Nature’s clones: what twins have taught us
Is it our experiences or our genes that make us who we are? Studying twins has revealed unexpected, and often unnerving, insights into the nature versus nurture debate. Tim Dean reports.
Imagine receiving a phone call out of the blue. You find the voice on the other end eerily familiar as it tells you some life-changing news: you are, in fact, a twin. And when it comes time to meet face-to-face, you find it’s like gazing into a mirror. You share a similar dress sense, hairstyle and even idiosyncratic gestures and expressions you thought were uniquely yours.
In 2004, Paula Bernstein found herself in exactly this situation when she was contacted by Elyse Schein, who broke the monumental news that they were twins, separated at birth and raised apart.
They met for the first time at a New York café, where nerves and trepidation soon gave way to warm conversation. “We had 35 years to catch up on,” says Bernstein. “How do you start asking somebody, ‘What have you been up to since we shared a womb together?’ Where do you start?”
“It was really surreal,” agrees Schein. “It was like meeting an alternate version of myself.” She found the similarity in mannerisms particularly spooky. “I always had the idea that [the way] I gesticulated – raised my eyebrows and so on … was due to my environment, but now I know it’s inherited.
We joke sometimes that we want to turn off each other’s mannerisms, because it’s unnerving to see them echoed back.”
And it wasn't only idiosyncratic gestures that the two had in common. Despite being raised by different families in disparate parts of New York City, they both followed an uncannily similar path in life.
“We were each editor-in-chief of our respective high school newspapers and then went on to study film theory,” says Bernstein, who is now a journalist, while Schein went on to become a writer and film-maker.
Cases like that of Bernstein and Schein are rare these days, but they give a compelling insight into the role that genes play in shaping the fundamentals of who we are.
In fact, Bernstien and Schein, who documented their experience in their book Identical Strangers, were separated as a part of a secretive scientific study of nature versus nurture conducted by child psychiatrist Peter Neubauer in the late 1960s. Such experiments wouldn’t be possible today for ethical reasons, but identical, or monozygotic twins still represent an incredibly valuable resource to science.
“They are a beautiful natural experiment,” says Nicholas Martin, geneticist at the Queensland Institute of Medical Research and editor of the scientific journal, Twin Research and Human Genetics. According to Martin, identical twins give scientists the perfect opportunity to tease out genetic and environmental variables from factors such as personality or susceptibility to diseases including asthma, depression, alcoholism and attention-deficit hyperactivity disorder (ADHD). And it’s all because identical twins are effectively natural clones.
As Martin points out, it’s long been observed that certain traits and diseases run in families, whether it be height, or asthma or even the preponderance of a particular profession. But it’s not always easy to draw conclusions from these facts alone.
“The trouble is that something running in the family is not sufficient to show it’s because of genes,” he says. “A high incidence of melanoma might be due to habits of being out in the Sun, because families share both genes and the environment.”
The trick is to disentangle the environmental influences from the genetic component. And this is where twins come in. The classic twin study uses both monozygotic and dizygotic (non-identical) twin pairs who have been raised together by their respective families.
This allows the researchers to eliminate shared environment – such as days on the beach in the hot Sun – as a variable, because it should affect both twins equally.
The researchers can then compare the variations between the two types of twins, knowing that the monozygotic twins share all their genes while the dizygotic twins share only 50%.
For example, it could be found that when one monozygotic twin develops melanoma then it’s 80% likely that the other twin will also develop melanoma. In contrast, if one dizygotic twin develops melanoma, there might only be a 40% chance the other twin will also develop melanoma. As a result, it becomes clear that genes are playing a significant role behind the scenes of this particular disease.
Twin studies of this kind have yielded some startling insights into how influential genes are in shaping our behaviour. “Well conducted twin studies are able to completely change the way we view problems and to reorient entire research programs,” says Martin. “For instance, autism had been attributed to a variety of causes, such as emotional coolness in the mother, until a landmark twin study found much higher concordance in monozygotic than dizygotic twins.”
Another case cited by Martin is ADHD. “It was believed ADHD was caused by food additives. So parents would limit their kids’ intake of red smarties to stop them from being hyperactive. But that was complete rubbish,” he says. “[Then] someone did a twin study and showed that monozygotic twins share ADHD 80% of the time and dizygotic twins only 40% of the time, which is almost exactly what you’d predict if it was entirely genetic.”
But it’s not only diseases that have been scrutinised by twin studies. Twin studies have also been used to shed light on the influence that genes have on our very personality. What could be more central to our identity than our personality – our beliefs, dispositions, quirks and all those little things that distinguish us from everyone else?
It seems self-evident that much of who we are is shaped by our past experiences. This was the view in the social sciences for much of the 20th century. Spanish philosopher José Ortega y Gasset summed it up in a 1941 essay when he wrote, “Man is what has happened to him, what he has done”. And that humans have no ‘nature’, all we have is ‘history’.
Yet twin studies place such a perspective on shaky ground. Recent twin studies have found a significant genetic component to a broad spectrum of behavioural and social dimensions, to the point where it seems experience plays only a bit part in shaping our destinies. Studies have revealed this phenomenon at work in attributes such as alcoholism, voting habits or even whether you’re a tea or coffee drinker.
“We almost got sick of finding behavioural traits that are genetic,” says Martin. “Everything from social attitudes, socialism, free sex, trade unions, apartheid – all those things the sociologists tell us are shaped by the social milieu – well, lo and behold, they’re more genetic than environmental.”
One 1996 study in the journal Psychological Science even found that happiness appears to be primarily determined by our genes. It discovered that socio-economic status, education, income, marital status or even religiosity could account for no more than three% of the variance in happiness. Genes, on the other hand, appeared to account for around 50%.
“Twins really do force us to question what it is that makes each of us
who we are.”
The signifance of this is not to be understated. For if we believe that our environment is the primary influence on our behaviour and personality, then we seek to change the environment to change behaviour. This was the principle underlying a broad range of social programs in the late 20th century designed to tackle deep-rooted problems such as unemployment and social disadvantage.
One example was the believed link between television violence and rates of violent crime, which were of particular concern in the United States during the 1970s and 1980s.
Yet, despite an increase in the availability of television channels and shows offering violent content since then, violent crime rates have dropped in the U.S., according to FBI statistics. As such, shaping the environment appeared to have little effect on real behaviour.
This is not to say that everything comes down to genes, or that identical twins are ‘identical’ in every way. In fact, one of the most vigorous areas of research today is exploring causes of differences between monozygotic twins. This is a question that is perplexing not only to researchers like Martin, but to twins as well.
Stephanie Lockery is an identical twin and a psychologist, which gives her a unique insight into the issue. She has lived her entire life in close proximity to her twin sister, Tamie, and despite their manifold similarities, Stephanie has often been struck by their subtle differences.
“People often assume that we are very similar. And on the surface, it may appear that we are – we’re both confident, outgoing, talkative and bubbly,” she says. “But people are often very surprised once they get to know us that we are quite different.” Lockery puts this down to their different ways of looking at the world; where she is more focused on facts and evidence, Tamie takes a more intuitive approach. This can be seen in their different ‘types’ according to the Myers-Briggs personality test.
They are both identical on three of the four metrics, but where Stephanie is ‘sensing’ – indicating she prefers concrete facts and evidence – Tamie is ‘intuitive’. And even this slight difference has led Stephanie and her sister on divergent career paths – into psychology and teaching respectively.
According to the Queensland Institute of Medical Research’s Martin, differences between monozygotic twins are not unexpected, but the question remains as to what causes them. Even in studies where environmental factors have been reduced to a minimum, there still appear to be unaccounted-for differences between twins.
“This has led me and others to question whether the ‘environment’ is actually something within the body, something endogenous,” says Martin. One theory is that it has to do with ‘epigenetics’, which looks at the variations that can occur in gene function without any changes in the DNA.
According to Martin, epigenetics is a growing area of interest for him and many other genetics researchers who are hoping to shed some light on the remaining unanswered questions of how genes influence our physical bodies and our behaviour.
So next time you have to make a substantial life decision, or you find yourself wondering how you got to be like you are today, spare a thought for those cunning nucleic acids and the authority they wield over who you are. Some might find it disconcerting to acknowledge that their identity is substantially shaped by their genes, but that doesn’t mean there still isn’t room for you to steer your life where you want to go.
“Twins really do force us to question what it is that makes each of us who we are”, says Paula Bernstein. “Since I met Elyse, it has become clear to me that genetics plays a huge role in shaping our characters. But although we share many of the same interests and personality traits, it’s a relief to discover that we’re distinctly different people.”