In this video I provide a basic introduction to twin studies and heritability scores. I explain how a heritability score tells us about the role of genes in explaining variance in a trait. I also discuss some common errors and emphasize that heritability is about the role of genes in general, not about specific genes, and that it only tells us about groups of people, not individuals. I also discuss some hypothetical examples that illustrate how differences in environments can influence heritability scores.
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Video Transcript:
Hi, I’m Michael Corayer and this is Psych Exam Review. In this video I’m going to be talking about heritability. So this falls under an area known as behavioral genetics. So behavioral genetics is an attempt to connect or find the relationship between genes and particular traits and behaviors.
One way that this is done is through twin studies. Researchers will compare sets of monozygotic and dizygotic twins for particular traits or behaviors and see whether the more genes they share influences the similarity in the trait or the behavior.
So for example, if I was interested in studying IQ, I might want to see, do monozygotic twins, who share 100% of their genes, do they have more similar IQs than dizygotic twins, who only share 50% of their genes. If the monozygotic twins are more similar, then that would suggest that genes are having an influence on the similarity of their IQ score.
You might ask, “well if dizygotic twins share 50% of their genes and siblings also share 50% of their genes, why don’t we just compare monozygotic twins to siblings, why do we like comparing them to dizygotic twins?” The answer is that dizygotic twins have more similar environments. So if you have an older brother who’s five years older than you, well things were quite different in your environments or they might have been quite different.
So even though you still share 50% of your genes, maybe your parents learned a few things about parenting by raising him, so you had a different experience. Or maybe you’ve moved to a different city since then, or maybe income level has changed or maybe you go to a different school than your brother went, any number of things in the environment might have changed we want to try to control for that and using dizygotic twins is a way to do that.
It makes the assumption that dizygotic twins who are raised in the same house are going to have more similar environments than siblings raised in the same house. So that’s why they’re preferable to studying just siblings. OK so that’s how twin studies are conducted we compare monozygotic and dizygotic twins for a particular trait and essentially what we’re doing in these behavioral genetics studies is we’re trying to find out why people differ.
So we start with this idea that people differ for some trait, whether it’s intelligence or how outgoing they are, or they differ in that some people develop a mental illness and other people don’t. So people differ. And we want to know why.
Why do people differ?
So the possible answers that we might have, people have different genes, that could be why they differ for a certain trait. We could say well people have different parents too, maybe that plays a role, people go to different schools maybe that’s playing a role. People have different levels of nutrition, there’s a lot of possible answers but we can sort of roughly divide them up into genes and then environmental aspects.
So if we want to know why do people differ, one of the possible answers is going to be genes. So we want to know how much do genes influence the differences in people. And this brings us to studying heritability.
So what is a heritability score?
A heritability score which is usually uses the coefficient h squared, a heritability says people differ, and it tells us the strength of genes in explaining that difference.
What exactly do I mean by that? A heritability score can range from 0 to 1. When a heritability is zero, that says genes have no effect on the difference. In other words it says people differ, people have different IQs, let’s say, but genes don’t matter. That would be if heritability was zero. Genes don’t play any role in explaining why this person scores 110 and this person scores 80, and this person scores 125. Zero would mean genes don’t play a role.
One, on the other hand, for a heritability score would mean genes are the only thing that matters. In other words, if we look at their genes we could explain all of the difference in IQ scores regardless of schools or nutrition or parents or anything like that. Now, as you might guess, these extremes are very unlikely to occur for most traits. Most of the traits we’re interested in are going to involve an interaction of genes and environment. So let’s use the example of IQ.
The heritability score for IQ is about 0.5. It actually varies in different populations. One sort of strange thing is that as people get older, if we look at populations of elderly people, the heritability score is higher, it’s about 0.8. You can think about why that might be and I’ll come back to a possible explanation in a few minutes.
So what does it mean to say that heritability score for IQ is about 0.5. It means that when we ask “why do people have different IQs?” about half of the explanation is “people have different genes” and the other half of the explanation is “people have different environments”. People go to different schools, they have different parents, they have different levels of nutrition and both of these factors matter and in this case, they’re about even.
Genes matter about as much as environment matters. OK so some things to keep in mind about this. The first thing is that the heritability score only tells us the relative strength. It tells us that genes are about half of the explanation for why people differ. But it doesn’t tell us which genes. And it doesn’t tell us which aspects of the environment. So when we say heritability is 0.5 that’s not telling us which genes are responsible, whether it’s one gene or 1000 genes that are responsible for that. Similarly it doesn’t tell us if parents matter more than schools, do schools matter more than nutrition, what are the important environmental aspects. This doesn’t tell us. We can’t get that from the heritability score.
So that’s the first thing to keep in mind. The second thing to keep in mind is that heritability scores are always about groups. We’re asking the question why people differ. People, multiple, it has to be about groups of people.
Heritability scores are not about individuals. It doesn’t tell you anything about your individual score. You don’t say, “well I scored 100 on an IQ test so 50 points came from my genes”. It’s not saying that at all. It’s saying you scored 100, somebody else scored 110, and about half of the reason for your differences is genes, you have different genes than that person. And the other half is environment. And of course, this for many people, large groups of people not just you and one other person.
OK so those are two key things to keep in mind for heritbility. It’s relative strength and it’s about groups not individual scores.
So, a good way to think about this is to imagine two extreme situations, these are hypothetical. These aren’t really going to happen. But hopefully they’ll clear up this idea of what a heritability score is.
So one hypothetical is we go to some town and everybody there is a clone. So this is the clone village here. So that means that everybody has 100% of the same genes. They share 100% of the genes in this clone town but let’s imagine they have different environments. Very different environments, some are rich, some are poor, some go to school, some don’t, some have libraries full of books they can study, others don’t have access to any materials, some have parents who treat them really well, some have parents who treat them really poorly, they have very different environments. But they share all their genes.
What’s our heritability score going to be in this situation? In this case, when we ask, “why did they get different IQ scores?” how much do genes matter? Genes don’t matter at all because everybody has the same genes. Genes aren’t going to add to our explanation in any way. So in that case the heritability score is going to be zero.
Let’s imagine another town, where we have identical environments. Now this is impossible but we can imagine it. Everybody’s raised exactly the same way, they have exactly the same teachers, exactly the same nutrition, but they have different genes.
What’s going to happen to our heritability score in this case? In this case, when we say “why do people differ?” the only thing that differs about them is their genes, so it must be that their differences in IQ can be completely explained by the fact that they have different genes.
So in this case we can see that actually the more similar environments become, the higher the heritability score. So this is kind of counter-intuitive: that as environments become more similar, genes matter more. Genes explain the variance more. They tell us why people differ. If environments are the same, then the only reason why people would differ would be genes. Similarly, if genes are the same, the only reason why people would differ would be environment.
Now this gets back to that question that I posed earlier which is “why do elderly populations have higher heritability scores?”. Well what you’re doing is when you look at elderly populations you’re comparing people who have more similar environments. Well how is that happening? Well what you’re doing is comparing only people who have survived that long. And that cuts out a lot of the really extreme environments people might have been in.
So the people who were really malnourished, the people who were exposed to lots of disease and illness, those people aren’t around anymore so those extremes of environment aren’t being included in your study. So what you’re doing is you’re comparing more similar environments, and so you shouldn’t be surprised that your heritability score goes up. Just like it would in our hypothetical example.
OK so I hope you found this helpful in understanding heritability, if you did, please like the video and subscribe to the channel for more.
Thanks for watching!
