How does the Zika virus cause microcephaly? Why do some people develop schizophrenia or mental disease while others don’t? Is our sexual orientation hardwired in our genes? As seemingly unrelated as these questions might sound, they can all be addressed using the same scientific tool: twin siblings.
Nancy Segal (Boston, 1951) has been chasing twins for the last 40 years. An evolutionary psychologist and behavioural geneticist, she travels the world in search of fascinating stories that help us understand more about ourselves. Some of these stories seem taken out from a film. For example, the case of Jack Yufe and Oskar Stohr, two identical twins born in 1933. While Jack grew up as a Jewish in Trinidad and became an officer in the Israeli Navy, Oskar grew up as a Catholic in Nazi Germany and was a member of the Hitler Youth. Despite having almost opposite world views, they both shared a number of habits, such as reading books from back to front or flushing toilets before and after using them. Interestingly, they both died of cancer, although with a 20 year difference. Another extraordinary case studied by Segal is that of the mixed-up brothers of Bogotá: two pairs of identical twins that were accidentally exchanged at birth and brought up as fraternal twins instead. While a pair was reared in a rural environment, the other grew up as urban middle-class and had access to better education and higher paid jobs. They found out about the swap by chance in 2013, when they were on their 20s. The case is specially interesting for researchers, as it gives them the possibility of testing multiple types of interactions: same genes, different environment; different genes, same environment; and different genes and different environment.
We recently talked to Segal and discussed some of her current work and future plans.
You’ve been working with twins for almost 40 years now. How has the field evolved during this time?
The field of genetics in twin studies is getting more molecular, people are looking at epigenetics, using twins to understand why genes turn on, why they’re expressed, things of that sort. So I think that the questions that are being brought to ask are different now. But I still think that the same questions that people have asked in the past are still important and that the classic twin method will always be useful, because we have to start at the beginning to know that something is genetically influenced if you’re going to pursuit it in a greater detail.
What are you currently working at?
There is this study that I have in mind that I’m trying to get set up. I’m very interested in religiosity, particularly because I ran into a pair of twins where one is a nun and the other isn’t. I was fascinated with this big difference between these girls, and how they’re very close but there’s now going to be a great separation. So I’m trying to gather other cases of identical twins where one is a nun and one isn’t. I haven’t been successful, but that’s something I’m very keen to do.
I’m also working in a prospective study of twins reared apart. These twins come from China and were separated indirectly because of the one-child policy. And they were adopted in the US, in Canada, and sometimes different countries. I have a pair where one is in Australia and the other is in England. How do the families find them? The parents don’t usually know. They post a picture on a website of parents of adopted Chinese children, and then another mother will see it and say “Oh my God, that looks like my child!”. So that’s how many of them find each other. What’s different about my study is that I track them over time. I have IQ tests and personality tests as they’re developing. The twins do see each other sometimes, but not always. I’ve published one paper on that and I’m going to do some more.
I’m also doing this case study on a family where there are three mothers: one in Australia, one in Canada, one in the US. They didn’t know each other, they all met together at a clinic in California for double conception, where each got two babies implanted that are not related to them, from a donor. But all six eggs came from the same couple. So they are all fraternal twins. The parents have never met, not since the one day they were at the clinic. But the families are going to come together next July in Disneyland, to bring the children. I give the parents questionnaires once a year, so that we can follow the development of the kids. So they’re fraternal twins but in different stomachs and different countries.
Where do you find all these fascinating cases?
I’m always open to many new things and people know that I study twin research, so sometimes I will come across a case and think “Oh my God I have to go there and study it“. But people also bring things to me. When I went to Spain, several years ago, to study switched-at-birth twins, someone had told me about them and in an instant I knew I was going. And then with the Colombian twins, somebody also told me about them, and they were not widely publicized at all. And so, I immediately knew I had to do it. My boyfriend calls me a twin hunter because I love these cases. They’re fascinating at the human interest level, they’re fascinating in terms of the science, and what they tell us about human behaviour and I find it so just irresistible that I’m compelled to go and study them. And sometimes people bring me interesting cases without realizing how interesting they are. I just got a hold of a case in Brazil where there were 24 sets of twins in five generations. I mean, the woman mentioned it and I was so excited about it!
You have recently started a collaborating with Brazilian researchers. What do you expect to find in Brazil?
Brazil has a different culture, they may have different ideas about how to educate twins and how to raise twins. And I think that to replicate existing findings in a different culture will be very fascinating. There has been some twin studies on the Zika virus too. There was recently a story in The New York Times about nine pairs of twins from Brazil, two identical and seven fraternal. Both identical twins were affected by microcephaly. In the fraternal cases, one of the children was affected while the other wasn’t, except in one case where both children were affected. They were very intrigued to find that out. The reporter made a mistake in the text: she said that the identical twins were both affected because they share the placenta but that’s not always the case. In the fraternal case they say they’re not the same because they have different placentas, but there could be a fused placenta. Or maybe the children are both genetically predisposed to be affected. So I think that needs a lot more investigation, but from these cases I think we can learn a lot more about how infective diseases are communicated between twins and why one twin is affected and one might not be.
Where is the field heading? What are the new challenges in twin research?
There’s going to be more work in epigenetics. We need to know more about love styles, about how people fall in love. There was one study done which didn’t show much, but I think we need to study more about that. We also need to know more about how twins make financial investments. Is there a genetic component to that? And we need to know more about political attitudes And more about religiosity. We need to know more about mental disease: about why one twin gets it and one doesn’t. I think twins can help answer many more questions that people have still been asking but we don’t yet have answers to.