Do you think that you could have

published this book 6 years ago?

>> I think that I could have, but it would

have been a less wise move.

>> [laughter]

>> I think it would have gone down

a lot worse 6 years ago. Things have

kind of cooled off a little bit, I

think.

>> Why is talking about sex differences so

controversial?

>> Well, it's a good question, and I think

that it's a few things. I mean,

sometimes it surprises me cuz sometimes

I think a lot of the differences that

we're talking about, they're really

quite modest differences. Um they are

often differences in preferences rather

than cognitive abilities. So, they're

kind of neutral as far as I'm concerned.

Um and if anything, I think probably

they put men in a worse light than

women. Um so, sometimes it surprises me,

but I think really the main reason is a

long history of sexism against women.

And I think that we've got quite a long

science as well has a bit of a sorted

history in terms of how we've spoken

about women, especially in the 1800s.

I've got a quote, one of my favorite

quotes, one of my favorite examples of

this is from um

scientist by the name of Gustave Le Bon,

and he he wrote that

uh sure, there are some women who are as

intellectually accomplished as your

typical man, but they're about as rare

as a as a two-headed gorilla. And so,

therefore, we don't really need to think

about think about [clears throat] them.

Um so, with stuff like that in our

background, it's I guess not surprising

that people are a little bit a little

bit nervous about talking about evolved

sex differences, a little bit worried

that if we start doing that, it's going

to open the door to that kind of

that kind of sexism. Um I think they're

wrong to worry about that. I think that

really the science of sex differences

has disproved those kind of ideas. Um

and I think I think as well that we can

treat men and women respectfully and uh

and as you mainly and treat them well uh

even if men and women are not identical

on average on every single trait.

>> Yeah, I suppose

the history of science is to sex

differences what the Nazi Party is to

behavioral genetics.

>> Right. Right. Exactly. Yeah.

Yeah. And I do think that science is the

solution to the early sexism rather than

a continuation of it.

>> Yeah, that's a great way to put it. All

right. Why is it important to study sex

differences then? Why why should anybody

care?

>> Uh I think lots of reasons. The main

reason for me is I think it's just

deeply deeply fascinating. I just think

it's

>> It's the [laughter] best. Yeah. It's the

It's the best, dude. Me and you fell in

love with the same book, The Moral

Animal by Robert Wright. He He changed

my life.

>> Yeah, likewise. It changed my life. It

um shaped the rest of my career. Um just

I think it's deeply fascinating. And um

Yeah, I remember when I read that book

uh that's when I fell in love with uh

fell fell in love with evolutionary

psychology. And in particular, fell in

love with the evolutionary psychology of

sex differences. And I got the same

feeling that I did when I was a a kid

and I first understood how the phases of

the moon work. And everything just

clicked into place. Suddenly, the whole

thing made sense.

>> Oh, that is why it looks that way. That

is why people are different in that way.

>> Exactly. Exactly. It all made sense. It

was clear that it was true. And uh yeah,

just uh sort of instant insight. And uh

yeah.

>> Interesting. Even as you're saying it, I

can hear this sort of

critic preparedness inside of my mind

going

"Why would you when you read a book Why

would both of us when we read a book be

so enamored by sex differences dot dot

dot That must be because you are

secretly a sexist who is looking to try

and keep people in their place and not

ever have any progress and da da da da

da You know what I mean?" Like the

conversation around this has become so

tarnished. Um that

>> Accusatory.

>> Yes. Yes. Yeah. That you can't have an

interest in something which is

fundamental and fascinating and really

important, right? So that you don't try

and

people aren't trying to fight against

something so that we don't fully account

for and understand why behavior unfolds

in particular ways that can allow us to

have a better society, a society that's

got outcomes that's more close to what

we want. Fascinating.

>> Yeah, it is. Um yeah, so I do

I agree with that and I think

uh yeah.

Sorry, I just

uh I just went blank. What was What was

I going to say about that?

>> No, no, no. Uh just why is it important

to study sex?

>> Oh, yeah, why is it important? Yeah, so

that is that points to another reason.

So, one big reason why it's important is

just the fact that it's fun uh

fascinating.

Um but I think also that like you say,

it is important in terms of steering

society in an positive directions.

And uh underst- understanding ourselves,

uh understanding interactions between

men and women. Um in terms of yeah,

trying to keep people in their place and

the like. That's certainly not the

philosophy that I come away with um from

having studied all this over the years.

Um I instead of it, my basic philosophy,

I think I can summarize in just four

words. And those words would be, let

people be themselves.

Uh so, if they if they fit with

traditional

uh gender roles and expectations, that's

great. Uh but if they don't, that's

great as well. It doesn't matter. The

fact that something has evolutionary

origin doesn't necessarily mean that

it's good.

Uh it doesn't necessarily mean that it's

bad, either. I think that it's it's

morally neutral and that it's up to us

to decide whether it's good or bad or

somewhere in between.

>> I think that's a big fear that people

have that by looking at the evolutionary

explanation for why certain traits show

up the way that they do, why certain

differences occur the way that they do,

it sounds like you're excusing away that

behavior.

>> Indeed.

Like a lot of people do worry that

that's the case and that people will

assume that it's the case.

>> It's a naturalistic fallacy in your way

through all of the bad behavior of men

or all of the worst behavior of women or

all of the most repressive ideas that we

have for how people should behave.

>> Yeah, yeah. And I don't think that it

does justify any particular behavior. Um

but I do think it's

right to worry that people might assume

that it does, falsely assume that it

does.

So, I do think when we discuss these

issues,

uh that we should take into account the

concerns that people have about this

whole topic of sex differences. And we

should discuss them carefully. And we

should always emphasize that most of the

gaps are not huge. Most of the

psychological sex differences are not

huge. Um that there's a huge amount of

variation within both sexes, tons of

overlap between the sexes.

And also that there's no

um

normative implications or

disimplications. So, no um

sense of

because something is evolved, because

something is natural, that that

therefore means that it's good

or even permissible. Again, it's up up

to us to decide that.

>> Right to say that the the differences

are modest at best, but when you start

to stack them on top of each other, you

end up with huge You basically end up

with two completely separate Venn

diagrams, right? It's not just

preference for violence and risk-taking

and parental investment and da da da da.

It's when you put all of them together,

you do have two very different groups of

people.

>> Yeah, those are called multivariate sex

differences when you sort of cluster

the differences together.

Um I think that

while that's statistically true, I think

we have need to be a little bit wary

about the concept of multivariate

statistic multivariate um

sex differences just because

often we are thinking about single

traits. So, often we're thinking about

sex differences in parenting, sex

differences in sexual interests and

aggression, risk-taking, uh and and so

on. Um

but also if you were to

divide up take take a group of people,

divide them into two based on a flip of

a coin.

Um they're not going to be identical on

every trait. There's going to be small

differences between them. But then if

you add up all those traits, you could

just as well stack up the differences

between these two randomly chosen groups

of people, stack up the differences, and

you'd find that they equally uh

massively discrepant from each other

based on the multivariate statistical

approach.

So, um

so, I think we you to be wary about

exaggerating the magnitude of human sex

differences based on that statistical

approach. Cuz you could just as well

exaggerate the sex differences between

New Zealanders and New Zealanders and

Australians or and the British and the

French or or like I say two randomly

>> British and the French do that enough

already. All right, there's been a last

few years there's been a huge debate

about what sex actually is.

>> Yeah.

>> What's the definition that you land on?

>> Uh well, there are a few definitions.

The one that I land on is the

traditional scientific definition uh via

which sex is defined in terms of gamete

size.

So, in the vast majority of sexually

reproducing species,

uh you have two different gamete sizes

and the small gametes are called sperm,

the larger gametes are called eggs.

>> Is that across the board?

>> That is across the board.

>> No way. Like so, every every

almost every sexually reproducing male

>> Yes, produces sperm and sexually

reproducing female produces eggs.

>> Right.

>> And and

>> might this might be like the most

facepalm thing in my life. Wow, I didn't

know that that was those are the only

two currencies at play.

>> [snorts]

>> Um I mean, it seems like so so in

biology, it's not like math, right? So,

in biology, there are very rarely uh

generalizations that apply to every

single species.

>> Mhm.

>> Um this is one of them, but the reason

is that it's a definitional it's a

definitional truth. So, we define males

as the ones that produce the smaller

gametes. Um the

empirical truth that in principle could

have gone another way is the fact that

in the vast majority of species, you

have where the gametes are not

identical, you only have two different

types.

And one is bigger and one is smaller.

And that's true right across uh you

know, all species except the minority

where you uh you have

So, that's called anisogamy where you

have two different sizes and you have a

few isogamous sexually reproducing

species, but the vast majority

anisogamous.

>> Can you

what is what's an isogamous

sexually reproducing species What are

some examples of them?

>> Uh there are some very uh I think no

animals and no plants, but I think maybe

some simple bacteria and things like

that. And I think that would that would

have been the starting position. So, we

all evolved from species that originally

were isogamous.

>> Mhm.

>> They They first of all sexual

reproduction would have evolved, then

isoga- and they would have been

isogamous. They would have produced the

same gametes and mixed them. But then

that's evolutionarily unstable and that

tends to fall apart into uh smaller and

larger gametes.

>> Why Why is it unstable?

>> Uh because when you when you have

same-size gametes,

uh

the selection for any trait that will

increase the chances of being one of the

gamete I I producing gametes that are

going to uh produce offspring that

survive. So, that creates a selection

pressure for maybe loading up your

gametes with some extra uh supplies to

help them survive.

>> Mhm.

>> Um so so some gametes uh get a little

bit bigger

because of that. But as soon as they get

big enough, the larger gametes, you have

a selection for smaller gametes that

seek out the larger gametes and kind of

ride on their coattails.

>> Mhm.

>> And uh so and you get this it pushes

them apart. And

>> uh from a

sexual survival or a gamete survival

perspective,

>> Yeah.

>> it's the gray area in the middle of the

barbell is where you go to die and the

black and the white on either end are

actually the ones that are most

effective.

>> Exactly right. Cuz the ones in the

middle, they're not big enough, they

don't survive as well as the bigger

ones, they pack a picnic for their

offspring.

>> Mhm.

>> Um but they're also

>> in their

motility mobility by the the smaller

ones.

>> Exactly. And and just numbers, you know,

so like if you are producing small

gametes, you can produce many more. So,

individuals that produce the medium-size

gametes, they don't survive as well,

plus they don't produce as many.

>> remember what the stat is. There's some

stat around. Jerry, can you look at

ChatGPT, um how many sperm are produced

per day, human sperm versus human eggs?

It's a It's such a hilariously big

difference. Yeah. Um Okay, so broadly

across the animal kingdom, in almost

every single

sexually reproducing species,

there's a binary.

>> Yeah, there's a sex binary.

>> There's a sex binary, yeah. There's also

a ton of variation in terms of the

phenotypic characteristics

within each of those sexes.

But yeah, there is there is a sex

binary. You know, you and you do get

intersex individuals.

>> Yeah, what about the challenges to this?

We we have

hermaphrodites, within-sex variation,

intersex conditions. Does that disprove

this?

>> Uh it doesn't. Um so the I guess the

easiest one is the within-sex variation.

There is tons of it. So there's

variation in terms of levels of

masculinity, levels of femininity,

um just uh size dimorphism versus size

monomorphism, where they're they're

about the same size. Um

but nonetheless, they are all either

producing sperm or eggs.

>> Yep.

>> The vast majority within each within

each group.

>> You've seen that uh

image, right, of the bell curve graph,

and you say, "On average, something

something." And then somebody says,

"Well, I know a person who is actually

more that than the other group. I know a

a woman who's

6 ft 7." And and and she's actually

taller than most men. Most men on

average are taller than women. Well, my

friend's 6 ft 2. You go, "Right, okay,

but that doesn't necessarily disprove

the way that

>> yeah.

>> Exactly. The gen- The fact that the

generalization's there. And even more

so, the fact that the gamete size is

definitional, as you say. Oh, here we

go. I don't even know what that number

is. Is that sperm Is that a billion

sperm?

>> 100 to 300 million sperm per day

produced by healthy adult male.

>> no, no, no, no. I want this globally.

Total global sperm production, 200

quadrillion. Eggs released, 70 million.

Step it up, ladies.

>> So.

>> So, I think [ __ ] lagging behind. We

need to up the egg production, so I say.

So much of the so debate falls around

nature versus nurture.

>> Yeah.

>> So much of it.

How do you even begin to start to

separate this out, right? No almost no

women have grown up in a society that

hasn't had other women with the

expectations of women and almost no men

have grown up in society that doesn't

have the expectations of men. How do we

know that all of the sex differences

aren't just socialization?

>> It's a good question and um I think the

first thing I want to say about that is

that evolutionary psychologists don't

deny that socialization has a very

important uh impact on the nature of sex

differences in any given culture.

We know that it does have a big impact

cuz we know that sex differences, their

size, and the exact details of them,

many of them, vary across cultures and

vary across times. Um but in terms of

how we know that it's not just

socialization, there are various lines

of evidence that point in that

direction. None of them are perfect,

none of them by themselves seal the

deal.

But the reason I'm persuaded is that

they all point in

uh when you get a sex difference and

they all point in the same direction,

um that just makes a very strong case, I

think, that that is not just due to

socialization. And in the book I I lay

out six different lines of evidence uh

for an innate contribution to any given

uh difference. Um see if I can remember

the name. So, the first one is um

that a lot of sex differences they

appear very very early in the lifespan.

Uh so early in fact that even though

it's conceivable that it could be a

result of socialization, it just seems

less plausible than that there's

actually also an innate push. So, uh the

aggression sex difference and the

risk-taking sex difference, both of

those appear basically as soon as kids

are mature enough that they can move

around the place and are capable of

risk-taking, capable of aggressing. Uh

you get that sex difference um

like immediately. And and

[clears throat]

good example of that is that uh

right from the get-go, right from early

childhood, more boys than girls end up

in the ER from doing more risk-taking.

>> Mhm.

>> Uh and and also if it appears at puberty

as well, um, like a lot of sex

differences innate sex differences with

an innate basis, they

>> What do you mean when you say innate?

>> Uh, unlearned.

>> Right.

>> Unlearned, yeah.

>> Right. Right.

>> So there's an innate contribution, an

unlearned contribution, and where that's

the case, often you find that the sex

difference,

um, you may you may get it to some

degree early in the lifespan, but then

post puberty, it just skyrockets and and

expands.

>> Right. Okay. Which would suggest that

there is a biological basis, or else

>> Yeah.

>> why would this big change occur when

puberty happens?

>> Exactly. And and do so right across

cultures.

>> Yeah. Is it a Would the suggestion from

social roles, socialization theorists be

that well, during puberty,

culture ramps up its pressure on people?

There's greater expectation on young men

and women to change their behavior?

>> That would be the way that you would

have to you'd have to try to explain it,

uh, something like that, yeah.

>> Uh, [clears throat] I think they're more

malleable. They're more they're more uh

>> Yeah, more plastic at that age, perhaps.

Yeah. You could you could do that as

well. Um, and that's sort of implying an

innate, uh,

contribution to the plasticity, I guess.

But yeah, not to the specific sex

difference.

Uh, the reason I don't buy that, though,

is that most cultures actually try to

suppress male aggression, for instance.

Uh, in every culture, male aggression,

uh, post puberty, young male aggression,

violence, risk-taking, homicide, uh, is

a big problem that the society has to,

uh, deal with. And so, uh, usually they

they try to try to clamp down on it,

rather than trying to, uh,

encourage it. And nonetheless, you get,

uh, this uh, increase in aggression.

That's true. And actually, this is the

second line of evidence. Is that often

these sex differences appear despite

culture, rather than just because of

culture.

>> Mhm.

>> Culture's trying to push down aggression

like right from early on. Uh, parents,

teachers early on, they they actually

tell off boys for aggression more than

they tell off girls for aggression. Not

cuz they disap- disapprove of girls'

aggression, but just simply because boys

are more aggressive.

>> Right.

>> So, they have more cause more cause to

do it.

>> Mhm. I think

fMRIs of developing fetuses in the womb

can detect sex differences at 3 months.

>> They can I think I think even younger in

some cases as well.

>> Yeah. What are they are these

boys and girls, these tiny un- minus

6-month-year-olds,

are they

learning

that their brains are supposed to

develop differently?

>> Yeah, they've been socialized to develop

different brains inside of the womb.

Yeah.

>> Okay, so early development.

>> Early development um

appearing despite culture rather than

because of culture.

>> Yep.

>> Uh another one is that um

they persist often for a long time. So,

for instance, uh sex differences in mate

preferences were really consistent right

throughout the entirety of the uh 20th

century and the in the US.

>> Mhm.

>> Uh sex differences in career preferences

as well or or career-related interests.

>> Mhm.

>> Uh they were evident from

like the early 1900s when people first

started measuring them.

And then they just stayed the same. So,

so the average difference, the fact that

men for instance on average more

interested in things and things-related

jobs, whereas women on average are more

interested in people and people-related

jobs. That was that was the case right

throughout the 20th century uh despite

the fact that there was quite a strong

uh

social there was a lot of social

pressure to try to get more women into

uh traditionally male roles and it

persisted nonetheless.

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What are the sociocultural explanations

that hold water?

Um well, I think a lot of them do.

Um

you know, it's it's tricky. So, I guess

we know because of cross-cultural

differences and

male-typical behavior, female-typical

behavior, and the size of the

differences, there's something that's

making the difference. But, it is

actually quite tricky to

to figure out what what it is that's

making the difference.

Um cuz we know, for instance, that

I guess like a popular explanation is

that it's how parents treat their kids.

And that seems incredibly plausible that

it's going to have an effect. There are

a few reasons to think maybe that it has

less effect than we might think. So, one

is that a lot of research in the West

finds that men finds that

parents don't particularly treat their

sons and daughters differently in some

important ways. But, they're equally

likely to encourage them to be

successful, equally likely to

encourage them to be independent.

Uh and another line of evidence against

parents having a big role is what's

called the second law of behavioral

genetics.

Uh and that's the finding that

um when two people, I guess like I say,

take identical twins. Two identical

twins that grow up in the same home,

you'd expect them to be more similar to

each other than two identical twins who

are separated at birth and reared apart.

And in childhood, they often are

quite a bit more similar than you would

expect by chance. Um by adulthood

though, they're often not much more

similar at all, and in some cases not no

more similar than they would be

um if they had grown up apart, which is

a surprising finding, but a very

consistent finding. And it suggests

maybe that um parental

treatment

affects individuals less than we might

think that they would. This kind of

pulls out the rug a little bit from the

idea that parents are having a huge

impact on sex differences specifically.

>> What about the gender equality paradox?

>> Uh well, that uh undermines another kind

of uh social cultural explanation, which

is the idea that um

the sex differences that we see are

products of the social roles of a

culture.

And shaped by

>> Is that social roles theory?

>> Yeah, social roles theory, exactly. Um

and patriarchy theory, the idea that

it's due to patriarchy, um

uh

that as well is undermined by the gender

equality paradox.

Um cuz both of those theories would

predict that in like like more

patriarchal societies

societies that are more patriarchal, uh

societies that have stricter gender

roles

>> Yeah.

>> you'd expect that it predict that the

sex differences would be bigger in those

societies

uh than they are in less patriarchal

societies, and ones that have less

strict social roles.

>> What would be some examples of how that

would play out if that was true?

>> Uh

basically any sex difference you care to

name, uh aggression sex difference, sex

differences in masculinity versus

femininity, all of those should be

bigger. Sex differences in

career-related preferences, they should

be bigger as well.

And the gender equality paradox is the

deeply counterintuitive finding, an

incredibly fascinating finding, that

often it goes the other way. Seems to go

the other way, that actually in more

gender equal societies, societies that

are less strict in terms of the gender

roles,

uh and societies that are less

patriarchal, you actually find larger

sex differences often rather than

smaller ones.

Uh

which is

>> in the direction that you would predict

as well.

>> Uh, yeah. So, so opposite to the

direction you predict if social role

theory was correct.

>> But, in the direction you would predict

if you were taking an evolutionary

perspective, I would imagine.

>> Um, well, actually, you know, taking an

evolutionary perspective, I I didn't

originally predict that. I I thought

that in more patriarchal societies, e-

even given that there's an innate push

towards certain sex differences, I still

thought that patriarchy and stricter sex

roles would pry the sexes apart even

more.

>> Right.

>> So, I was actually very surprised that

it doesn't and that it does the reverse

is the real surprise.

>> What are the rebuttals to socio- that

socio-cultural

explainers have for the gender equality

paradox?

>> Uh, there are a few. One is that, um,

people in more patriarchal societies,

they tend to the the men and women have

little to do with each other.

>> Right.

>> And so, that they have a more sort of

exaggerated picture of,

uh, the other sex. And so, when they

fill out personality inventories, for

instance, they they're comparing

themselves with with other men if

they're men or other women if they're

women. And they therefore see themselves

as being more similar to the

average

for their comparison group. But, their

comparison group is just their sex.

>> they're just in they're so siloed off in

these patriarchal societies that they

don't know what the [ __ ] the other

side's doing.

>> Exactly.

It is. [laughter] It is.

>> You're just in a bunker so much that you

don't know what's going on.

>> Exactly.

>> It makes sense. It does.

>> It does, but, um, there's also a

plausible rebuttal to that rebuttal,

>> [laughter]

>> uh, which is that, um,

>> It's [ __ ] love it. So good. This is

my [ __ ] This is my [ __ ] [laughter]

>> Uh, so, it doesn't just apply to

self-report things like personality. You

also find it for, uh, some cognitive

abilities. I think spatial ability, for

instance, you find it.

>> Yeah.

>> Um, you find it as well for physical

traits like height.

>> Are you saying that in more gender equal

societies boys get even better better at

throwing.

>> Um I don't know if that is actually the

case, but that that would be an example

if there's research on that. I'm not

sure there is actually cross-cultural

research on that.

>> Throwing's my favorite one, dude.

>> It's huge huge difference.

>> my It's one of my absolute favorite. Did

you ever look at that study where um so

the the the difference in throwing uh

velocity, accuracy is a pretty big sex

difference. But one of the um

appropriate pushbacks was, well, uh

men's shoulders articulate differently,

uh the length of the forearm means that

you've got more leverage, etc. etc. I

think the the sex difference at 8

[clears throat] years old in throwing

velocity and accuracy is basically the

same as adult men and women being able

to detect adult women and men just by

their face.

>> Right.

>> It's a 90% It's a huge huge huge

difference. So, there was this My

favorite study on this was uh in order

to negate the articulation argument, the

issues of sort of biomechanics, uh they

got a tennis ball serving machine and

fired tennis balls at students.

>> Yep.

>> And got the boys and the girls to see if

they could get out of the way. And I

don't think any of the males were hit.

And I think that I This is before you

wouldn't have been able to get this past

an ethics board. Um but yeah, just the

sort of spatial uh awareness, that

spatial rotationy stuff is um

the sex difference is is is pretty

large. And apparently Maybe maybe in a

more gender-equal society, even more

would be women would be hit by the

tennis balls.

>> Well, that's possible, yeah. Um so, the

gender equality paradox, it doesn't

apply to every trait. So, it might not

necessarily be that. But um yeah. But

but you are right. I think that was

probably a 70 I think it was a 70s

study. I'm not sure you could do that.

Yeah, in fact, you couldn't do that. You

couldn't aim projectiles at uh

participants in studies anymore, sadly.

>> What's the largest sex difference

between men and women?

>> Um well, the largest ones are physical

differences.

Physical sex differences. So, um the

very largest uh sex differences in

reproductive anatomy, where basically

all males have penises, all females have

vaginas, and so on.

Um

but other physical sex differences um

are also very very large. So, the sex

difference in upper body strength is is

huge, and there's very little overlap,

close to no overlap

uh for that one.

Um the sex difference in voice pitch is

another very very large one as well.

That's huge, uh which is quite

interesting because uh

the voice pitch sex difference other

apes

isn't isn't as big. It's nowhere near as

big. And that's even the case for um

for other species that are much more

dimorphic than us in terms of the size,

like gorillas and and orangutans.

So, that's a bit of a puzzle, not quite

sure why that is, why that's the case.

Uh when it comes to psychological sex

differences, the vast majority of them

are much more modest,

um but there's one that stands out as

being as being almost as big as some of

these physical ones.

I always ask people if they can guess

what it is. I think last time I was on

your show I asked you if you could

guess, and and you didn't, but then no

one does.

>> I've read the book now.

>> You've read the book now, so

>> Yeah. Yeah. Yeah.

>> So, you've got a head start on that.

>> what it would be, Jared? The the single

biggest psych psychological sex

difference between men and women?

>> Hm.

Is it aggression?

Um

>> It's not aggression. I'll give you a

clue, and that's that it is

it's related to reproduction pretty

closely.

What was that?

>> Diversion.

>> It's Well, actually, that is a big sex

difference, but

>> Not as Not as big as this one. You speak

to yourself.

>> Speak for yourself, dude.

>> [laughter]

>> It's the sex difference in

which sex each sex is primarily

attracted to. So, men most the vast

majority of men are primarily attracted

to women, the vast majority of women

primarily attracted to men. And when I

say it, it's obvious right it's so

obvious that it's a big sex difference,

and um

but but it's curious, people don't think

of it. Even um scientists often don't

think about it. I've read a bunch of

papers that summarize evolved sex

differences, and they don't tend to

mention that one. They often neglect to

mention that

that very very large one, but also that

one that has quite a clear evolutionary

rationale. That's very interesting.

>> because if that wasn't the case, we

wouldn't be here.

>> Exactly.

>> Literally.

>> We literally wouldn't be here. Yeah.

>> Uh okay, so what are the biggest sex

differences? Let's get down to some

brass [ __ ] tacks. What are the

biggest sex differences when it comes to

sex?

>> When it comes to sex,

>> Yes.

>> one of the largest ones is the sex

difference in interest in casual sex and

sexual variety. And

so basically so sociosexuality, the

trait of sociosexuality. No strings

attached sex.

Do I need to mention which direction it

is?

>> Uh

yeah, yeah, yeah, yeah, yeah.

Someone can intro it I guess.

>> Yeah.

So so men on average

uh score higher.

However you measure that trait, they

score higher than women on that trait.

The difference is there maybe though are

not not as huge as people sometimes

assume.

Um we're looking usually at an effect

size of

uh you know, about

Cohen's d equals one. So what what that

means is there's about one standard

deviation between the average for men

and the average for women.

Uh so so yeah, between eight and one.

And that is pretty big. That that is big

for a human sex difference.

But there is still quite a bit of

overlap. So there are plenty of women

who have a higher sociosexuality than

plenty of men.

I guess an intuitive way to put it, a

way to make it intuitive, would be to to

say to imagine plucking two people at

random, one man and one woman.

With effect size of

or or one, probably two-thirds to 70% of

the time the man will score higher than

the woman on that trait.

Um so that is a majority. It's not an

overwhelming majority though. In a

significant minority of cases, you'd

find that actually the woman scores

higher than the man.

>> Mhm.

>> Yeah.

>> Why?

>> Why is that sex difference? That is

getting right into this the central

question of why there are any sex

differences at all.

>> we we we probably need to talk about

like the evolution of sex differences,

why they why they exist in the first

place.

>> Yeah, um so we should jump to that.

Uh

Can I just as an aside though, can I we

we mentioned three of the six lines of

evidence for innateness. Do we want to

jump back and do the other three and

then maybe get into the sort of thing.

Um

So, what are we what are we

So, we had the developmental sex

differences, we had the resistance to

cultural pressure, we had the um

persistence across time. So, another one

is uh hormonal correlates of sex

differences.

And in particular, um you find

associations between prenatal hormonal

exposure and traits found later in life.

Uh so, higher levels of testosterone in

the womb for instance are associated

with higher levels of aggression later,

higher levels of risk-taking,

lower levels of parental inclinations.

>> Mhm.

>> Uh and so on. So, a lot of

>> regardless of sex.

>> That is regardless of sex, yes. So, you

can look at within sex differences as

well.

>> Mhm.

>> And uh yes, so one actually very strong

line of evidence for that claim is the

fact that uh women who who when they're

in the womb are exposed to very very

high levels of prenatal uh testosterone

uh due to having a condition called

congenital adrenal hyperplasia or CAH.

Uh they

as girls and then as women, they they

exhibit a lot of uh male-typical

traits.

Uh for instance,

>> in playing with dolls.

>> Exactly, yeah. Less interest in uh

getting married and having kids, uh more

interest in things-related professions

as well.

>> [clears throat]

>> So, that's pretty interesting.

>> them more likely to be bisexual or

lesbian?

>> Yeah, yeah, it does. It makes them more

likely to be interested in in other

women sexually as well, either as

bisexuals or as lesbians, yeah.

>> Wow.

>> So, so like a wide range of traits seem

to be influenced by testosterone.

>> is is correlates with hormones?

>> Yep. Uh the fifth one is uh

cross-cultural universality.

So, most of the key sex differences that

we've been discussing so far, they are

found either in every culture

or in the vast majority. And if it were

all just down to culture,

you wouldn't expect that. You'd expect

there to be more variation across

cultures than what we found. Um but you

don't you know you just always have like

in every culture, you find that men are

more aggressive than women, and you find

uh at the extreme of aggression in

particular, the differences are

particularly big. And you find that the

vast majority of homicides are

perpetrated by by guys in every single

nation for which we had data.

Greater risk-taking, more accidental uh

fatalities among men than women.

Uh women across cultures are more

involved in parental care, and so on and

so on.

And then last but not least, and maybe

this is the one I find most plausible on

its own, is the fact that a lot of these

sex differences, you find them as well

in in other species.

You don't find them in all other

species, but importantly you find them

in species um

that were subject to the same

evolutionary selection pressures as as

ourselves.

Uh and actually the selection pressures

in question uh bring us to the ultimate

roots of evolved sex differences.

Uh so, basically

most sex differences emerge from the sex

difference in in what I like to call the

maximum offspring number,

the male animals versus female animals

can produce. And in most species, uh the

males potentially can produce more

offspring than females.

Um that that average number of offspring

has to be the same. At least in species

where you have similar sex ratio, the

average has to be the same, but the

variance in the number of offspring

produced can be greater in one sex than

the other, and it's usually greater in

males.

>> Why does the average need to be the same

if the variance can be different?

>> Uh because it takes two to tango.

>> Yeah.

>> And so every

uh

offspring that's produced is produced by

one male one female.

And in most species it can be

>> have one male

servicing 20 females, why not have a 20

to one ratio?

>> Well, that'll be a 20 to one ratio in

terms of the number of parents.

But the [snorts] average number for each

sex will be the same.

>> Right.

>> Because you got the one male with 20

times more

>> Mhm.

>> uh than any female.

>> Mhm.

>> And then 19 and so that's already a high

number. But then the other 19 bachelors

who never get to have kids, uh that

brings the average for males down.

And then it's the same for all the

females. So you average out across all

of them and and the average is the same.

>> I I get that.

>> Sure.

>> Maybe I'm being stupid. Why is it not

the case given that men given that males

are able to produce more children than

women can

and females can,

>> Yeah.

>> why is it not the case that there is a

bias toward fewer males in the animal

kingdom?

>> Right. Right. That's a great question.

Um and that is one of the sort of early

mysteries

of evolutionary biology.

And if it were for the if selection

acted for the good of the group, that is

what we'd find.

We'd find if if you only need one one

male for every 20 females and

selection were were for the good of the

group, you get that because

if you have 20 of each,

>> Mhm.

>> then the other 19 are surplus and

they're using up these resources and

they can uh competing with each other

and fighting with each other. It's not

good for the species as a whole. Uh but

selection favors what's good for the

individual. And for the individual,

>> I could be the one.

>> You could be the one, exactly. And and

the average is the same. So the the

Um it's I guess it's useful to think

about

Think about it from the parents' point

of view. Like is it better to have to

have sons or daughters?

Um and if there were

20 females for for one male,

it'd be much better to have sons

because each son is going to just have

There's going to be tons of females. And

so that would mean that selection would

slowly favor an increase in the number

of sons.

>> well, the [ __ ] you can't beat

evolution.

>> Exactly. And the reverse, if there's 20

20 males and on for every female, then

it would be better for parents to

produce daughters because then they get

to pick the cream of the crop and and 20

males probably they're going to have a

low number, so it's better to have

daughters. So

the equilibrium point is 50/50.

>> Yeah, obviously as well, there's

survival and not just reproduction.

We're not living in this perfect

hermetically sealed safe environment.

>> Yeah.

>> Males do other things than just

reproduce regardless of whether they can

do 20 to 1 or not.

>> And they

Yeah.

>> Getting food, helping to protect,

getting rid of other tribes, etc., etc.

>> Yeah. Yeah, exactly. And you know, there

are plenty of species where the male uh

his his role

in reproduction is just the reproductive

act and then he then he's off looking

for other females, but we're not one of

those species.

I mean, that does happen occasionally.

There's occasional male that would do

that. Uh but generally we're quite

involved in child care.

Uh yeah, so okay, so

the big difference there is you you get

this difference in the variance.

>> Mhm.

>> Um some males can have tons of

offspring.

Uh and they can have many more offspring

than any female can. Where that's the

case though,

there's going to be some males that have

no offspring

um or maybe a few a few or none.

And where that's the case, you get

strong selection on the males to be one

of the lucky few males that has many

offspring

>> Mhm.

>> rather than one of the the few that has

none.

>> Yep.

>> Or one of the many that has none.

>> Yep.

>> Um and so that selects for traits like

greater aggression, within sex

aggression to try to beat up other males

for either directly for females

>> Okay.

>> or for the territories,

the status, or the resources that are

necessary to attract female attention.

Uh a tendency to show off, a tendency to

grow a great big tail. If you're a

peacock, you might want to try that.

Grow a great big flashy tail to seduce

the peahens.

Um, or just a tendency to seek multiple

mates. And uh, and seek no strings

attached sex, basically. So, that's the

answer to your earlier question. Is that

what That's why you have selection for

like stronger selection on males than

females for an interest in multiple

partners and sex without commitment or

that kind of thing.

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>> How does parental investment influence

the evolution of sex differences?

>> That is a major contributor. And the

reason is that So, so this variance sex

difference is mainly a a result of sex

differences in parental investment. So,

the reason that um, some males can have

many, many more offspring than any

female in in a lot of species is that

females invest more in the young.

So, in mammals for instance, to produce

any one offspring, uh, a female has to

have a have a long pregnancy, has to

give birth to the young, and then has to

nurse the young uh for for a while,

whereas uh a male to produce an

offspring just, in principle, only needs

to um engage in in one sex act.

>> Mhm.

>> So, he can mate with multiple females

and have tons of offspring cuz he

invests

potentially little in any given

offspring.

>> Mhm.

>> Uh whereas the females on the other

hand, they invest a lot in each

offspring. That lowers the ceiling for

the number of offspring they can

produce.

>> Therefore, you have to be choosier.

>> You have to be choosier about your

mates, exactly. Sleeping with

uh or or mating with lots and lots of

males less useful for females than for

males.

Um

but

so so parental investment is really the

core um to

>> Is that the Is that the key driver?

>> Uh well, I think the key driver is the

difference in the maximum offspring

number or or the reproductive variance.

>> Right.

>> Because it's not just shaped by parental

investment.

>> Behaviorally, that shows up as parental

investment.

>> Well, that's a big part of it, yeah. So,

the parental investment is is the major

cause of the sex difference in in

maximum offspring number. But other

things do come into it in different

species.

>> periods too is like a biological

imposition on parental investment,

right?

>> Well, look, is actually is is

how long it takes to just make a baby

considered part of parental investment?

>> of parental investment, yeah.

>> Right.

>> And parental care is and nursing is

>> Yeah.

>> Um

>> Obviously those two, but I wasn't sure

whether or not it's classed Parental

investment is classed as actually the

making of the baby.

>> It it is cuz it forecloses the option to

make other babies.

>> Understood.

>> Yeah. Yep. Yep. Yep. Um but ecological

factors come into it as well. So, one

example of um effect other than parental

investment that influences the

reproductive variance uh is ecological

factors. So, you'll have some species

where the males where they're quite

solitary. So, the males and females

might pair up, and there aren't many

other individuals around. Um even if the

female invests a lot more in the young

than the male does,

their maximum offspring number is about

the same because just simply cuz they're

isolated from other uh members of the

species, the males don't have many other

opportunities to sow their wild oats,

basically.

>> What's Bateman's principle?

>> Bateman's principle,

so that is the idea that reproductive

variance is greater in males than

females in most species.

>> Right.

>> And then Trivers sort of refinement of

that was that the main cause of that is

parental investment. So so Bateman

put put put a lot of it on anisogamy. He

explained the difference in in

reproductive variance in terms of the

fact that

um

females invest more per egg than males

invest per sperm.

Um

Trivers said, Robert Trivers, who died

recently, he said it's not just that,

actually it's it's all kinds of

investment that you put into into the

young, not just the gametes, but also

gestation and parturition, which is

giving birth, uh nursing, parental care,

all those things um are what matters.

And and I would just add that it's

parental investment is the big one, but

other things as well determine

reproductive variance.

>> What is the It's about 40% of males of

human males ancestrally reproduce and

about 80% of females, is that right?

>> I have heard that number bandied about.

I actually think that probably

in our species,

my guess is that it's not it's not that

big. I've also seen other evidence

suggesting that um

it's called reproductive skew,

>> Mhm.

>> and suggesting that the reproductive

skew in our species is quite a bit lower

than you find in most mammals.

And I think the reason for that is that

>> So the 40-80 number you think is is

wrong?

>> Yeah.

>> Right, okay.

>> so. I think it's I think it's a small

one. I well I think it's smaller and I

think because males in our species very

often invest

quite a lot in the in the young. So it's

not just the sex act. Um they often

typically help out with the young as

well.

Uh and so we have we have high levels of

biparental care in our species. That

reduces the maximum offspring number for

males relative to females and

brings it down brings it closer to what

we have for females.

>> So if other animals have less of a

reproductive skew but also have less

male parental investment, how would the

reproductive skew for us be so great if

we have more male parental investment?

Is that what you're saying?

>> That that is what I'm saying.

>> Bingo! I [ __ ] know evolutionary

psychology, dude. Don't test me on this.

I [ __ ] understand this. It's the only

topic I understand.

Um

>> Yeah.

>> So it's the variance in reproductive

success.

>> What

>> What do you What do you reckon it was

then if it's not 40/80? Feel free to

just bro-science it.

>> I think I

I might

>> [laughter]

>> Um I probably shouldn't put a number. I

I uh

other than to say I think it's smaller.

I think it was it was smaller.

I know. I know.

>> That's why I'm in the room, dude. Let me

just [laughter] find and put this thing

on for the rest of the conversation. Um

>> Well, you've got these as well.

>> That's also That's if you want to say

something insane.

>> Okay, cool. Well, I'll keep it here cuz

I may well want to

>> insane, you're allowed to put that on.

It's actually saved me once already. I

I got I got clipped talking about a

slightly highly postulated theory. Put

that on. It's going to be interesting on

you because you already have the

mustache and the glasses.

>> It will indeed, yeah.

>> Nothing's changed.

>> Nothing's changed.

>> [laughter]

>> Yeah. Okay, so we've got a

It's like an interesting I guess an

interesting

pivot on what people think. The That's

sort of 40/80 number. As far as I was

aware, like to actually

So you think the skew is different? What

I thought might be true is that the

overall number might be different, too.

That the whole To me, infant mortality

>> Right. Right. Right.

>> [ __ ] ruthless. So

the only way that we can work out how

many

of our ancestors reproduced presumably

is due to some kind of genealogy stuff.

And that means that it's not how many

reproduced, it's how many survived.

>> Yeah, survived to reproduce

>> yeah. The grandparent optimizing machine

from your

previous book, which is fantastic and

everyone needs to go and look at.

>> Um yeah, everyone has said the universe.

>> 80% of women having

grandchildren producing or

80% of women being grandchildren

producing people

>> Yeah.

>> seems insanely high to me.

>> Uh I guess the thing is that they would

have had a bunch of kids and only about

50% would make it, but 50% would make it

and so if they, you know, have sex for

instance.

>> That's what it is, around about 50%

>> Yeah.

>> that are born go on to reproduce.

>> With unbelievably high, right?

Uh what about 50% um infant mortality

rate. So, like half of them didn't make

it. Yeah. One of the great things I

think about civilization is that we have

lowered that number to very close to

zero

cuz that was I think a massive cause of

misery throughout most of human history.

>> Was it Darwin lost a bunch of kids?

>> He did, yeah. He had uh

What was it? He He had 10 and three

didn't make it to adulthood, I think

that's the number. Yeah.

>> Is it your book or was it I think this

might have been The Moral Animal Yeah.

where

Robert Wright talks about the fact that

Darwin lost multiple children.

But the one that was the most

psychologically painful Yeah. was his

daughter who was 12 or 13.

>> Yeah.

>> And the reason for that

the evolutionary reason that Robert

gives for it is that

parents are able to detect

the moment at which the life cycle of

their child would have allowed them to

just about become the grandparent

optimizing machine that evolutionarily

we're driven to be. And that if a child

dies at two years old, that's

horrible, but

they won't they had a long way to go and

many things could have occurred between

now and being able to have kids. If a

child dies when they're old, then

they've had all of the opportunities to

have had kids and that didn't

didn't go well or maybe did go well. But

if you are just on the cusp of being

able to reproduce biologically and you

die, that is from an evolutionary

perspective the most painful. Am I

making that up or is that the story?

>> No, that is the story. Yeah. And

there's

pretty good evidence on it as well. That

grief tracks reproductive value of the

individual.

>> No [ __ ] way. So this has been

reproduced cuz that book's 30 years old

now.

>> Yeah. Yeah.

Um yeah, go the grief average grief

levels go up and up and up as

reproductive value goes up as as kids

move from the very vulnerable

historically very vulnerable early

infancy to childhood, it's kind of peaks

at about the age that reproductive value

peaks. So couple of years after puberty

and then starts coming down again as

people get older.

>> [ __ ] fascinating. How is everyone not

obsessed with evolutionary psychology?

It's so cool. It's so fun.

>> And sheds light on some of the most

fundamental stuff in life.

>> Yeah, I mean obviously it's very

bleak in some ways too. It's very stark.

It doesn't paint a particularly

flattering picture of human motivation.

Like the reason that Darwin was

particularly sad about his 13-year-old

daughter dying is that he had this weird

algorithm running in the back of his

mind. He's like, "Oh, so you're just

saying that all the children are that

produced for you to be happy to do

things like

I do that you you don't need to lord

our ultimate evolutionary drives

>> Yeah.

>> in order to understand and respect them

and accept the fact that they are

important influences on our behavior.

>> I completely agree and even you know,

and I do feel the pull to what like it

is a bit bleak. I remember when I was

first reading about that grief theory,

it does seem a bit a bit dark. But then

when you think about it though, it's not

very flattering exactly. I mean, I guess

one thing is that um

we don't actually we're not actually

thinking about reproductive value.

Uh we just we just have a built-in

tendency to react as if we are. But

we're not actually thinking in the back

of our minds, this is sad because

I this this kid was going to close

[laughter] to having kids and then I

wouldn't have been

>> then. Well, this is

this is sort of my least I don't know

actually. I I I probably could write out

a hit list of my least favorite internet

rebuttals to anyone talking about

evolutionary theory, but one of them is

a lack of understanding about the

difference between proximate and

ultimate motivations for behavior.

>> Yeah, exactly. So one of the big ones.

>> Yeah, like the

>> big misunderstandings.

>> why you

feel the desire to do a thing

and the evolutionary advantage of you

doing the thing. The fact that those two

things are different. Um you eat food

because it tastes good. But the reason

for eating food is to keep you alive.

You have sex because it feels good, but

the reason to have sex is not the reason

that it feels good.

>> Exactly right. And and it's quite

annoying, right, when people say, "So

you're saying that um like sex and

uh having like short-term relationships

or long-term relationships is just about

babies, but that can't be true cuz often

the last people the last thing people

want is to have kids."

>> I don't want to have kids. I'd love to

have sex.

>> Yeah. Yeah, yeah, exactly. So it doesn't

make sense. They've just misunderstood

the idea cuz it's not a psychological

theory.

Uh like the the genes I view

explanations the the evolutionary

explanations those are not motivations.

Those are just explanations for why we

have the the proximate

psychological drives that we do.

>> Not the ultimate.

>> Exactly.

>> Yeah, yeah. And you know, with the

the

what's it referred to as that theory of

the sexual not sexual value?

>> Um

>> of

uh

Darwin's daughter dying, what's the

>> Oh, yeah. Our reproductive value.

>> Reproductive value.

No one is how horrible to say to a

parent, "Well, it's actually

Mrs. Thompson, the reason that you're

grief Mrs. Darwin, the reason that your

grief is so high is because of the

reproductive value that your daughter

almost didn't cash in on."

Horrible, horrible thing to say. But it

doesn't need to

it doesn't need to be something that's

in your mind to like go how could you

claim that this mother is feeling that,

thinking that at the time and that's

contributing to a grief? No.

>> Yeah.

>> No, she just loves the kid. Yeah. It

would would be the same it would be the

same as going

why is it that people get depressed when

they're lonely? I'm not thinking about

potentially being kicked out of the

tribe. You go, "Well, yeah, like we're

in a modern environment. Why do you

think it is that depression and anxiety

correlate with isolation?"

>> Yeah. Yeah.

>> If it didn't matter. Well, it does

matter. It doesn't matter anymore, not

in the same sort of a way, but that

doesn't negate the fact that our system

is basically ancient programming living

in a modern world.

>> Exactly. And even though it does seem

bleak sometimes, I don't see that other

explanations, socio-cultural

explanations would be any less bleak,

you know? If they were telling me

>> I'm at the mercy of the memes and trends

of the modern world.

Some guy decides to start a new cool

movement somewhere and then that

completely sort of blows the wind of

whatever it is that I'm supposed to care

about.

>> Exactly. Or or you might say, "Okay, the

reason that you grieve this individual

is just because you've been socialized

to do so."

How's that any less any less bleak?

>> It's the same. This is why I think

there's a lot of similarities between

evolutionary psychology and behavioral

genetics that if you start to strip away

the science and the evidence underneath

it, what you end up with is an equally

or maybe even uglier perspective. No, if

if for instance, if you deny that there

are genetic influences on outcomes in

life,

>> Yeah.

>> what you're left with is this world

where

everybody did start off

equal.

And you are exclusively at the mercy

of the entire world.

>> And and injustices.

>> Yeah. Yeah. The the only reason that

this didn't come out is all on your

shoulders. All on your shoulders because

you didn't try hard enough or your

parents didn't try hard enough.

>> Yeah.

>> Horrible. Horrible.

>> And I guess so the thing is that

whatever hard truth

and they're not all hard either, but

whatever hard truths

ultimately we just got to um accept

what's true, whether we like it or not.

As you know, sometimes it is positive.

So so in natural selection has given us

a whole range of less than noble traits,

but it's also given us some really

lovely traits as well. Like the capacity

to fall in love and to love our children

and to love our friends.

>> Mhm.

>> You know, these are all great

things. Yeah. Yeah.

Exactly. To be able to do stuff.

>> Okay. So I guess one just to round out

the We haven't even got into the sex

differences yet, which is the meat and

potatoes of this, but I don't care. Um

Is it

Are humans unique in this regard because

we've got mutual mate choice mhm and

we've got biparental care.

So when we're talking about evolution of

sex differences, parental investment,

ability to produce children, volume of

children across a lifespan, Bateman's

principle, variance in reproductive

success,

but humans do seem to be somewhat of an

outlier in mammals and especially in the

animal kingdom.

>> Yeah.

Yeah.

>> What wrinkle does that throw in?

>> Uh well, yeah. So we are

So the question, are we unique? Sort of

yes and no.

So the way in which we are unique, we're

very unique among mammals. Like you say,

in the fact that we have high levels of

the tendency to pair bond, sort of fall

in love and form relatively durable pair

bonds and for both sexes rather than

just the females to invest in the young.

Um that is found in maybe 5

to 10% 10% at the most of mammals.

So the overwhelming trend among mammals

is that the females do all the

investment in the young and the males

are basically deadbeat dads.

In birds on the other hand, they

are an outlier family of animals among

animals in general and that around 90%

of birds form pair bonds and have

biparental care. And so there's a funny

thing I find this

just very very interesting is that

in our primary reproductive behavior,

humans are more like the average bird

than like the average mammal.

>> No way.

>> Yeah, that's cuz of our pair bonding and

our biparental tendencies.

>> Is a

reinforcement that the

parental investment is the key driver of

sex differences.

>> Yeah, exactly.

And and of the

difference in most birds hang on a

second. So

most birds are kind of like dinosaurs

and humans are closer to birds than they

are to most mammals, which means that

humans are kind of dinosaurs.

>> Yeah, so

that's great. It's great. Do you want

this or

>> Yes, I don't need that. I don't need

that.

>> No, no, you don't cuz I I agree. So we

we're mammals that are

more like dinosaurs in terms of having

the the primary reproductive arrangement

of of today's dinosaurs.

>> go.

>> It's wild, right?

>> Let's [ __ ] go.

>> And so yes, so we're an exception among

mammals in that respect.

A way in which we're not an exception

though

is that the the same evolutionary

principles that apply right across the

animal kingdom apply to us as well. So

the fact that sex differences emerge

from the maximum offspring number, the

the level of sex differences and

reproductive variance

that is true across the animal kingdom

and

the rules about that apply to us as well

and the fact that we have we do have a

sex difference and reproductive

variance. Men do have and have always

had greater reproductive variance than

women, but it's relatively low compared

to most mammals.

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>> Isn't there an interesting inversion of

this in spiders where female spiders are

bigger than the males?

>> Uh actually, in most insects, it's that

way around. And actually, there are more

animals where the females are bigger

than where the males are bigger. Um but

that's not because of difference in

reproductive variance. It's because

um

in a lot of species, the main driver of

sexual dimorphism is just the number of

eggs that you can produce. And females

bigger females can produce more eggs.

There's a selection pressure on them to

get bigger and bigger and bigger.

They're typically bigger than the males.

The males

don't need to be much bigger.

>> need that much room for sperm.

>> Uh yeah, exactly. Yeah. So, so the um

gamete size difference, sperm are bigger

than eggs, is reflected in the

physiology of the species.

>> so [ __ ] cool. That's so cool.

>> we're the weirdos. We're so used to

males being bigger than females, but