machine, which was proportional to how much money they actually got.

- Real money. - Real money. If you ignored the pictures, you'd go home with like $17 extra, compared to if you were influenced by them. And the females did really well economically. So they pretty much kind of ignored the pictures of the males, even though they were rated, even the ones that were super hot, they were not very concerned with that.

For the males, it was exact opposite. So the males are giving up essentially. They're paying, and they had thousands of trials. They're paying somewhere between a half and three quarters of a cent to see images of women who were rated in the top, like third of attractiveness. They also would wait significantly longer and they would work really hard.

It was like rats pressing for cocaine, quite literally to keep those pictures up on the screen. Okay, so that's the setting we've established in monkeys and in people, similar economic principles that are guiding social, you'd call it attention, social valuation, whatever. So we're like, okay, let's go look in the brain.

So we did an MRI experiment, fMRI experiment, measured blood flow to different parts of the brain. We only tested males because they were the ones who displayed differential preferences there. And what we found is that kind of parts of the visual system that are involved in encoding faces, but then the reward system was activated and tracked linearly how much money these guys were paying to see images.

There's basically the trade-off value, the currency, the translation of pictures into money, okay? Then in monkeys, we studied all the same areas, but now we could record from individual neurons in those areas, rather than looking at blood flow, which is a crude proxy. And we found exactly the same thing, which is that neurons in the reward system were spontaneously and strongly activated by those pictures, you know, that made sense, right?

So the pictures of the perinea of females by dominant male faces. And that correspondence, I thought, was pretty compelling. (upbeat music) (upbeat music) (upbeat music)