Studies of twins provide fertile ground for the nature versus nurture debate. Biddy Passmore looks at the arguments

Take this case. A baby girl is adopted by a working-class family with an overweight, insecure mother who resents her daughter’s attractiveness and regards her as an outsider. The baby’s identical twin sister is adopted by a well-off family with an attractive mother who dotes on her. Which twin fares better?

Actually, both fare equally badly. The alienated Amy sucks her thumb, wets her bed and develops into a shy, role-playing hypochondriac. But so does the cherished Beth.

With this story of Amy and Beth, which kick-starts his fascinating study Twins, Lawrence Wright demonstrates the awe-inspiring power of genes to determine what we become. In theory, if Amy had been raised in Beth’s family, she should have been a happier, more mature person, yet “in nearly every respect . . . Beth’s personality followed in lockstep with Amy’s dismal development”.

Twins, and especially identical twins - and even more especially identical twins separated at birth - provide a rich field for psychologists and geneticists to till. The evidence they provide is essential ammunition in the “nature versus nurture” debate. But stories like that of Amy and Beth make us all feel uneasy, because they strike at the very heart of the notions of identity and free will. As Lawrence Wright says: “We think we are who we are because of the life we have lived. We think we shape the character and values of our children by the way we raise them. We think that we are born with the potential to be many things, and to behave in an infinite variety of ways, and that we consciously navigate a path through the obstacles and opportunities that life presents us with, through a faculty we call free will. But when we read about twins who have been separated at birth and reunited in middle age only to discover that in many respects they have become the same person, it suggests that life is a charade . . . and that the injunctions of our parents or the traumas of our youth . . . may have had little more effect on us than a book we may have read or a show we may have seen on television.”

Because twin research affects deeply held philosophical, political and religious beliefs, the academic debate over it has been unusually bitter. Lawrence Wright describes its history as “one of the most appalling chapters in science”, twin studies having been used in their time to justify Galton’s aristocratic notions of the natural worth of the English upper class, the evil experiments of the Nazi, Josef Mengele, and racial injustice in America. “Today,” says Lawrence Wright, “few on either side of the debate between geneticists and environmentalists would argue that we are exclusively the creation of nature or the reflection of nurture. The discussion has evolved into a statistical war over percentages.”

It is as if there were a vast football pitch on which the opposing teams were struggling to push the ball over the half-way mark and thus claim victory for their side. On the evidence given here, the geneticists are pushing towards goal. There has been nothing on the environmental side to counter the power of twin and adoption studies.

But the picture is not clear-cut. The position is perhaps best summarised by the phrase “genes drive experience”. In other words, we are genetically programmed to experience life in a certain way, which in turn affects the way people treat us, which in turn becomes our environment . . .

Lawrence Wright, a staff writer with the New Yorker, takes the reader on an entertaining and fair-minded tour of some pretty complex research. What, at the end of it, becomes of our fond belief that we have made the choices that have made us what we are? And what does the evidence say to us as parents? The news is both discouraging and liberating. We may not be able to do anything about the fact that our daughter is a compulsive worrier and our son a feckless ne’er-do-well - they were probably born to be that way. But equally, it probably doesn’t matter that we are irritable and never spared the time to brandish flash cards before their baby eyes. They would have turned out just the same. All that seems to be needed for children to realise their genetic potential is fairly basic comfort and care.

In one or two areas, however, as common sense suggests, the home environment does seem to make a difference. Lawrence Wright cites evidence that, although a tendency to alcoholism is probably inherited, children of alcoholics adopted into families where drinking is not a problem rarely become alcoholics themselves. And, while a tendency to anti-social behaviour is inherited, a criminal family environment is far more likely to turn that into criminal behaviour.

So we can keep our children off the booze and out of gaol. That’s something.