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Studying twins, studying personality

Updated Tuesday 25th May 2010

 John Oates meets twins - and those who study them - to explore if there is anything we can learn about personality and links to genetics

Marian and Vivian Brown Creative commons image Icon DC Atty under CC-BY licence under Creative-Commons license
Marian and Vivian Brown, celebrated twins of San Francisco 


Copyright The Open University


John Oates:
Studying twins has long been a favoured approach for psychologists interested in exploring influences on people’s personality. Identical or monozygotic twins are particularly interesting because they share the same genes; unlike fraternal or dizygotic twins who are no more similar genetically than siblings with the same parents.

If identical twins grow up apart, yet remain very similar in personality through their development into adults, there must be genetic influences causing these similarities - so the argument goes.

In making this audio I met with several leading researchers in the field and also with some identical twins themselves. At Imperial College London, I met with Nancy Segal. She’s been researching personality in twins for many years.

Nancy Segal:
I’m here in London to attend the International Twin Congress which draws together researchers from all around the world to discuss their specialities in the medicine, psychology, behaviour of twins, and studying identical twins raised apart is one of the most effective ways we can use twins to understand the origins of personality.

And it is striking, I’ve studied reared apart twins for ten years, and we find that despite the differences in the homes the twins are quite similar in many personality traits from extraversion to dominance to aggresivity, and this does strongly underline the fact that personality is to a substantial degree, although not entirely, influenced by our genetic factors.

And I think that for parents raising twins or for people with siblings we can better understand why we’re alike some people and why we differ from other members of our family.

I think that people are fascinated with identical twins because it challenges so many of our traditional beliefs about the way that the world works. Most of us are taught to believe and do see evidence of individual differences in behaviour, in physique, in interest, and we come to expect it, and so when we suddenly encounter a set of identical looking and identical behaving people, it fascinates us, it occupies our interest.

It may even be disturbing to some people because I think in an evolutionary way we need to distinguish who our friends are, who our family members are. So I think it’s the novelty of this that really draws our attention.

And, you know, if you try a little exercise, take a picture of a pair of identical twins, and maybe they’re not all that good looking individually, but then, and cover one, then take your hand away and there’ll be two, and they will be absolutely irresistible - it works every time.

John Oates:
Daphne and her identical twin Barbara were born in Britain and adopted into two different families. This is Daphne.

I believe I was about six weeks old when I was separated from my sister. I was adopted through a child agency, and then I went to live in Luton. I was told when I was 11 that I was adopted. Which I think at that time it was pretty good because children were never told or, but I was told, and my grandma said you have a double, which I took to be a twin and an identical twin.

But when you’re 11 you get things all muddled up in your head, and I’d been doing piano lessons and my teacher was emigrating to Canada, and she wouldn’t recommend me to anybody else and said, my mother said to her, Daphne’s mother had gone somewhere, and I took it into my head it was Australia because a great aunt had gone to Australia, and I thought she’s gone to Australia, she’s taken my twin with her, that’s all right, I’m very happy as I am.

Funny enough it was my father’s second wife. He’d been married before. He’d got two older daughters a lot older than I was. And I didn’t discover until many, many years later that he said to his wife that she could take one twin or no twins.

Evidently there were twins in the family that were terrors, so whether that put him off, and he was about 26 years older than my mother, so I suppose being his second marriage he was an older man and couldn’t face the prospect of having twins.

John Oates:
After several failed attempts to contact Daphne, Barbara finally succeeded in tracking down her lost twin.

Well, I think we felt comfortable at meeting on sort of mutual ground. I mean I was in Yorkshire, Barbara was in Kent, and my husband had a trip to go to America. So we travelled down to London and of course Barbara travelled up. At good old King’s Cross Station, that’s where we met.

I came down on an Intercity which is a very, very long train. I opened the train door, I didn’t have to look anywhere for Barbara, as I opened the door she was standing on the platform immediately in front of me, and we just said hi, walked down the station busy chattering to each other.

Forgot that our husbands didn’t know each other, they hadn’t even spoken on the phone to each other. We left them humping all the cases off the train, and we went off nattering down the platform; felt like meeting an old friend that you hadn’t met for a long, long time.

I think our sense of humour is one of the biggest things. I mean to say we can just look at each other and laugh and know exactly what we’re laughing about. I mean ,we’ll read the same books, we’ll knit the same things, we’ll cook the same things.

Personality-wise, we don’t always express our feelings I don’t think. We laugh at things and possibly that’s hiding something.

I know it sounds stupid but we both think that we are quite shy people because we don’t necessarily ask questions to people. We’ll wait until somebody asks us, and we’ll answer. But I think a lot of the time we are hiding things when we laugh, and you can get away with a lot by laughing.

John Oates:
Do you think that this similarity, and the things that you’ve talked about are things that often are similar between people anyway, do you think that you’ve become more similar since you’ve met up again?


Not necessarily. I tell you for why, because when we were at school, I know my schooling was very, very poor. Never passed the 11 Plus, hated maths - and I believe Daphne didn’t like maths at all - didn’t mind art, I didn’t like physical exercise.

So even then I think some of the things were very similar. I mean to say I used to like reading.

Daphne had possibly better books than I did, but I still used to go to the library and get books. So I think sometimes even then, although Daphne had a brother, I was on my own, but I was quite happy to be on my own. So I think possibly going back some of the things could have been similar then.

John Oates:
Nancy Segal has studied these twins in some depth at the University of Minnesota. How does she explain the similarities between them?

Nancy Segal:
Barbara and Daphne seem to have an intuition, an intimacy, a rapport, a very close understanding of one another, that really evolved quite quickly, that might take many people a long time to develop. And humour was a big part of it, and no one quite understood what about their interactions set this laughing off, but they both said that with other people it never happened, but when they were together something about their interaction just triggered the scale of giggles.

The issue here is that identical twins raised apart of course share no environments in common, but when it comes to personality they are as alike as identical twins raised together. Now that is somewhat counterintuitive finding because one would think that if you’re with somebody you should be more like them. But what this is telling us it that the reason identical twins are so similar is not because of their shared environments but because of their shared genes.

Now we still need to account for the other 50%, and we suspect that is explained by differences in their environments, what we call the non-shared environment, and that’s why the similarity is only 50% and not 100%.

Because the environments even of twins raised together is not exactly the same; there may be minor differences, maybe even associated with prenatal differences that might make one of them more outgoing, one less so, maybe one twin had an accident or an illness. Amazingly, if you think about the conjoined twins, the twins who failed to separate entirely at birth, they have very different personalities and very different environments, and yet again this is somewhat counterintuitive to our thinking.

But when you talk to these twins and really ask them to closely outline their environments for us, they will tell you that they are quite different. As an example, I interviewed one such pair where one went to college and the other one did not - these were conjoined twins - and the one who did not go to college said that she simply sat in the classrooms and thought of other things.

So when you think these things through a little bit carefully I think that you can see that the environments of identical twins are not exactly the same. Now, when you think about reared apart twins, I think that’s what’s happening there, that within the range of their different environments the twins are selectively seeking out or gravitating towards similar things in their environments. That I think would explain why, for example, we found two twins who were avid readers - in fact they were from the UK. One twin came from a home with a lot of books; the other one came from a home with very little. Both twins, however, were great readers and had even read the same kinds of books because one went to the library. So she in a sense created or fashioned her own environment. But I think we have a lot of research left to do in apportioning genetic and environmental factors with respect to behavioural traits. And the reason I say that is twofold. First, with the human genome project coming to a conclusion near in time we’ll be better positioned to associate various genetic factors with certain behavioural traits. Secondly, we don’t know a lot about what environmental factors are relevant for behavioural development.

We can say now yes this is shared, this is non-shared, but what is non-shared? Is it the school, is it your friends, is it the books you read, is it the accident, is it the contest you’ve just won - what is having the effect? So we have a long way to go for better understanding of that set of influences.

John Oates:
Thirty-seven years ago Dorothy was surprised to learn late in her pregnancy that she was carrying twins, and even more surprised when they were born to find out that they were identical. Her twins Linda and Sharon grew up together and now as adults they’re striking in their similarity.

We both like the same sort of clothes, same colours, same styles, quite plain really, but both just got the same tastes. I don’t know whether we would have had the same tastes had we been sisters or whether the fact that we’re twins has contributed to that in any way.

I think also, well, because we’ve been grown up together, we’ve been in the same bedroom, even though we had a choice of going into a spare one, for eleven years we still decided to be in the same room. We’ve grown up with each other.

You know, we’ve been in the same classroom at school throughout our whole school years. You know, I think we’ve just got the same tastes and, you know, I wonder if that sort of, whether you could say that about a husband and wife that have lived together for all these, you know, for many, many years.

Well, it was quite a shock because I didn’t know until three weeks before. It would have been say seven weeks, but they were born a month early, so it was a shock.

Especially as I’d, only expecting one, but I had two other daughters and there was always an argument who was going to hold the new baby first, and when we found out my husband just turned to them and said there’ll be no arguments you can have one each, and that’s how it was.

A nurse came and told me in the early hours and said I thought you’d like to know that your twins are identical there was only one afterbirth, and I didn’t even know until then that that meant that they would be identical. And I found them almost the same.

I did when they were getting a little bit like say to the crawling stage that Sharon could creep before Linda. So Sharon had to go into the playpen and creep around, this one sat outside, so she didn’t go anywhere. And then you found that they’d, their personalities would swap, they’d change over. So one would be more forward than the other, and then the other one would take over, and that’s the way it seemed to be.

But as they grew older if they hurt themselves, or one of them did, you’d find within a short time the other one would do just the same in the same place. I remember Linda having a big really nasty scratch, really bad wasn’t it? And didn’t you do the same just afterwards? And, you know, and same with knees, whatever, whatever one done you would find after a little while the other one, it would happen to the other one just the same. But I didn’t have any tantrums, no two year tantrums, not at all.

But the nice thing is with me and Sharon we don’t have to explain anything; she knows just what I mean when I say something, and I know what she means. You know, sometimes with friends you have to be a bit careful, you know, you don’t want to offend them. No, never anything like that between us two is there?


We know how to take each other.

We do, yeah, and we finish each other’s sentences off.

John Oates:
It would be tempting to assume that Linda and Sharon’s striking similarities have been made more so by the fact of them growing up together. But Robert Kruger: another twin researcher from the University of Minnesota, believes that this shared environment may not, in fact, be the main explanation.

Robert Kruger:
: The thing that’s surprising to everybody is that growing up together in the same family seems to make people similar only by virtue of the fact that they share a certain amount of genetic material. The very idea that they’re growing up in the same family doesn’t seem to be the source of their similarity in terms of things like personality.

People often find that surprising, but I think we can think about it as perhaps less surprising and perhaps speaking to the flexibility of people when they leave these environments that people are more flexible than you might think.

So that the key environments might be those environments they happen to find themselves in right now, those things that are making them unique, in spite of the fact that they grew up in a certain set of circumstances or have a certain set of genes, that those unique experiences that people are going through are very relevant to understanding their personality at a given point in time.

And to my way of thinking that really speaks to human flexibility so that we aren’t just simply driven by these genetic effects, and we aren’t simply driven by the particular circumstances we grew up in. Instead there’s a lot of sort of unpredictability that’s not coming from those things that suggests the importance of looking to particular things that are happening to people at any given point in time in the way that they’re dealing with those things.

And so that I think is an important area to start looking into more, these kinds of non-shared environmental effects that make people unique.

That’s the key thing for people to understand is that the stories are intriguing, and they can suggest things that one might want to study systematically, but it’s the systematic study that should be used as the evidence for or against genetic effects on things like personality. And again when one looks at the systematic evidence that suggests that yes genetic effects do play a role in differences among people, but they’re certainly not the only story.

Again differences among people are also traceable to the unique experiences that they have over the course of their lives.

So even identical twins who are reared apart are not exactly the same person, so there are interesting aspects of them that are different, and I think now a major focus needs to be on what’s the source of that stuff. Why is it the case that these persons who share their genetic material are, nevertheless, not exactly the same people psychologically?

And it appears that that might have a lot to do with the unique experiences that they have over the course of their lives. We only have limited control over what it is that we can do or experience through our lives and yes genes may drive, to a certain extent, the kinds of experiences we seek out, but we don’t have complete or perfect control over that.

So romantic relationships may be a good example of something like this where you don’t have complete control over who you’re going to end up with or the impact that that person might have on your life; nevertheless, those impacts can be quite profound. And that lack of control over the impact of the other person that, nevertheless, affects you is a good example of a potential non-shared environment, something that can’t be shared among people within families, and they can’t be completely under genetic influence because it’s just simply too random. And I think those random kinds of experiences may be important to understanding differences among people.

And again the example of this that might drive that kind of inquiry is that identical twins are not exactly the same people, and that’s I think an important, and again, anecdotal observation, but it drives the scientific inquiry where you say well are there systematic differences among identical twins that can be linked to differences in terms of their experiences of these particular environmental events, and that I think is an important area to begin doing more work on.

John Oates:

Kay has identical twin boys, Andrew and Craig, and she has been struck by their dissimilarities rather than their similarities.


Even as babies, definitely, Craig has always been the quieter one. Andrew was always the one who made the most noise, did the most crying, that sort of thing yeah. I mean each child sort of determined how they were treated I suppose. Andrew was more demanding. So, you know, he took up more of the time I suppose.

But Craig was quite happy to sit and wait. If it was dinnertime Andrew, you know, demanded to be fed, whereas Craig would probably just cry a little bit to say I’m hungry too.

Andrew loves football, all sport. He will watch sport on the television, enjoys sports. Craig has no interest at all in football. I don’t really know what Craig enjoys. He likes art and reading and watching the telly and PlayStation games and things like that. They both enjoy the PlayStation games but Andrew’s the sporty one definitely.

If I go to check them when they’re asleep and their sleep positions are invariably exactly the same. If one of them's got a leg up, the other one, because they’re in bunk beds, the other one will have that same leg up. Things like they’ve both broken the same elbow.

It was three years apart but they both had the same fracture of the same elbow, that sort of thing so. I don’t know whether that’s to do with being identical twins or, you know, whether they have that certain connection, even when they’re asleep, you know, that they do the same things.

John Oates:
Here in Britain, the Institute of Psychiatry in London is carrying out a major programme of research into behaviour genetics and Robert Plomin has a major study of twins under way.

Robert Plomin:
When I came to England in 1994 I decided I would do a large scale twin study of development. So we’re studying all the twins born in England and Wales in 1994, ‘95 and ’96, so that’s about 15,000 pairs of twins, and we’re studying them for language development, cognitive development and the development of behaviour problems.

Those are the three most common domains of problems in childhood. In fact, the most common reasons why parents would take their kids to a paediatrician is more common referred problems than say medical problems.

So we wanted to use the twin method to ask whether or not there’s genetic influence early in development and the extent to which genetics explains why these three domains are interrelated the way they are, it’s called comorbid. Kids who have language problems very often have cognitive problems and very often have behavioural problems.

And so nobody’s studied these issues early in development, so we’ve been doing that for the last five years, and we’ve just now finished testing the oldest cohort kids, born in 1994, at seven years of age after their first year in school. So that’s the best time that some of these behaviour problems really become more predictive of long term problems of the kids. So that’s the main study that I’m doing now.

It’s called TEDS, the Twins Early Development Study, the study of 15,000 pairs of twins, where we’re following the kids from early childhood to middle childhood and eventually to adolescence.

This sort of research that began in the early 1920s has I think convinced most people that genetic influence is important. And it’s not to say it’s all genetic, and it’s not to say that it’s one gene that accounts for say personality; we’re talking about many genes. It’s... these are probabilistic propensities; we’re not talking about hard wired genetic determinism.

And I think that genetic research of this sort, twin and adoption studies, have made the best case we have for the importance of the environment. Because if less than half of the variance is genetic that means the other half is environment. So this research has been very important. I think it will continue to be important.

There’s a lot of questions we can ask about development, the interface between nature and nurture, that go beyond just estimating heritability, that’s no longer very interesting because there’s nothing to my knowledge in psychology that has reliably been shown to have no heritability, and I think most people accept that message now.

The vast majority of twin research on personality involves self report questionnaires, and I was interested in looking at parent ratings, and that didn’t pan out so well for reasons I could explain but basically parents don’t have a good sense of how their kid compares to other kids because all they know is their kid.

Teacher ratings are something that I think is better, but teachers aren’t so good at measuring personality as much as behaviour problems.

If the teacher tells you this kid is really off the wall, you can believe it because the teacher has seen hundreds of such, of kids, and they have a good anchor of comparison. But a new study was published a couple of years ago from a German group that studied adults, and they administered self report questionnaires, but then they also did an interesting thing, they asked each of these twins to have their two best friends or two people who know them best, not including the other twin and not overlapping for the two members of a twin pair.

So these are peer ratings of personality. And I was really holding my breath, the study took a long time to do, and they had about 800 pairs of twins so it was very large study and it took several years. And I really wondered whether or not those peer ratings of personality would give you the same results as self report questionnaires. And they did almost exactly.

So I thought that was very impressive. I think the biggest finding doesn’t, from twin studies, doesn’t involve genetics though, it involves the environment, and this is the topic of non-shared environment. Parents environmentally don’t have as much of an effect on their kid’s personality development as they might think. You know, if you say X the kids do Y, and their peers become enormously important. And you see that in terms of fashion, you know, it just hits you over the head with adolescents how much their peers influence what’s cool or not. So I think you know it’s possible if you’re, that extra familial influences like peers are more important than our theories of socialisation would have led us to believe because our theories focused on the family environment. So it’s an exciting area of research and behavioural genetics I think has made its biggest contribution here not in terms of nature but in terms of nurture.

And it suggests that the way forward, if you want to know why kids in the same family are different you’ve got to study more than one kid in a family, and the other implication of behavioural genetic designs is that you really need to embed studies of the environment in genetically sensitive designs like twin studies or adoption studies because, otherwise, you completely confound nature and nurture.

Genetics accounts for maybe half of the variants, that is of the individual differences in personality, but the other half is not genetic. There’s no genetic explanation for it, so we call it environment. In behavioural genetics what isn’t genetic is called the environment.

Now that doesn’t mean it’s the environment that psychologists have tended to study, it doesn’t mean tender loving care of your parents necessarily. It could be prenatal factors, it could be illnesses, and it could even be DNA mutations aren’t inherited, yet they can cause differences in people.

So it’s a very broad concept of what we mean by the environment. But the interesting finding is that, although environment’s important, it’s not the environment that psychologists thought was important for personality development. All of our theories of socialisation from Freud onward focused on what we called shared environment.

That is, they assume that the way the environment works is on a family-by-family basis. One kid has authoritarian parents and another kid has permissive parents, and so it’s that family environment provided by the parents that’s responsible for personality development. Well behavioural genetics has shown in twin and adoption studies that that type of environment is not what’s accounting for personality development.

It’s not growing up in the same family with another kid that makes you similar for personality. Genetics makes you similar but the environment makes two kids in the same family no more similar than kids reared in different families.

John Oates:
Identical twins Sharon and Linda shared a lot when they were growing up. I asked them about what happened when they started to have boyfriends?

Sharon liked the men that were well groomed, in a suit, looked as though they earned millions. I liked the man who didn’t mind or didn’t look as though he minded getting his hands dirty. So we were very different like that. But over the past few years we seem to have met somewhere in the middle of that. Our taste in men is the same now.

I’ll point somebody out, Sharon will almost undoubtedly agree, and vice versa.

Yes. I mean we’ve both been with our husbands since we were young. I’ve been with my husband since I was 15, I’m now 37 - I need a medal. You’ve been with yours since you were 13.

I was 13.

So at that age you didn’t really know, it was like young love wasn’t it.

And actually do you remember, the one day after school, I met my husband at the senior school, and I’d been seeing him for quite a little while and really he should have known the difference, but one day after school we planned, and Sharon took his hand and walked all the way home holding his hand, and he didn’t realise.

He didn’t know. That just shows how much he looked at you really doesn’t it because - I think he was dragging me along wasn’t he because he’s nearly six foot.

I think that’s what it was yeah. But he did not realise that he was holding Sharon’s hand and not mine, and he was very embarrassed when we pointed it out.

But he did realise.

He went to kiss you.

He went to give me a kiss, and I thought I’m not having this, and I think I put my hand up and he wondered why and I had to say, “Well, Nigel, I’m not Linda.” And he was, well I think he was so shocked, he could not believe.

He wanted the ground to swallow him.

He did, he did. But we laugh about it so much now. That is one of our stories we’ll be telling until we’re, you know, got a few more wrinkles.

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