Making Sense with Sam Harris - #211 — The Nature of Human Nature

Episode Date: July 17, 2020

Sam Harris speaks with Robert Plomin about the role that DNA plays in determining who we are. They discuss the birth of behavioral genetics, the taboo around studying the influence of genes on human psychology, controversies surrounding the topic of group differences, the first law of behavior genetics, heritability, nature and nurture, the mystery of non-shared environment, the way genes help determine a person's environment, epigenetics, the genetics of complex traits, dimensions vs disorders, the prospect of a GATTACA-like dystopia and genetic castes, heritability and equality of opportunity, the implications of genetics for parenting and education, DNA as a fortune-telling device, and other topics. If the Making Sense podcast logo in your player is BLACK, you can SUBSCRIBE to gain access to all full-length episodes at samharris.org/subscribe.

Welcome to the Making Sense Podcast. This is Sam Harris. Just a note to say that if you're hearing this, you are not currently on our subscriber feed and will only be hearing partial episodes of the podcast. If you'd like access to full episodes, you'll need to subscribe at samharris.org. There you'll find our private RSS feed to add to your favorite podcatcher along with other subscriber-only content.

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and we grant 100% of those requests. No questions asked. Okay, just the briefest housekeeping here. Just to say that we have finally posted the bonus questions I have long been promising to subscribers. Those can be found on my website if you're logged in, or also in the subscriber feed near the related episode. And I haven't done these for every episode, but there are many going back quite a ways for people like Nicholas Christakis, Donald Hoffman, Eliezer Yudkowsky, And so if you look in your subscriber feed going back, you will find those, and as well on my website, if you are logged in to your account. Okay. Today I'm speaking with Robert Plowman. Robert is a professor of behavioral genetics at the Institute of Psychiatry, Psychology, and Neuroscience at King's College, London. He previously held positions at the University of Colorado Boulder and at Pennsylvania State University. He's also been elected a fellow of the Academy of Medical Sciences

and of the British Academy for his groundbreaking work in behavioral genetics. And he's the author of the fascinating book, Blueprint, How DNA Makes Us Who We Are. And Robert and I get into many of the interesting and fraught questions here. We talk about the birth of behavioral genetics, the taboos around studying the influences of genes, in particular in human psychology, controversy surrounding the topic of group differences, the first law of behavioral genetics, the concept of heritability, nature and nurture, the significance of non-shared environment, which is genuinely perplexing, the way genes can shape our environments, epigenetics, genetic influences on complex traits, dimensions

versus disorders, the prospect that this will land us in some Gattaca-like dystopia. Heritability and Equality of Opportunity, the implications of genetics for parenting and education and other social policies, DNA as a fortune-telling device, and other topics. Anyway, it's a fascinating conversation. This is Important Science, and now I bring you Robert Plowman. I am here with Robert Plowman. Robert, thanks for joining me. Well, it's my pleasure. It seems like I've known you because I've listened to so many of your

podcasts. Nice. Well, I have read your book. Let me properly introduce your book first because it's a fantastic introduction to everything we're going to talk about, and there's no way we will exhaust its interest. So people should read your book. The book is Blueprint, How DNA Makes Us Who We Are. And we'll track through the case you make here pretty systematically. But first, I should say, you really are one of the most revered people in this field of behavioral genetics. And this is a field that is still somewhat under the radar for people, I think, intellectually. I mean, people know that we had the Human Genome Project some decades ago, and I think there's this vague sense still, somewhat analogous to the sense everyone had

that artificial intelligence never amounted to anything, and then all of a sudden it amounted to a lot. But people have a sense that this genetic revolution hasn't really arrived, and yet behavioral genetics is this field in which we're discussing the role that genes play in determining who we are in the most basic sense, I mean really the nature part of human nature, and your book is just a great introduction to that and its implications directly for psychology. But before we get into the data and your argument, maybe you can summarize your background a little bit. How did you get into this work? Depends how far we go back, but I'll start at university. You know, I think one of the things

I hope we get to talk about, I've heard on several of your podcasts, is about the role of chance. I've heard on several of your podcasts about the role of chance, and genetics has a new kind of spin on chance. And I went to the University of Texas at Austin because I was an inner-city kid in Chicago. None of my family went to university, let alone graduate school, but I had this wonderful advisor who helped me apply to graduate schools. And being a good inner-city kid, when the University of Texas offered to pay for me to go, I said, well, that sounds like a good deal. So I went to the University of Texas in psychology, but they, unknown to me, had the only program in behavioral genetics in the world. It had just started at that time in the early 1970s. And this is one of these chance events that everyone in those days, I don't know about when you were in graduate school, but in those days you had to take core courses. So you had about two years worth of

courses you had to take in clinical and perceptual. And everyone had to take this course in behavioral genetics. 40 other students were in there and it floored me. I just saw this evidence for the importance of genetics. Most of it was from animal studies at that time. And I just knew right away, that's what I wanted to do the rest of my life. Yet, none of the other students took it up. So what is that about? You know, I don't know, but it was really a turning point in my life. And I went, it was very lucky because most of the behavioral geneticists in the world were there at that moment. And I was really at the beginning of the application of genetics in psychology. And, you know, back then it was actually dangerous to be doing genetics in psychology because psychology was completely dominated by environmentalism and nurture.

So I kind of grew up with the field and, you know, I learned a lot of stuff about genetic influences. I'm sure we'll talk about genetic influences on environment and developmental changes. There's a lot we learned. And I thought, great, that was terrific. And I'm happy with my career. And then along came the DNA revolution. And that's what's really changed everything. And it's all relatively new, so I don't think that's what's going to have the impact on people, because you can argue with these twin studies and adoption studies, but you just can't argue with DNA. And that's what's new, and that's what's really going to make a difference. Yeah, so we're not just talking about things like height and weight, obviously. We're talking about personality characteristics, things like how nice a person you are and how outgoing and how neurotic and how happy, how empathic, how prone to violence, and also just core capacities like intelligence. And whether you think about that narrowly in terms of IQ or

much more loosely in terms of educational achievement. We're really talking about everything we can care about in ourselves and our children and in people we interact with in society. And the punchline here is that, as you say in your book, that DNA isn't all that matters, but it matters more than anything else, and it matters more than everything else put together in determining who we are, which is, on its face, again, a very provocative statement. Even today, I mean, in the beginning of your book, you write two sentences that fairly floored me because I'll have to read them, you say that you delayed writing this book in part due to cowardice because you recognized how dangerous this used to be. And you say it might seem unbelievable today, but 30 years ago, it was dangerous professionally to study the

genetic origins of differences in people's behavior and to write about it in scientific journals. It could also be dangerous personally to stick your head up above the parapets of academia and talk about these issues to the public. Now, Robert, either you are a time traveler from the future and you wrote this book in 2050, or you're living on Mars right now. Because in my world, anything less than a full commitment to the blank slate is still taboo. I mean, there are people who are trying to cancel JK Rowling right now for just admitting that biological sex is a thing. This is the environment we're in, at a minimum, on social media.

So do you really not perceive this to be a fraught territory now? Well, as I say in the epilogue to the book, I was very nervous about this book coming out. My friends said it was a professional suicide note. But I saw lots of signs that things are changing over the years. Back when I was in graduate school, the textbook said that schizophrenia was caused entirely environmentally and even worse by what your mother did in the first few years of life. Genetics never got a look in. So you had to be very careful about even suggesting that something might show genetic influence. But in the 40 years since, there's been a mountain of evidence from twin and adoption studies and family studies that have

convinced, that's convinced most scientists that many traits, in fact, I would say all traits in psychology show significant genetic influence. And it's not just statistically significant. We're talking about a lot of influence, like explaining about half of the differences between people. So I think things have changed a lot. And I've experienced that when I've talked to the public. Mostly the reaction I get is that not hostility, but just ignorance. People say, well, I didn't know about that. It makes great sense. In fact, most of the public I talk to are surprised there's a big controversy. They say, you know, it sounds so reasonable. And there's a lot of evidence behind it. So

I think things have changed. And so I was wondering who the people are you've been talking to that are still blank slaters. Again, there's resistance in some quarters on the far left politically, generally, that biological sex is even a thing, right? I mean, this is what J.K. Rowling has just run into, or that intelligence has anything to do with IQ and is whatever intelligence is, whether IQ or not, that that would be at all heritable. And then when you start talking about group differences for any trait we care about,

it just becomes utterly toxic politically. And the truth is that there's no ethical or perceived ethical sweet spot here, because if you ascribe differences between groups to, you know, again, to take the most fraught topic here is that, you know, IQ differences across racial groups, however defined, you know, Charles Murray's territory, This is just the plutonium of social science. And even acknowledging that these differences exist is taboo in some circles. They have to be artifacts of testing or any other metric you'd be using. But once you get past that, then they have to be due to racism. And once you look past that, let's say comparing Asians to whites on IQ tests, are we now alleging that there's some anti-white racism that is benefiting Asians on these tests?

That begins to look a little weird. But now everyone, again, is jumping out of their skin with political discomfort. And the truth is there is no way of accounting for these group differences that people are comfortable with. I mean, genes are the worst answer, but environment and culture and family situation, that's also a bad answer. People just don't want to say that they don't want to draw any invidious comparisons between groups on any level, we will inevitably touch this territory if only to comment on why we're not wading further into it. I just want to offer a warning to both of us and to our listeners that there is no avoiding these topics on some level because, again, with the best of intentions, with no interest in specific things like IQ

differences among groups, say, the moment you begin to study things like intelligence or anything else you care about at the level of the genome's implications for how people develop later in life, or just begin to tease out the difference between contributions from the environment and contributions from DNA, you get ambushed by these topics that make people incredibly uncomfortable. And this is something we'll get to toward the end of our conversation, where we talk about the social policy implications of all of this, but in a world where we have completely solved our political and social problems, let's just deposit a world where there is no inequality. There's zero inequality of opportunity. Everyone gets to go to the best schools, and everyone is equally wealthy and has equally conscientious parents, and there is nothing wrong

at the level of society, well then in that world, every difference in outcome between people will be ascribable to differences in genetics. And that hardly seems fair to people either. So it's very difficult for people, given certain assumptions, to find any spot of comfort in this conversation. And I think you and I can see some daylight past all that and talk about how we're comfortable with what we're learning about human nature here. But I just want to warn us and warn our listeners that there's a kind of uncanny valley that we have to pass through here where things seem to be threatening at the level of ethics and politics. Could I speak to that? Please. You've raised an awful lot of issues there, but just a couple of the main ones. And you're right, the third rail is group differences.

And in the paperback edition, which came out last year of Blueprint, I have an afterword where I describe, talk about my reactions to the response to the book. And one of those is why I didn't talk about group differences. And I just mentioned briefly in the book, but I discuss it more in the afterword, that the most important point to realize is there's no necessary connection between the causes of average differences between groups and individual differences. So individual differences in a trait like intelligence could be very highly heritable. That doesn't necessarily imply that an average difference between, say, ethnic groups is also heritable. But more than that, the reason I've stayed away from group differences, there's sort of three reasons. One is that there's much more variance. I assume your listeners know variance is just a statistic measuring how much people vary. The vast majority of the variance on these traits is within groups rather than between groups.

And so much so, like, you know, boys are better at math than girls, and girls are better at verbal. That accounts for 1% of the variance. That means if you know whether a child's a boy or a girl, you don't know anything about their verbal ability or their mathematical ability. So differences between groups. But in contrast, we have very powerful methods for understanding the causes of individual differences within groups. And then the final reason is, I don't think I have to study everything. And that's not just, you know, I'm not just being facetious there. I think it's an important point.

In your discussion with Murray, which I thought was brilliant, by the way, you know, it's what ought to happen. You know, these are difficult issues. I thought you discussed them very fairly. But towards the end of your interview with Charles Murray, you asked him, but why do you persist in studying these average differences between groups? I think you even said something about it seems to be, you didn't say prurient, did you? But you did ask him about that. I thought his answer was very unsatisfactory. And so early on, I said, look, there's lots of important things to study. Why are some people schizophrenic and others not? And most of the variants that we're trying to explain with genetics is within groups.

So why focus on the politically explosive issue of average differences between groups when we don't have powerful techniques to definitively answer the question of the etiology of those differences. And that's why I think there's so much heat and so little light there. Yeah, yeah, I agree. But just again, a point of caution, and I think there's no avoiding this. The reality is, you know, I am still digging out from the consequences of having had that conversation with Charles. Is that right? Yeah.

Oh, really? Yeah. So it's like that's at least a year and a half, something like that. That was number 73 and you're up to 210. Yeah. Okay. Yeah. So maybe it's, you know, it's two years, but, you know, he spent the last 25 years of his life not overcoming the effects on his reputation of having written The Bell Curve. You know, at this point, I'm reconciled to never coming out from to the limits of their abilities, essentially saying that I was a racist for having had that conversation and what I said in it. And it's maddening, but that's the environment we're in now, where people who certainly are discussed as being real journalists and who you would think would have reputations for some sort of integrity and intellectual honesty to protect, will smear you as essentially

a Nazi for even touching this topic. And the point I was making with Charles, which was really the reason why I spoke to him in the first place, It was not born of real interest in IQ, much less racial differences in IQ. But I'm interested in our inability to speak honestly about facts as we understand them. And for years now, I've been seeing that there's certain things that will just spring out of the data that we can't avoid, right? Whether you're looking for them or not, if you want to understand intelligence, and you're not at all interested in differences between people per se, you certainly don't want to put any ethical weight or moral weight on human worth based on differences in intelligence, but the topic is still going to be forced upon you. And so we just have to get comfortable with that. And I'm very comfortable that we understand what the political right answer is in the end. We know we want people to have equal opportunities, and we know we want people to be treated as moral equals at the level

of fairness in our society and in notions of justice. And we want to correct for the greatest disparities in good and bad luck insofar as we can do that. And so much of this, the ethical punchline for me is that this is all due to luck in the end. I mean, you don't pick your genes. You don't pick your parents. You don't pick your environment either, right? There's nothing that you pick, you know? And so if you're a good person who cares about the well-being of others, and you realize that, you know, there but for the grace of happenstance, you could have been in any other

possible situation on Earth, it's through no wisdom of my own that I wasn't born in the middle of a civil war in Congo, then you should be committed to making the world as good a place and as fair a place as you can make it. And that dictates a certain kind of politics and a certain kind of ethical commitment to treating people fairly. But people don't see that you can be, I mean, honestly, there are people who listen to this conversation, and despite what I just said, and I could rattle on in this vein for an hour and a half, and the punchline will still be, those two guys are Nazis. That's the environment we're in, and it's a very dispiriting reality. And it's only because I have taken elaborate pains to inure myself to the blowback to these kinds of conversations that I even can have them. Honestly, in any other role in society, I mean, had I been a professor at a university, had I been a normal journalist

who had a boss, I think I would have lost my job based on the blowback from my conversation with Charles Murray. And that's just a sobering reality of the environment we're in. Well, I'm so sorry to hear that, though, because, I mean, your whole podcast is about just having honest conversations about topics. Now, you know, that is probably the hottest topic you could pick. You can talk about genetics of schizophrenia, and people don't get upset about that. You can even talk about cognitive abilities, but if you talk about reading disability, nobody sweats that, no problem. disability, nobody sweats that, no problem. So intelligence just is like a red flag to a bull in some ways. And then by getting into average differences between ethnic groups, I mean, there you've got it. So that's the worst, well, the best case for your podcast to be able to talk about difficult topics. But I don't go there because of the reasons that I mentioned.

And there's an awful lot to learn about individual differences. And in the end, I think they're very important. You know, why are some kids reading disabled? And why do some people become schizophrenic or not? So you really did go to the third rail on it. And I am amazed to hear, though though that you're still getting blowback. I avoid it because I don't do social media. And in the academic press, things are really going the genetic way. If you look at grants funded, for example, I mean, genetics is, there aren't

that many behavioral geneticists, but they dominate research funding in psychology. They dominate the most highly cited papers in psychology. So I am an optimist, though, and I have a sense that you're not quite as much of an optimist as I am, but I can look at this history. Or I spend too much time on social media. Maybe that's right. I mean, I just don't do it for that reason. I mean, it just gets you down. I don't even, long ago, I decided I wouldn't even respond to emails or to even publish criticism of my work because I found, even back then, 30 years ago, before social media, a lot of the critics weren't honest critics. I mean, they would say,

well, what about this? And you say, okay, well, we've done research on that. And that's, yeah, but then what about this? And what about that? And a lot of them, I realized, had nothing better to do. Whereas I had science I wanted to do. And I did feel in the end, if psychology was going to be an empirical science, in the long run, if you take a very long view, getting the data is what matters. And I hope in the end, you know, students of psychology will read about behavioral genetics and nature and nurture and say, well, what's all the fuss about? I mean, of course genetics is important. So I am an optimist and I do look at things kind of with my rose-colored glasses, but

I see huge change. I haven't been called a Nazi for 20 years. That's great. Let's let that be either the motto or the epitaph for this conversation. All right, so I'm going to don your rose-colored glasses here, and we will proceed, because there's fascinating science to talk about, and if people don't understand our intentions here, they will be unreachable by the powers of human speech. What is the first law of behavioral genetics? The first law of behavioral genetics is that

everything is heritable. By that, I mean individual differences in traits, cognitive abilities and disabilities, personality, mental health and illness. Those traits, those individual differences all show significant and substantial genetic influence. Right. And so we should clear up some confusion that people naturally have around this concept of heritability. And then we're going to go into how we know all this based on adoption studies and twin studies and all the rest of the actual science. But let's talk about this concept of heritability. How are people confused about it? Yeah, well, it's great you brought that up because that six-syllable word is the most misunderstood word around because it includes the word heritable. It somehow involves genes and DNA.

So people have a lot of different notions of it. But in behavioral genetics, and I should say by behavioral genetics, I mean what we call quantitative genetics, like twin and adoption studies and now DNA studies. It's the same techniques you'd use if you were studying medical disorders, for example. So it's not peculiar to psychology or behavior, but these are the epitome, in a way, of the complex traits and common disorders. That's the focus of the DNA revolution now. So, heritability describes the, it's a descriptive statistic. And like all descriptive statistics, like means and variance, it can change in populations over time. But it describes the extent to which differences that

we observe in a trait, say like body weight, body mass index, to what extent are those differences due to inherited DNA differences between people in this population at this time? There are many misunderstandings, and probably the most common one is for people to think, well, they confuse what is with what could be. So we're describing what is in a particular population, the extent to which people differ in body mass index, and to what extent does that do to diets and exercise or inherited DNA differences? So we're talking about differences. And we find, people might be surprised, that about 70% of the variance of body mass index

in the Northern European populations that we study is due to inherited DNA differences. So that's often a shocker for people. We've done surveys, and people think there might be some genetic influence, but they think it's more like 30% or so. But 70% is a lot. It's not 100%. But a lot of the differences between people in body mass index are due to inherited DNA differences. But that's what is, and it doesn't imply what could be. So, you know, one of the most interesting things I found about doing my DNA and getting these polygenic scores that we'll talk about later is that I have a very high polygenic score for body mass index. I'm quite heavy. I'm at the 70th percentile of

weight. But what's interesting about this is some people say, well, if you learn that you got bad news and your genetic risk for alcoholism, or in this case for obesity, you'll just give up and say, oh, well, there's nothing I can do about it. But the point is we're describing what is, not what could be. And certainly, if you lock me in a room and didn't give me any food, I'd lose weight. Or more than that, if I had a bit more self-control or motivation, I might not eat like a pig the way I would do, you know, given free access to food. So, you know, the differences between what is and what could be, and the other caveat, there's a bunch of them, but the other caveat I think that's important is we're dealing with

the normal range of genetic and environmental variation. That is, the range of variation that we can study, which is fairly representative populations, maybe 95% of the population. But it doesn't include the genetic extremes of single gene mutations, for example, nor does it include the environmental extremes, say of abuse and neglect. There are many wrinkles here. I guess the two further points I would want to make about this concept of heritability that are related to what you just said. So even if something were highly heritable in general, in any specific case, it may not in fact be expressed. I mean, you take like alcoholism, I don't know what the contribution of genetics is to alcoholism, I don't remember if you

mentioned it in your book, but... It's not real high, but it's say 40%, something like that. Yeah, let's say even if it were 100%, right, even if it were just determined by DNA, in a world without alcohol, it would not find expression, right? So the role of the environment in any individual's case, or even in any group's case, if you find an island of proto-alcoholics, but where alcohol has not been discovered, you'll see 0% alcoholism among people who have the genome that would determine 100% alcoholism in another context, right? So... Yeah, but you know, but not even going to that extreme, the differences between

what is and what could be. So when we say alcoholism or alcohol abuse is 40% heritable, we mean of the genetic and environmental differences that exist in this population at this time, inherited DNA differences contribute about 40% to that liability, you know, the variance in alcoholism. And even if you say, even as you said, if it's 80% or 100% heritable, if I say, okay, I know my genetic risk for alcoholism is high, but I also know you can't become alcoholic unless you drink a lot of alcohol. So I could take that information, say even from DNA, risk for alcoholism and say, well,

I've got to be more careful because if I drink as much as other people, I'm more at risk for becoming alcoholic than they are. And you can't become alcoholic if you don't drink a lot of alcohol. Yeah, yeah. So this covers things like alcoholism. It probably doesn't cover everything that interests us, but it's relevant. And also we should talk about or mention the fact that differences between people we're talking about when we're talking about heritability, we're not talking about things that everyone shares, which are also genetically determined,

which is having a head or having arms and legs, right, or being bipedal, or having some bilateral symmetry. These are things that virtually everyone has who is intact at birth, and we don't talk about the heritability of having arms and legs, right? Yeah, that's such an important point. And we have, say, 6 billion base pairs of DNA, and 99% of those are the same for all of us. And that's what makes us human. We're talking about the 1% of DNA sequence differences, base pairs of DNA,

the extent to which those differences between us make a difference? And the answer is they make a big difference, but it is differences. So if you say height is 80-90 percent heritable, it doesn't mean I grew to six feet because of my genes and the other four inches were added by the environment. We're only talking about differences between people, why I'm very tall and other people are not so tall. Genetics is largely responsible for those differences between people. It really is a critical point, so thanks for bringing that up. Yeah, yeah. So we're talking about in the case of the three billion base pairs in each half of the genome. So we're talking about 30 million base pairs that account for the difference between us. And as you say, we're

99% identical to one another, although we are, if I recall, we're 50% identical to the banana. So I don't know how much comfort to take there. Exactly right. So I don't know how much comfort to take there. Exactly right. So let's talk about the confusion that is even more common on the concept of nature and nurture and how to differentiate those. And one of the more fascinating points in your book comes in this discussion of the nature of nurture. What's the confusion here around nature and nurture and how we can demarcate them? Well, there's that larger issue of just separating nature, that is inherited DNA differences, and nurture, that is environment. And there's a lot we've learned there. But the topic of nature of nurture is a different topic. So what would you prefer to start with?

Let's differentiate nurture and environment because people think it's one thing and then the parents are either horrified or happily exonerated when they learn the punchline here. So let's talk about first, what are the contributions to individual differences beyond DNA? We'll talk about nurture and unshared environment, and then let's talk about the nature of nurture. Yeah, great, good. Well, we talked before about the first law of behavioral genetics, that everything is heritable.

And we can get more precise than that and say, on average, across all the traits that have been studied, about half of the differences between people, half of the across all the traits that have been studied, about half of the differences between people, half of the variance of these traits can be ascribed to inherited DNA differences. Now, 50% is a lot. This is effect size, the idea of how big of an effect it is, not just is it statistically significant. In psychology, it's rare to find anything that explains 5% of the variance. So 50% of the variance is off the scale, but it's a lot less than 100%. And the other 50% is actually not due to genetic differences. But what we've learned

is that it's not nurture in the sense that people have always assumed it was. From Freud onwards, nurture was thought to be what happens in families, particularly parents, and what they do to the kids, like schizophrenia is caused by what your mother does to you in the first few years of life, was the line when I was in graduate school. So what we've learned is, I think, almost more important what we've learned about nurture than nature, because that other 50% is not due to systematic effects of the family environment. So it's probably best if I just give you one piece of data on that that makes that point. Just take BMI and parents' body mass index.

Parents and their children correlate about 0.3 is when the kids grow up. I mean, at birth there isn't any correlation at all, but they correlate about 0.3. Is it nature or nurture? Well, it was always assumed to be nurture, and that's not a dumb hypothesis. I mean, parents give the kids the food, they model be nurture, and that's not a dumb hypothesis. I mean, parents give the kids the food, they model lifestyles and that sort of thing. But the adoption studies showed that when parents adopt a child who's not genetically related to them, the correlation between those parents and their kids for body mass index is zero. Similarly, children growing up in the same family correlate about

0.3 or so in body mass index. It's a reasonable thing, it's nurture. But if those children are genetically unrelated, their correlation is zero. The other side of the adoption design is to take genetically related people adopted apart. These adopted children who correlate zero with the body mass index of their adoptive parents correlate 0.3 with the body mass index of their birth parents whom they never saw after the first week of life, who had no influence over their environment. So that's the sort of evidence that for decades

was used to say genetics is important, but then people realized, you know, in the 70s and 80s, that it's telling us something very important about the environment. Whatever the environment is, it's not making kids in the same family similar to one another. It's not making kids similar to their parents. And that's what I called, in 1987, non-shared environment. It's not making kids similar to their parents. And that's what I called in 1987 non-shared environment. It's important. It's making a big difference.

But it's not what we thought it was. It's not due to shared family environmental influence. So what is it? Well, for 30 years, we've been trying to figure that out. Like, what is it that's making two kids in the same family different? You know, for example, parents don't really treat their children the same. I don't know about you and your daughters. If you ask parents, they say they do.

But if you ask the kids, you'd swear they're growing up in different families. And if you videotape interactions between parents and children, you do see that parents aren't treating their kids the same. I mean, like your friends, when they're sufficiently high, would probably admit to this. I mean, some kids are just more lovable and cuddly than others, you know? So anyway, there are these possible parental differences in treatment. And it turns out, we did a 10-year study of this called NEAD, Non-shared environment and adolescent development, and we find, yep, sure enough, differential parental treatment correlates with differences

in children's outcomes within a family. So you take siblings, you know, and so the parents who say are more, if you look at the relationship between parental harsh discipline and children's antisocial behavior. In a family, the child who is more antisocial, the parents are more harsh in their discipline. Well, as always, these correlations in psychology have always been assumed to be environmental, but I think all your listeners know the adage that correlation does not imply causation. Is it necessarily the case that the parent's discipline of the child caused the antisocial behavior, or is it possible that the parent's behavior is reflecting the children's behavior? And you can put this in a behavioral genetic design, and what you find is

that about half of those correlations are due to genetic differences. So this is where the nature of nurture comes in. And it kind of took me off the track, though, of non-shared environment. And the punchline there is after 30 years of trying to find these systematic sources, we haven't been successful. I know in one of your conversations with Paul Bloom, you mentioned Judith Harris's book in the 90s, which really popularized a lot of these concepts, but really results, but then also proposed maybe peers are important. And that's another reasonable hypothesis. You know, your daughters probably won't end up having the same friends. Maybe one of them has more academically oriented friends and the other has more athletically oriented friends. That could be a source of difference. But since Judith Richeris

proposed that, people have also looked at that. And again, there's correlations there. Kids who are more antisocial have friends who are more antisocial, you know, in a family. So the sibling who is more antisocial is more likely to have friends who are also more delinquent. But again, is it cause or effect? And it turns out that, you know, kids select friends. If they're antisocial, they select friends who are like them in that score. And about half of that is due to genetic differences. So that's what we mean by the nature of nurture. But it's also why we haven't found systematic sources of non-shared environment. Whenever we find something that looks like it's causing differences between kids and

a family, it ends up being a genetic difference in disguise. So after 30 years, I came to this what we call gloomy conclusion that non-shared environment is essentially idiosyncratic, stochastic, not systematic, so that half of the variance for psychological traits are due to these environmental factors, but they're essentially random chance. Stuff happens. Okay, so this is all, I think, more important than may be obvious to people at first pass here. So I just want to linger on this topic. So first, what you're saying here is that virtually half of everything we care about in human nature, in our psychology, whether it's susceptibility to various psychopathology, and we'll talk about how we think about disorders and whether the disorder framework is the right framework here.

But for virtually everything in psychology and in human difference, one could care about, from intelligence to big five personality traits to susceptibility to things like depression and schizophrenia. The punchline here is something like 50% of human difference. It's often on either side of that halfway mark. I mean, sometimes it's 60%, sometimes it's as high as 80% later in life for things, is accounted for by genes. And the other half is environment, but it is not the environment that parents or anyone else can systematically control. And for the environmental component of things, very often half of what is ascribed to the environment is actually genes in disguise because people, based on their own genetic proclivities,

wind up shaping their environment. So I think this is an example you use in your book. You could ask someone, how often does it rain where you live? If ever there were an environmental someone, you know, how often does it rain where you live? If ever there were an environmental variable that has nothing to do with DNA, well, you know, the weather is certainly that. But then you ask yourself, well, you know, people are free to move, right? People can pick the climates in which they live, and maybe some of that is being driven by genetic proclivity, right? There's some people who just hate living where it rains, right? I count myself as one of those people. It's not an accident that I don't live in Seattle. And so it is with everything else. How much TV do you watch as a kid? How often do you read? How often do your

parents read to you? This all seems like it's a pure statement of an environmental influence, i.e. nurture, and yet when you strip out the influence of genes, you find that genes are accounting for half of those so-called environmental differences among people. I should just pause there, Robert, to ask, did I summarize that point correctly? Yes, I thought that was great. And the point for people to take home is correlation does not imply causation. So parents who read a lot to their kids have kids who do better at reading at school. And if you don't think about these issues, you might say, sure, it's environmental.

But I hope after this discussion, people at least pause a minute and say, well, wait a minute now, you know, who are these parents who read a lot to their kids? And who are these kids who do better at reading? It could be due to genetics, or increasingly, I think it's due to parents responding to genetic differences in their kids. I have six grandchildren, and I thought, with the first two, I thought, you know, what they're supposed to do is sit there and let you read to them. I remember you talking about reading Harry Potter to your older daughter. Well, that's what I thought grandchildren were supposed to do. And with one grandchild, exactly right, I could read to her all day long and she'd say, oh, please read some more to me. But I've got another grandson who it would almost be abusive for me to make him sit there and let me read to him. He wants to rough

and tumble play. So increasingly, I think as parents, we're responding to differences we see in our kids. And given that you have two kids, I wonder if you experienced that, this wonderful phrase that's been attributed to six different people. Parents are environmentalists until they have more than one child. With the first child, you can explain anything environmentally. That's the problem with environmental hypotheses. You can't explain anything after the fact. But then you have a second child, and almost every parent notices that there's big differences between these children. And you say, I didn't do that. Have you experienced that? Well, there's also, there's one enormous environmental difference too, which is the second child is growing up in the presence of the first,

whereas the first had, in our case, five long years of being an only child. So it's hard to figure out how to factor that in, but that's a non-negligible influence there. But yeah, I am noticing they're impressively similar in some ways, but they are clearly different people. The genetic deck got shuffled. Yeah, that's exactly right. In case people haven't realized this, you know, the first-degree relatives like parents and offspring or siblings are 50% similar genetically. That means they're 50% different genetically. So genetics predicts that kids in a family will be different. These socialization environmental theories, they have a lot of trouble explaining why are two kids in the same family

with the same parents so different, when presumably it's these parents that are causing differences in the kids' development. Yeah, So again, there are implications for parenting here and social policy, education. We'll get to those in the back half of our conversation. But all of this, again, is a bit of a high wire act to talk about these things without having people freak out. But we are really just talking about the facts of human psychology insofar as we have come to know them. And no doubt we will be wrong about certain things, certain assumptions will be proven wrong in the fullness of time, but the idea that genetics doesn't account for a lot of what we care about in human nature, the door seems to be closed

to that thesis. I mean, the blank slate thesis is no longer on the table. And, you know, it's empowering in some ways, it pushes your intuitions around in others, and we'll talk about those effects. But we should talk a little bit more about how we know this. Before we get there, could I just kind of summarize what we were saying? Because we covered a lot of topics and a lot of those are very big issues for people, you know. Yeah, go for it. And so, what Blueprint is saying is there's three main points. First is, everything's heritable, so inherited DNA differences account for a lot of the differences.

Of the rest of the variants, it's not genetic, it's environmental, but it's not the environment we thought was important, it's this non-shared environment. And then when we find correlations, like between parents reading to kids and kids' reading ability at school, you can't assume that's environmental. They're often genetic effects in disguise. So I find what helps people put this together is if I tell you that if one of your daughters had been switched at birth in the maternity ward and raised in a different family, she would have grown up to be very similar to who she is, even though she was raised in a different family. And that's not hypothetical because we have studies of identical twins reared apart. And this wonderful documentary that won an award last year called Three Identical Strangers,

about three identical twins and just how similar they are despite being raised in quite different family environments. So it's a dramatic illustration of this point because your daughter would be her identical twin. She's still 100% genetically who she is, even though she's raised in a different family. So I think that helps people to understand it, that we'd be very much who we are, even if we had been raised in a different family with different parents. Right. Okay. Although we're going to have to land back on this topic and give some account of why being a good parent still matters. So we'll have to get there. So, okay. But before we do, let's talk a little bit more about these studies, adoption

studies, twin studies, adoption studies with twins. And we have, so let's just remind people of the biology here. We have two different types of twins. There are monozygotic twins and dizygotic twins, identical and fraternal twins. And identical twins share the same DNA, they're a hundred percent identical, barring some surprising mutation genetically. And whereas fraternal twins are like ordinary siblings, they share 50% of their DNA, but they just share the same environment all the way down to the womb. So in these studies where you can compare identical twins to fraternal twins, and you can really strip out the influence of shared environment, because again, you're looking at one group that has identical DNA and one group that has only 50%

similar DNA, and yet shared environment. And then you have these other studies where you have identical twins separated at birth and raised in different families, and you are a pioneer in doing this work. And maybe, I don't know if you want to talk about the Colorado Adoption Project, or I don't know how you want to enter this, but let's talk a little bit more about the logic of these experiments and why they have been so compelling. Well, that was a great description of the twin method. But the punchline there is that if a trait, take like musical ability, which hasn't been studied very much, and it's hard to measure,

but what we'd be saying is if genetic influences are important, you'd have to predict that identical twins would be more similar in their musical ability than non-identical twins. And the extent to which they're more similar than fraternal twins. I say non-identical because in UK, they call them non-identical rather than fraternal. So if a trait like musical ability is heritable, you'd have to predict that the MZ, monozygotic twins, are more similar than the dizygotic fraternal twin. And the extent to which identical twins are more similar estimates the magnitude of genetic influence. So, as you said, that's a pretty powerful test of genetic influence, but the main assumption there is called the equal environments assumption.

What if identical twins are treated more similarly than non-identical twins? Well, that's been studied. It seems to be a fairly safe assumption. You get identical twins reared apart are just as similar as identical twins reared together, for example. But it still is an issue. But the neat thing is we have this other method that's completely different called the adoption method. And that's a wonderful situation to be in because the adoption method also has its possible, well, it has its assumptions and possible flaws, but they're completely different. And these two methods, the twin

method and the adoption method, converge on this conclusion that everything that we study in psychology is heritable. So the adoption method, though, is in some ways more powerful. You can really see it with identical twins reared apart, but they're very rare. So much more typical are biological parents who adopt their child away at birth, and then you can study those adopted children and their adoptive parents who give them their family environment, but not their genes. They're not genetically similar to them. So it's another powerful way of getting at genetic environmental influences. And I gave you the example of body mass index and how adopted children don't correlate with their adoptive parents in terms of body mass index, even though they share

food and lifestyle. Whereas parents who share genes and environment with their children correlate about 0.3 for body mass index. And the real killer data is that these adopted children correlate 0.3 for body mass index with their birth parents, who they never saw after the first week of life. So I think together, that's a very powerful indication, not only of genetic influence, but of the unimportance of what we call shared environment, you know, that traditional view of nurture. Right. We should just pause to acknowledge how counterintuitive this is. We're talking about parents who have their own eating

habits, which they then lavish upon their children from birth onward. And it turns out those habits stripped of their underlying genetic cause is not what contributes to the body mass index of a child as he or she grows up. Yes, exactly right. Yeah. So now what about epigenetics here? Is there anything to say about what we know there? Yeah. When I give a public lecture, it's sort of the first question I get is, yeah, but what about epigenetics? You know, the environment changes genes. Well, you know, as I say in the book, you only inherit DNA, differences in DNA sequence. You start life as a single cell with half, three billion base pairs of DNA from your mother

and three billion from your father. And that DNA is the same DNA in the trillions of cells in your body. We do pick up some mutations as we go along. But the genes that are expressed of those six billion, The genes that are expressed of those 6 billion, say, 3 billion DNA difference, nucleotide bases of DNA and the double helix of DNA, we don't have the same DNA expressed in all of our cells. You know, the cells in your liver do different things from the cells in your blood and from the cells in your brain. And that's gene expression. Different bits of DNA are turned on and off in response to the environment. But what we inherit are the DNA differences. And if a DNA difference correlates with an outcome

like schizophrenia or alcoholism or reading disability, then that means that that DNA difference was expressed somewhere, and it's making a difference. But some, you know, people have really used epigenetics, which literally means above genetics, beyond genetics, to try and argue against Mendelian genetics. And I think there's, after the initial excitement about epigenetics, I think people are calming down about it and realizing, yeah, gene expression is important. Everything between inherited DNA and behavior is important. We call that expression transcriptomics and tabelomics and the brain. Everything in between DNA and behavior is important to understand, but it's important to realize all we inherit are DNA sequence differences. Right, and if they're making a difference in terms of traits, and if they're

correlating with differences, individual differences in traits, well then they're being expressed on some level. Yes, that's right. And the neat thing about DNA is you don't need to know anything about what goes on in between the DNA and the behavior to be able to make these predictions. But that's not to say all of these other things are not important, but I'd like to argue against this idea that epigenetics somehow invalidates genetics because it doesn't. Right. Yeah, there's another detail here which is interesting and has important implications, and it's that we're not tending to talk about single genes having some overwhelming trait effect. We're talking about

thousands of genes contributing tiny effects to any one of these traits, whether it's a susceptibility to schizophrenia or intelligence or anything else that interests us. And that has some significance. Well, you tell me what significance you see. I mean, the one thing that jumped out for me immediately, which I believe you mention in your book, is that it gives a somewhat less than hopeful picture that any single drug target will be a high leverage target for us in improving ourselves in whatever way we might hope to. Yeah. Well, the most important thing we've learned from the DNA revolution in the last 10, really, five years, is that genetic influence on complex traits and common disorders of the sort

we've been talking about are not due to one gene, certainly, we've known that for a long time, but they're not due to 10 genes or 100 genes, they're probably due to thousands of tiny, tiny DNA differences. Now, first, I'd like to say, though, that there are thousands, some people say 7,000, 10,000, single gene disorders. These are like Mendelian hardwired deterministic disorders, like Huntington's. They're necessary and sufficient. So, if you have the gene for Huntington's, you will die from Huntington's unless something kills you first. And you only have Huntington's if you have the gene for Huntington's disease. And that's the problem. Everyone learns about genetics from Mendel. And

Mendel was studying disorders in pea plants, like wrinkled seeds, you know? And so they're hardwired and deterministic. And he showed through that that that's the way genes work in heredity. But what's important to realize is that despite these thousands of single gene disorders, many of which are extremely debilitating, lethal for the people who have them, they're very rare. One in 100,000, one in 500,000. So, they're very rare, fortunately, and don't really contribute much to the heritability of the traits that we study. The heritability of complex traits and common disorders, medical as well as psychological, are due to thousands of tiny DNA differences.

And that's a drag in some ways. If you're trying to do a bottom-up approach, as neuroscientists, you know, would want to do, where you go from genes to brain to behavior, it's going to be very hard if each of those DNA effects are so tiny. you know, you're going to definitely have to get away from a modular approach to neuroscience where you think, you know, this gene does this, and then that has that effect. You know, it's going to take more like a systems network sort of approach to be able to deal with the brain from this perspective, which we talk about as polygenic, that is, every trait is influenced by many, many genes, and that would include traits in the brain, you know, neurotransmitter levels, whatever.

But the other word that's important is pleiotropy. Every DNA difference has many, many effects. So, you know, you name these genes based on a disorder you know like this gene caused diabetes but then you find out that gene affects hundreds of other things so this polygenic point that you're making is critically important and it's really hard for people to understand because they're still thinking about genetics from a single-gene, hardwired, deterministic perspective. Yeah, so there's a lot there. So let's break apart a few of these concepts. So yeah, there's an analog point to make about the brain. There are very few parts of the brain that only do one thing, where you can say this is the part of the brain that, you know, recognizes

faces, right, and this, it does nothing else. Well, even fusiform cortex does other things. So the real picture is of pleiotropy, where any one gene, in this case, contributes to many traits. And also, and this is a point you make in the book, our concept of disorders like schizophrenia is itself misleading, and it is a point you make in the book, our concept of disorders like schizophrenia is itself misleading, and it makes sense to talk more in terms of dimensions for traits as opposed to these kind of terminal disorders. And you use an analogy which really drives home the point with height, and maybe you want to talk about height and the imaginary problem of

giantism to clarify this concept. Okay, it is just a hypothetical example, but it does make the point that suppose you decided you've got a new disorder here, giantism. So people over six feet five inches, they're giants, and everybody else is normal in height. And yet you find that all the genes, there's been thousands that have been identified, you know, thousands of DNA differences. They all work. They don't, there aren't like a separate set of genes that cause people to be giants and different from the rest of people.

All of this is quantitative. It's a matter of more or less. That is any DNA difference that is more prevalent in the giants, it will be distributed in the distribution. So people who are higher than average are more likely to have that DNA difference. So the DNA research, I think, puts the nail in the coffin of diagnoses. Now, you might say, well, that's just a stupid example. I mean, why would anyone divide height, which is so normally distributed, into a dichotomy?

But I think that's what we're doing with most other disorders. You know, depression, no one thinks depression, you wake up one day and you're depressed. Depressive symptoms are almost, they're quantitatively distributed. And you never find genes for a disorder. Any gene you find is distributed through the population. Like more concretely, one of the first of these effects that were identified using these new approaches called genome-wide association was a DNA difference that was associated with body mass index. So this gene had an A and a T, the four nucleotide bases of DNA, A, C, T's and G's.

And in the old, in thousands of years ago, we were all TT. But then someone got a mutation that was an A. And that A seemed to have been adaptive. The story used to be that it allows you to conserve fat. And in the Stone Age, that would be a good thing because you never knew when your next meal was coming. But now that makes you more likely to become obese in a fast food nation. So if you have two A's, you're three pounds heavier than someone who has one A. And if you have no A's,

well, that one A makes you three pounds heavier than someone with no 1A. And if you have no A's, well, that 1A makes you three pounds heavier than someone with no A's, TT. So there's a six-pound difference between TT and AA. That's what we mean by an association. So that was found for obesity initially. But then they found that that DNA difference works quantitatively throughout the distribution. That is, if you and your sibling, you have an A and they don't, you're likely to be, if we get a lot of siblings like that, three pounds heavier on average. But that only accounts for 1% of the variance of body mass index. And when that was published in Science in 2007, people, well, 1%, I mean, what's that? Turns out it's one of the biggest effects that we can find for

complex disorders, complex traits and common disorders. So it's so important to realize that these polygenic scores, that is, you can put these thousands of DNA differences together, because any one of them just doesn't account for enough variance to predict or to try and understand it mechanistically. But you can put them all together, aggregate them in a polygenic score, and make pretty substantial predictions. Like we can predict 25% of the variance in height and about 10% of the variance in weight by putting all of these together. So for weight, this one DNA difference I was talking about accounts for 1%, but then these other DNA differences account for 9%. So altogether, you can predict about 10% of the variance. But these polygenic scores are all necessarily perfectly normally distributed, because it's

the central limit theorem of statistics. You know, you flip a coin, and you flip 100 coins, and you get this normal distribution of heads and tails. And that's what you're doing. You're flipping alleles. You know, you either have one allele or the other allele or two of them. So these polygenic scores are perfectly normally distributed so that the genetic liability for everything, any disorder, autism, schizophrenia, coronary heart disease, it's perfectly normally distributed. So I think that is really, I think, ought to put the nail in the coffin of diagnoses. Because I really believe in psychiatry and psychology, these diagnoses have held us back tremendously. And all of the DNA studies, these genome-wide association

studies, are case control studies. So the whole game is to find these people who meet these, what I think, are arbitrary diagnostic criteria, and you call them cases like schizophrenics, and everybody else is a control. And that's really held us back because it's just simply not true. Yeah, because everybody else who's normal, so-called normal, could be just like the six-foot-three person who's not classed as a giant, but still shares all of these increased height probability genes. And it means that we all have thousands of genes for schizophrenia. Right. It's just quantitative. And I think, you know, if you have a very high, we call it polygenic score for schizophrenia,

it probably takes, we all have stresses that would freak us out. And as you've mentioned several times in your podcast, if you did have a genetic propensity towards schizophrenia, you probably ought to be careful about some of the psychedelics, for example. Or some of the evidence suggests the high THC sort of marijuana could also be dangerous in that situation. It's like alcoholism, you know, if you have the genetic propensity, it doesn't mean you're going to become alcoholic or you're necessarily going to become schizophrenic, you're just more likely to be. And given the stresses and strains of life, you're more likely to be tipped over the edge than someone else. Yeah. And these genes are very likely contributing to who you are in noticeable ways that put you on the spectrum, which has schizophrenia as its terminus. So these genes for height that would render a giant 6'5 or beyond are also operative in you at the height of 5'10.

It's just you have a different complement of, and we're now talking about many, many genes for any one of these traits. Exactly right. The picture is, I think the phrase you use is that the abnormal is normal. We're all on every spectrum that we could posit exists in the population. We're all somewhere on it, and whether we have a symptomology that's interesting or not is the only difference, right? So it gives us a finer-grained way of thinking about human human difference and the boundaries between what is considered normative or normal and pathological. I mean, it's common to, I think you referenced this in your book, ever since Aristotle, the analogy between madness and genius has been drawn, and I think probably too much has been made of that. But this is susceptible to, in the end,

a genetic analysis. I mean, we can look at the genotype of whatever we want to call genius and the genotype of whatever we want to call madness and just see how much genetic real estate they share. That's really right. One implication I find quite interesting, too, is I'm basically saying there are no disorders, they're just quantitative dimensions. And one implication of that then is if there's no disorder, there's nothing to cure. It's not like you're cured, yes or no. It's all quantitative. It's a matter of more or less. We're alleviating symptoms rather than curing a disorder. It all has to do with psychology aping medical sciences where, you know, a lot of this does work if you have a simple cause, a simple environmental cause.

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