What Twins Separated At Birth Teach Us About Human Genetics - Nancy Segal

How would you describe what you work on when somebody asks?

Well, I tell them that I'm a developmental psychologist, but my interests also cover behavior genetics and evolutionary psychology. And I'm passionate about twins because twin studies are a simple and very elegant way of combining all three, developmental psychology, behavior genetics, and evolutionary psychology, different levels of analysis and application. But also, I'm a twin myself, and so I really think that when you're personally invested in something, you have greater, um, interest and passion for the topic. And so my whole career, I liken to a candy store. I've just had a wonderful ride, and the ride is continuing.

That's how I feel when I get to speak to anyone from the EP world or the behavioral genetics world or the sex research world. It's, uh, yeah, it's, it, it's the absolute most fun. Uh, William Costello, the guy that's doing all of this stuff on incels at the moment, famously keeps on reminding me that research is me-search. It's a little bit different for him doing it on incels and you doing it on twins. Given that you're a twin, I don't know what it says about William, given that that's what he's studying. Um, what is the evolutionary psychology angle of, uh, twin research? I c- I can see how behavioral genetics comes in. You know, we're, we're splitting apart genetically and environmentally what the contributing factors are. Uh, but I- I'm less sure about, about the others.

Right. So one of the, you know, very provocative theories that was launched was kinship genetic theory by William Hamilton, who asserted that we feel greater cooperation, altruism, investment in people with whom we share higher degrees of genetic relatedness. We don't consciously do the calculations in our head, but we behave as though we do. And so I've always known for years that identical twins are more cooperative, m- m- closer socially than fraternal twins are on average. And so I felt that this was another level of analysis that I could bring to twin studies because many people say, "Well, they're, they're closer 'cause they look alike or people encourage it," but I- I don't believe that's the case because behavior is not built on appearance. Behavior is built on the brain. And so I always felt that there was some deeper level to this. And so I've been able to use twins, I've been able to use unrelated lookalikes, I've been able to use virtual twins, many, many types of relationships that vary in genetics and in environmental connectedness. And the findings are, are beautiful 'cause they really converge on the same thing. You know, I remember when I was doing my doctoral dissertation, my first major study, and I used seven to 11-year-old children, twins, and what I did was I put them into a puzzle completion situation, a kind of semi-naturalistic study where they all completed the same puzzle under the exact same circumstances with the exact same instructions. And without a word from me, and I kept their mothers out of there, the identical twins just did this orchestrated dance, beautifully coordinated. There was little rough play. Everybody was very cooperative, and they were very successful at puzzle completion. But the fraternal twins, on the other hand, tended to grab different pieces of the puzzle to their side of the table and work on it independently, and the whole thing looked like a mess, and there were lots of elbow jabs and snatching of pieces. And so that really told me that twins can tell us so much just by being themselves.