In this video I address the question of what causes IQ differences between people. This relates to the nature/nurture debate in considering the strength of genes and environment and these interact to shape intelligence. Twin and adoption studies allow us to assess these relationships and estimate the heritability of IQ. While heritability of IQ is generally estimated to be around .5, the Wilson Effect refers to the fact that heritability scores for IQ tend to increase throughout development. This snowballing effect of genes may relate to few extreme environments being included in most IQ research, the increasing freedom of people to seek out their own environments with age, and the possibility that different genes change their influence throughout development.
The Wilson Effect: https://www.cambridge.org/core/servic…
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Video Transcript
Hi, I’m Michael Corayer and this is Psych Exam Review. In this video we’re going to try to address the question of what causes IQ differences. So in the previous video we talked about IQ variation and we saw that IQ varies, right? People have different IQ scores. We saw the distribution of all of those scores across the population. So now we might ask what’s causing those differences? How much of this is due to the fact that people have different genes? And how much is due to the fact that people have different environments, right?
This is the sort of nature versus nurture debate that I’ve talked about previously. And of course you probably already know that the answer is going to be that it’s an interaction of genes and environment. We’re not going to have cases of, well, people’s differences in IQ are solely to the genes or solely due to environment but rather that genes and environment interact in order to influence a person’s intellectual development. Ok, so how can we go about attempting to answer this question of how much do genes matter or how much does environment matter?
Well, what we can do is look at twin and family studies, right? So we can look at people who share different amounts of genes and how much that seems to influence their intelligence and people who share or don’t share environments. Now one of the problems you might have if you just look at parents and children is that often if the parents are raising their own children then not only do they share genes with the children but they also create the environment that the child is living in. And so that kind of muddies the waters a bit in terms of saying, well how much of this is due to genes and how much is in fact the parents are creating the environment?
So one thing we might look at is adopted children. So that sort of gets around this problem because we can look at, okay, we have children and we have their adoptive parents; how similar are their IQ? So the parents are creating the environment but they don’t share genes with the children. So how much does that, how similar are their IQs? And in the case of children and adoptive parents we get a correlation of about 0.19. Alright, so that shows that there’s some similarity in their IQ scores but it’s not particularly strong. It’s a fairly weak correlation.
Now we can also compare that to the children and their biological parents. So these adopted children, how similar are their IQs to their actual biological parents? And in this case we see that the correlation is 0.24. Now this suggests that genes are playing a role here, right? If you’re an adopted child your IQ is going to be more similar to your biological parents than it is going to be to your adoptive parents who are raising you. Now you might say well the parents are creating the environment but we really want to know about the environment that the child is actually living in.
So this might want us, might lead us to want to compare siblings. So we can compare adoptive versus biological siblings. So that’s the next thing we’ll look at here. I’ll say ok, we have adopted siblings. so they’re not genetically related, right, but they’re living in the same home environment. How similar are their IQs? In this case we get a correlation of about 0.25 so their IQs have some similarity even though they’re not genetically related but they’re being raised in the same home.
Then we could ask okay how about biological siblings who are raised in different homes? So they’re adopted separately, one child goes to one home the other child goes to another home, but then we test each of their IQs and we ask how similar are their IQs? And in this case we get about 0.24 so it’s fairly similar to siblings who are adopted in terms of the correlation of their IQ scores. So that would suggest that, again, genes are playing a role. These children are related to one another genetically but they’re being raised in different houses and yet we still have some correlation in their IQ score.
Ok, but the most interesting examples to look at are with twins because when we look at these sibling environments when I say well, they might not be exactly the same age and things can change in the home environment over time, and I mean just having older or younger siblings, I mean, that’s maybe going to be playing a role in the child’s development, you know?
So we want to look at dizygotic twins because these are siblings, they share 50% of their genes, but they’re being raised in a more similar environment because they’re the same age in that environment at the same time. So, you know, if you have an older brother who’s five years older than you and the way that he was raised by your parents and the way you were raised, even though it’s the same home, might have changed over those five years. Whereas with dizygotic twins they’re being raised the exact same time and they’re the same age. So we can look at dizygotic twins who are raised in in separate homes versus dizygotic twins who are raised together. And when dizygotic twins are raised apart we have a correlation of about 0.5 so this is more similar. And part of this might be when we think about environment, it’s also important to remember that part of environment includes the womb.
So these dizygotic twins have a more similar prenatal environment. I mean they have essentially an identical prenatal environment. That’s not exactly true, but it’s close enough and so that might explain why it is that they have this higher correlation than just normal siblings do, even though normal siblings also share 50% of their genes. Ok, then we can look at dizygotic twins who are raised in the same house. So they’re being raised together and in this case we have an even stronger correlation about 0.6. Alright so that would be suggesting that, well, the environment is making their IQs even more similar. That would be sort of supporting environmental influence.
Ok, but the most interesting thing to look at is the monozygotic twins. So these are twins that have 100% shared genes. They result from a single zygote like mono zygotic, a single zygote, so one fertilized egg that splits and becomes two people. So their genes are identical. So these are identical twins and what we see here is that when monozygotic twins are separated and raised in different houses, which is fairly rare we don’t have a lot of cases of this to study, but these studies have been done with, you know, as many separated monozygotic twins as they can find, and in this case we have a correlation in IQ of 0.78. Quite strong relationship here, right? Their IQs are very similar even though they’re being raised in different houses and so that would suggest a much stronger influence of genes.
And then we have monozygotic twins who are raised in the same house and in this case we get a correlation of 0.86. Now this is really high, now you don’t, I wouldn’t expect you to memorize all of these numbers here, but to get an idea of how things, you know, are more or less related but this is worth remembering right here this idea that monozygotic twins have a correlation in their IQ scores of 0.86. This is very high and the reason this is really worth remembering goes back to the idea of test retest-reliability that I talked about in a previous video. Because you might wonder well what’s the test retest reliability for IQ tests? If a person takes an IQ test and then they take an IQ test again, you know, six months later or something how similar are their scores, right? And what’s the correlation between those two scores? And it turns out for IQ tests the correlation between those two scores ranges from about 0.8 to 0.9 and this is actually very high. IQ tests are quite reliable compared to any other psychological trait assessments.
This is very good reliability for IQ tests and what we see here is that monozygotic twins raised together are almost like the same person taking an IQ test twice. So if you were a monozygotic twin and you were going to take an IQ test again you could have your identical twin take the test for you and get probably about the same score that you would get, right? So this is strong evidence that genes are playing a role in the development of intelligence.
Ok, so this brings us to calculating heritability. So I’ve made a previous video explaining heritability, so you might want to review that if you’re struggling with this concept, but the basic idea of heritability is it tells us how much of the variance of a particular trait is due to the fact that people have different genes, right? So what’s the extent of genetic influence on the variance in the trait, right? That’s an important point. It’s about why people differ. It’s not about specific scores, okay. So I mentioned in the previous video that the heritability score for intelligence is generally estimated to be somewhere around 0.5 and this means that when we ask the question “why do people have different IQ scores?” the explanation, about half of the explanation, is they have different genes. And the other half of the explanation is they have different environments, right? So the important point is to remember that this is about populations not individuals. It’s asking why do all people differ in terms of IQ, not why did this person score 110 or something like that. So it’s about populations not individuals. That’s important to keep in mind and it’s also important to keep in mind that the heritability score can vary, right?
It can vary in different environments. So I gave this example in the previous video where I suggested that you imagine a clone society, right? A clone colony or something where we have everybody has 100% the same genes; they’re all clones. And we would ask well why do they differ for something? And the answer would always have to be, well they only differ because of environment. Heritability in that case would be zero because genes don’t explain anything because everybody’s genes are the same. On the other hand, if we imagined a place where everybody had different genes but they all had exactly the same environment which, you know, is impossible to do, but we could imagine that they had an identical environment and in that case when we ask well why do they get different IQ scores the answer would have to be, well, because they have different genes. Because they don’t have different environments, so in that case the heritability would be 1.0 right? So remember this score ranges from zero to 1.0 so those are two extreme populations that we could imagine, right? They don’t really exist but it’s to emphasize this point that heritability can vary and it can change over time as environments change, become more or less similar, then that is going to influence the heritability scores.
So this brings us to what’s called the Wilson Effect, and this is something that somebody actually mentioned in the comments on my previous video on heritability, and what the Wilson effect is, is the idea that heritability changes for IQ over time. So that estimate of about 0.5 is sort of an overall estimate of looking at all these different studies of similarities in IQ, but if we look at different ages, we look at people throughout development we see that the heritability score actually changes over time. So you can think about, which way you might think that would happen but the idea of the Wilson effect is that heritability for IQ changes throughout development and this is named after Ronald Wilson who did some research suggesting this in the 1970s. But it was, the name the Wilson effect was proposed by Thomas Bouchard and I’ll link to the paper in the video description where you can read about the Wilson effect. And Thomas Bouchard is actually one of the researchers who conducted some of these studies of monozygotic and dizygotic twins that were raised together or raised apart.
Ok, so you might think, well maybe you know environments get more different as people get older and older, right? They are exposed to more and more new things and so we would think that genes would weaken over time but the Wilson effect is actually the opposite. It shows that heritability increases throughout development, that as people get older their IQ the heritability for IQ gets higher and higher, suggesting that genes actually exert greater influence over time rather than less influence. It’s the opposite of what you might expect, right? It’s a bit counterintuitive. So why might this be? What, what could be going on? How is it that heritability could increase?
So when I say it changes here, the idea is that it’s increasing throughout development. If you look at very young children, right? You calculate a heritability score, it’s going to be below 0.5 if you look at children, which a lot of IQ tests look at children, you’ll get a heritability score of around 0.5 and then if you look at adults you’ll get heritability of around 0.8 that’s the estimate. Genes are the major part of the explanation why do people, why do adults have different IQ? The answer, 80% of the answer is, they have different genes. Alright, so why might this be?
Well, one potential criticism that Bouchard mentions in the paper is the idea that we’re not really looking at a lot of extreme environments. So when we talk about people’s IQ and the people who are doing these assessments were generally not looking at people who are malnourished, people have had, you know, horribly traumatic living environments that they’ve been exposed to, and so we’re kind looking at more moderate environments and so we’re reducing some of the environmental influence because those really drastically negative environments would certainly exert strong influence. If you’re malnourished as a child, right, if you’re, you know, exposed to all sorts of pathogens and disease and things like that, that’s going to influence your intelligence for sure. But generally we’re not looking at people who have experienced that. So that is kind of reducing the extreme environment, so that’s going to make genes seem slightly more important.
But another thing is this, this snowballing effect of genes, part of this might be that people seek out different environments. When you’re younger your environment is sort of imposed upon you, right? Your parents kind of choose what you do with your time and what you’re exposed to and even in school, but as you get older you get more and more freedom to express yourself. And it might be the case that you have a genetic predisposition for certain environments and as you get older you have more and more opportunity to allow that to express itself, right? Your genes are going to influence what environments you seek out and you have more opportunity to seek out those environments the older you are. And by the time you’re an adult you’re sort of in control of which environments you expose yourself to. Whereas when you’re a child, you didn’t have that option. And so that might be part of the explanation.
And another part might be that genes change over time, in terms of the idea that different genes are going to be activated at different times. So when you’re a child and then you go through puberty and you have all sorts of changes and there might be new genes that are turning on and expressing themselves that weren’t being expressed when you were a child. And so it could be the case that those genes are now exerting greater influence on your cognitive ability.
Ok, so that’s the general idea of the Wilson effect and I do want to briefly go back to these correlations that we looked at. And this one here with adopted siblings and this correlation here of 0.25 turns out, if you look at that over time, if you look at as these siblings get older and older this drops to zero, right? There’s essentially, there’s no relationship between adopted siblings in terms of the correlation of their IQ by the time they’re adults. So when they’re children, right, their IQs are more similar. As they get older their IQs diverge more and more until there’s really no correlation between them and that relates directly to this Wilson Effect. This idea that heritability is increasing over time and so all of these explanations that I gave for this Wilson Effect could be explaining this drop in the correlation of adopted sibling IQ.
Ok, I do want to talk about specific gene interactions and it’s important remember when we talked about heritability we’re talking about genes in general, we’re not talking about any specific genes. So a heritability score doesn’t tell us which genes are influencing development. It just says genes matter this much and so in the next video we’ll look at specific genes that are associated with intelligence. I hope you found this helpful, if so, please like the video and subscribe to the channel for more. Thanks for watching!
