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Who's in the Video
Dr. Kathryn Paige Harden is a tenured professor in the Department of Psychology at UT, where she leads the Developmental Behavior Genetics lab and co-directs the Texas Twin Project. She[…]
In Partnership With
John Templeton Foundation

The relationship between genetics and the environment is incredibly complex.

The term “heritability” refers to the proportion of variation in a population that is due to genetic differences. But people misinterpret the word to mean “genetically determined,” “unmodifiable,” or “innate” — but it doesn’t mean any of these things.

Just because a version of a gene might be linked to a particular outcome in one group of people does not mean it is linked to the same outcome in another group of people.

PAIGE HARDEN: There's this classic thought experiment about two gardens. So imagine one garden and it has the perfect amount of sunlight, and then you plant different flowers in that garden. And you can look and say, "Well, the differences between them in these plants' heights is about the seeds that I planted. They all have the perfect environmental conditions. The differences between them are genetic." Now imagine another garden, and it is neglected. If you looked at that and you said, "All those plants are stunted and dead," and then you said, "Ah, the difference between these two gardens is that these plants in the neglected garden are genetically shorter than these plants in the rich, well-resourced garden." 

We can't then say that all the differences between tall and short plants across very different environments are in fact due to genetics. That's a really obvious thing when it comes to plants, but it's a really difficult thing for people to keep in mind when we're talking about individual versus group differences. This relationship between heritability and response to the environment is a complicated one. 'Heritability' is the proportion of variation in a population that's due to genetic differences between people. It's a confusing term because people misinterpret heritable to mean 'genetically determined, unmodifiable by the environment, or innate.' And it doesn't mean any of those things. But it's hard for us scientists to describe why. 

The thing about factoring in environment in genetic studies is that we already face an enormous, statistical power problem. Do you have enough people? Do you have enough observations? Do you have enough data to be able to reliably say that what you're picking up on is real and not just random chance? Genetic studies that ignore the environment and are just looking for the "main effects" of genes are already requiring sample sizes in excess of one million people. So when you start to factor in gene-by-environment interaction, the power problem gets even bigger. Statistics is hard, biology is hard. 

A lot of times people haven't had exposure to this since high school, if at all. In the larger conversation, we're trying to make sense of information about race. We're trying to make sense of information about genes, about education, about inequality. I think it's really understandable that people have a hard time thinking about how all these things go together. 'Portability' is this idea that what we've discovered in one sample or one group of people, how much can we port that over to a new group of people, and trust that things are gonna work in the same way? So we see this all the time with our social science effects. 

A minimum wage change in one state doesn't have the same effect as a minimum wage change in another state. What I think people often miss is that genetic causes can operate in the same way. So, just because a gene influences the probability of an outcome in this group of people, to the extent that that's probabilistic rather than deterministic, to the extent that that genetic effect is moderated by the environment, we're not gonna be able to expect it to be portable when we study a new group of people. 

When I say that something's heritable, I mean that within a group of people that have experienced roughly-equivalent environmental conditions, genetic differences between them are related to the phenotypic differences between them. The genes of the seeds are related to the height of the flowers. But that doesn't allow us to make the leap that differences between groups that have experienced really different environmental legacies are due to genetics.