by Diana Day
Twins have long fascinated those who study human nature, whether present in medical theories of Hippocrates or in stories as old as the Bible’s Jacob and Esau or Romulus and Remus, the mythological founders of Rome.
Sir Francis Galton, a distant relation of Charles Darwin and a scientist, scholar and author, was a pioneer in the use of twins to study questions of nature versus nurture in the late 1800s. Today there are any number of twin studies and twin registries all over the world. The results of twin research have helped form the basis of our modern understanding of human behavior.
Dr. Laura Baker of the Southern California Twin Project at the University of Southern California is a long-time twin researcher. She recently spoke with BeTwinned.com about twin registries and how twin studies are set up. She also gave us a sneak preview into early results from her current twin study.
BeTwinned.com: Please tell BeTwinned readers a little bit about yourself and what you do at USC.
Dr. Laura Baker: I am an associate professor of psychology at the University of Southern California. I’m a twin researcher, and I study twins as a vehicle for understanding the nature-nurture question of human behavior. Twins are a great resource for us to understand how genes and environment impact our behavior, so it’s great to have so many twins in Los Angeles.
I have been at USC since 1984, when I began doing research on twins. I started a twin register when I first arrived here. The way we started out was with volunteers, just advertisements through newspapers and the radio and through word of mouth, slowly gathering names of twins who were interested in participating in research. And then we gradually moved toward hospitals and school districts, and we recruited twins through those sources as well.
So I now have a fairly large register of twins that’s, for the most part, kids, but we still have an interest in studying adult twins as well.
BeTwinned.com: There’s a difference between doing research about twins to find out about twins themselves and doing research using twins in order to understand humans in general, right?
Baker: That’s correct. I am, of course, fascinated by twins themselves as a group of individuals. There are lots of interesting things about twins, but mainly, people doing twin research in my area — the area of behavioral genetics — are interested in the larger picture of human behavior. Twins give us an insight into how humans function in general. They really help us to have an understanding of the larger scope of human behavior.
BeTwinned.com: Why are twins and multiples uniquely situated to do that?
Baker: Because there are basically two kinds of twins. There are identical twins (monozygotic twins — from one fertilized egg that split) that came from the same fertilized egg so they are essentially genetically identical, and there are fraternal twins (dizygotic twins) that come from two different fertilized eggs, but they happen to share the womb at the same time.
The way that twin studies generally work is through the comparison of the two kinds of twins to each other. What we do is to look at the similarities of twins to one another and we look to see if, for example, the identical, or monozygotic, twins are more similar than dizygotic twins, and if so, that would be suggestive of genetic influences because the increased genetic similarity could explain their increased phenotypic, or behavioral, similarity.
So the nice thing about twins is the fact that there are these two kinds of twins, the genetically identical and the non-identical twins. But, they both share environments, they both share the womb at the same time, they are the same age, they grow up with this contemporaneous relationship. And so they serve as a comparison group for one another.
There are, of course, also studies of identical twins raised apart. Those are a really fascinating and unique group of individuals to study as well. It turns out that they provide a really powerful design for separating genes and environment. But it’s not necessary to have twins who are separated at birth. You also don’t have to have adopted twins to get a handle on gene-environment influence. By looking at the two kinds of twins who are raised together you can make many of the same comparisons.
BeTwinned.com: What are some of the big discoveries that researchers have made using twin studies?
Baker: In the last 25 years the study of twins and adopted children together really changed the way that social scientists thought about human behavior. Twenty-five or 30 years ago, it was really thought that learning and experiences were the primary explanation for individual differences, but the systematic study of twins and adopted children and their families all really changed the way people think. We now have moved away from a primarily nuture model to thinking that genes and the environment are both important in explaining individual differences.
In fact, it’s now the case that it seems that just about every realm of human behavior that you look at shows evidence for some genetic influence. Twins have really helped us to understand that very fact. Identical twins routinely are much more similar than fraternal twins on just about everything that you look at, from their actual physical appearance, to their general cognitive abilities, to their very specific talents, whether they’re conservative or liberal, the degree to which they are religious and even their social attitudes.
All of these things have some genetic influence, which was a surprise to a lot of social scientists. Everyone thought that social learning would be the key factor, but in fact genes seem to have these pervasive influences in the way that we think, feel and behave, in general.
It’s almost become kind of a joke to try to find something that’s not heritable, that’s not influenced by genetic factors. For a while, people were searching for something that wasn’t genetic, and it was really hard to find something. There are a few things that came up – it does appear that how spiritual or how extensively one participates in their religion seems to be influenced by genetics, but the type of religion that one participates in is probably more a family-learned cultural experience. So, which religion you participate in seems to be more cultural or environmental, but the degree to which you participate in it seems to be influenced by genetics.
BeTwinned.com: You obviously spend a lot of time with twins in your work. On a personal level, what have you learned about twins from working with them?
Baker: Twins are a really special group because they have this contemporaneous relationship. Twins can be, and often are, extremely close, more so than non-twin siblings. The bond that you see between twins can be remarkable. The love and affection and devotion they have for each other is really heartwarming.
Sometimes you’ll meet twins who are so similar that it’s hard to keep track of who’s who, and you get really confused when you try to hold a conversation with the two of them together. And a lot of times you’ll have twins that finish each other’s sentences, and it’s as though you’re talking to one person, but it’s really two. It can be a really bizarre experience, but fascinating.
I think that that bond, that really special emotional connection and level of communication between twins is really special and unique, but it also varies across twins. Some twins are closer to each other than other twins. And you also see some twins who are really competitive, and that close bond can actually be a problem for them because they feel almost not recognized as unique, functioning individuals. They can resent each other because of the competition. So at the same time that you can see this close, loving bond, sometimes it creates competition that’s even more drastic than between non-twin siblings.
Sometimes I’ll see twins who, during adolescence, will grow to almost loathe each other. They want to go their separate ways and be their own person and stop living in the shadow of their twin, and they’ll sometimes just go their separate ways as young adults and not see each other for years. A lot of times I’ll find that they come back to each other once they get rooted in their adult lives, and then they rediscover each other and they have this close connection.
BeTwinned.com: I’ve heard and read recently where it’s supposed to be common knowledge that twins are underachievers, except in athletics.
Baker: I’ve never seen any evidence for twins being underachievers. I’ve never seen any evidence for adult twins, at least, being any different in terms of IQ, educational attainment, or psychopathology. There’s no greater instance of psychopathology in twins. They seem to function just like everyone else.
There are some studies that show some early delayed cognitive development in twins, but those studies are kind of mixed as well. But, it seems to disappear by the time they get out of school, or even by the time they get into school. So sometimes twins will have language skills that will develop just a little bit later, and that can just depend on the twins themselves, how many other kids there are in the family and what’s going on with the parents. Of course genetics can play a role in their development as well, and premature birth too. So all of those factors might lead to some delays in twins, but they tend to disappear by the time they’re well into elementary school.
So it’s my understanding that twins are really the same psychologically and physically as the rest of the population, perhaps with this exception of having this special bond with each other. But they’re no more psychic than other people, for example. They’re not psychic. It might seem that way sometimes. You hear these freaky things sometimes, like one feeling what the other felt when they were across the world. Those are compelling stories, but what you don’t hear is all the times they don’t feel those things. Some of them really think they can read each other’s minds and that they can feel this connection from a distance. It’s hard for me as a non-twin to believe that. My feeling is that they just have this special communication and that they really understand each other in a way that no one else can. They have this intense communicative bond. But I don’t know how that would explain their experiences from a distance.
BeTwinned.com: When people say “twin studies†does that imply multiples as well?
Baker: It does imply multiples as well. Of course, twins make up the bulk of multiple births. But in our research, we do include triplets as well. We’ve never had anything higher than triplets. But we have nine sets of triplets in our study out of 605 families. It becomes tricky, trying to figure out how to statistically analyze the three. But in some ways you can think of triplets as a bonus because you can get three twin pairs out of them. You’ve got A-B, B-C and A-C, so triplets are wonderful because they give you three times as much information.
BeTwinned.com: What can people do who wish to sign up their twins or multiples on a registry?
Baker: People can go to websites. I have a website where I mention that I’m doing twin research, and there’s a link where people can send an e-mail if they’re interested. We’ll accept any twins that want to be in our register, but that might mean that they have to wait for a while before there will be a study that they will qualify for. But they still get our newsletters, and we keep them posted on things.
Sometimes other people write to us wanting to know if they can find twins for their studies, and we’ll send out letters to people on the registry saying to contact this person if you’re interested. People try to be protective of their twins and while we want to make them available for studies, we also don’t want to go sending their names out to anybody without their permission.
So people can look online. There’s lots of twin registers, and people can just put their names in them. There’s actually some effort right now at the NIH to put together a national twin register, to find all the twins in the U.S. and get them into a database somehow.
BeTwinned.com: What does a study entail?
Baker: It could be anything from doing nothing to coming in and participating in a laboratory assessment from anywhere from a couple of hours to a couple of days. The people in Minnesota, in their study of twins, had them in there for a week, so that’s really unusual. There are a lot of studies that are done with mail surveys and phone interviews, so there’s a range of things.
We’re doing a study right now with someone from the University of California Davis who’s a political scientist, and he found out about my twin register. He called me to say he was interested in voter behavior. He said we could do the study by just matching the twin register for the adults to the voting records in the County of Los Angeles, and we could look at concordance for their voter behaviors without having to contact them, because it’s public information.
But we felt uncomfortable even though there’s no harm to anyone. We just felt that out of courtesy we should just write to the people in our register and say that we’re doing this, and if you have a problem with that, just let us know. So we did do that, and so far most people have written back enthusiastically, and in fact they started telling us about their voting behavior.
I find most twins that sign up are really eager to participate, and they like to feel special. The only problem sometimes is that families with twins get really busy. It’s hard with kids, and even with one child, we tend to overschedule and having two must be a nightmare. So it can be hard to schedule time to get them to come in and participate in the study. So that’s one of the challenges we face.
BeTwinned.com: Are there any hot new discoveries?
Baker: Here at the Southern California Twin Program, we are actually looking at the development of both social and anti-social behavior. We’re interested in the range of problem behaviors that kids might experience.
We’re particularly interested in adolescent rebellious behaviors. It’s always been thought that that’s the norm almost, for adolescents to become rebellious and anti-social, that that’s what they do as they try to break away from their home ties and become independent.
We’ve been investigating the development of aggressive, anti-social conduct problems in twins. We started at the age of nine and 10. We’re looking at the whole community, so we’re not just looking at kids that get in trouble. We’re looking at all kids, and we’re looking at the range of behaviors, and we’re looking at why some kids become more aggressive than others and why some people are more rule-adherent, versus others who may eventually break laws.
One of the things we’re finding is that that there’s a big range of behaviors that kids display from the age of nine to the age of 14 — our study started at pre-adolescence and now we’re moving into the adolescent phase. We do find some evidence for genetic predisposition towards aggressive rule-breaking, anti-social behaviors. So, conduct problems do seem to have at least a partial genetic basis, but there’s also a huge environmental influence. And things like socio-economic factors, the level of stress in the home that the mother reports, certain kinds of life events, all do seem to contribute to conduct problems as well.
We’re really interested in following our kids into adolescence, where one question is: How much do peers actually influence kids to become more anti-social? Is there going to be a greater effect of environment via the peers than genetics, or are the genetic factors going to become even more important in determining who can resist those peer influences and who can’t? Those are questions we have, so we have studied these 605 families of twins and other multiples. We’re about to follow them up when they are in their teens, from 14 to 17, so that’s the next five years of our study. We’ll be recruiting some new sets of twins, so twins in California who were born between 1990 and 1995 and who are interested in this particular study can contact me.
Additional resources about twin research and twin registers.