The notion that the rules of attraction are influenced by biology gained fresh support this week. Testosterone levels are well known to affect aggression and assertiveness, but they also help to shape the face: the hormone thickens the jaw and broadens the nose. Research led by Lynda Boothroyd of Durham University has shown that women use these traits as a clue to men’s characters.

Although social factors are obviously important to mating, nature also plays a part. This is starting to be accepted more widely: most people understand it would be odd if evolution had not influenced sexual tastes. Another of the week’s scientific publications, though, has made a more controversial proposition, that biological influence on behaviour is not limited to sex. Our genes may also sway our choice of friends.

This conclusion comes from a twin study led by Kenneth Kendler of Virginia Commonwealth University. As identical twins share all their DNA while fraternal twins share only half, comparisons can tease out the relative contributions of genes and the environment to behaviour. Kendler used this to investigate how people pick their peer groups, and found that genetics has a surprisingly large role.

While fraternal twins each tended to make different kinds of friends as they grew older, that did not apply so strongly among identical pairs. Their social worlds remained remarkably alike. As both types of twins shared childhood environments, this suggests that genes become more important as people grow up.

Genetic factors, indeed, were the strongest influence on the characteristics of the peer groups of young adults, followed by environmental factors that affected one twin but not the other. Shared family background came last: it was significant when the twins were children, but not by their late teens.

This looks counter-intuitive in the extreme. Nobody thinks genes are the sole root of personality, and thus of the kinds of friends we make. Even those who make the case for a genetic contribution accept that culture and the environment matter, too. Surely, as we grow older and have more experiences, these should become more important, and our genes less so? That, though, does not seem to be the case.

Many people find this dispiriting. They say it suggests we are prisoners of our genes, automatons formed by an arbitrary, inherited nature from which we cannot escape. It certainly threatens the postmodernist view that holds social conditioning to be vital to behaviour. A large role for genes, though, need not threaten the cherished idea that individuals are free to make their own decisions. It actually does the reverse.

What seems to be happening is this. When we are children, we are heavily influenced by those around us – our parents, teachers and peers – and tend to behave according to their social norms. But when we become teenagers and then adults, we gain progressive freedom to act as our own natures and temperaments dictate. We can and do throw off social pressures created by others when we find they do not suit us, and find it easier to carve out individual life paths of our own.

That genes should have a large effect on personality, behaviour and our choice of company should come as a relief to anyone who values individual liberty. It means we cannot always be conditioned by education or society to act as others want. The Jesuit maxim – “Give me the child for his first seven years, and I’ll give you the man” – does not always hold. Our genes allow us to be ourselves.

Mark Henderson is Science Editor of The Times

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