A more subtle approach involves a comparison of the degree of similarity of identical twins with that of frater-nals. As both kinds of twin are brought up within the same household the extent to which they share an environment is, or so it appears, the same. Any greater resemblance of identical twins to each other must then, it seems, show genes are involved.

This approach could be powerful but has its own problems. Both types of twin are brought up together, but identicals may copy each other's behaviour. That makes them appear similar for reasons unconnected with biology. The fact of being identical twins — perhaps with similar names and dressed in identical clothes — may predispose to mental disease.

One of the fundamental assumptions ol twin studies — that identicals and non-identicals differ only in ilu- extent to which they share genes — is not always justified. Lite before birth can be tough; and more so for identic;)) twins than anyone else. Many illnesses of adult onset — heart disease being one — are, at least in part, due to difficult conditions in the course of the pregnancy. Twin pregnancies are always more of a strain than those of a single foetus. As a result, a shared and hostile environment may impose more similarity on a pair of twins than expected on genetic grounds.

Identical twins come in two forms. AH arise from the splitting of an early embryo. Some are mirror-images of their sib; to look at one is to see the other in a mirror. Such individuals divided quite late in development, when the left-right pattern of the embryo had been set down. A late split increases the chance that the twins will share a common placenta and will have to fight for a share of their mother's blood. Such twins survive much less well than those who have a placenta each and the survivors are born ten days or so earlier than those with a less difficult time before they are born. So intense is their struggle that one may steal blood from the other, so that one grows up large while the other is small and anaemic. To complicate matters further, some non-identical twins — no more similar in their genes than are brothers or sisters — also exchange cells early in development, so that each is a chi-maera, made up in part of their sibling's tissue.

Nevertheless, to compare the two kinds of twin has had its successes. Members of a pair of identical twins are twice as likely to suffer from coronary heart disease than are those of a pair of fraternals. For diabetes, the figure is five times. Even tuberculosis is shared to a greater extent between identical than fraternal twins as a hint of an inherited susceptibility. Other characters, such the age when a baby first sits up, are also more similar for identical twins.

The argument about nature and nurture is of more than scientific interest. It has been rehearsed endlessly by those with one or other political axe to grind. Genetics once used an axe sharpened in the fires of Social Darwinism. Now that it has hit the headlines, there is a new acceptance of biological theories of human behaviour. Arson, traditionalism and even zest for life have all been blamed on the DNA. The nineteen-sixties were the decade of caring and a child's inability to concentrate was blamed on poor teachers. Then there was the 'working-mother syndrome' in which a parent's absence was held to be at fault. Now some psychologists have invented a whole series of behavioural ills coded in the genes while others place renewed weight on friends (and not parents) as the main agent of a child's development.

Psychology's obsessive need to dissect biology from experience is alive, well and as simplistic as it ever was. One study finds that students with hay fever are unusually shy. This proves that 'there is a small group of people who inherit a set of genes that predispose them to hay fever and shyness'. That is naive; but family and adoption studies do suggest that some aspects of personality, from introversion to the speed of response to a sound has an inherited component. From there to the discovery of any genes involved is a long step; but psychologists have not been shy about taking it. Announcements of the discovery of single genes for manic depression, schizophrenia and alcoholism have all quietly been withdrawn.

One form of behaviour has always raised passions about gene and environment but makes an excellent case that genetics and social attitudes ****- little to do with each other. Homosexual attraction is almost universal at some time in every lifetime. Some people continue in prefer their own sex. Exclusive homosexuality is a convenient subject of study for those interested in the genetics of human conduct as it is easy to identify, quite common, and no longer much concealed.

One study of American male homosexuals hinted at an association between such behaviour and a group of genes near the tip of the X chromosome. First, the brothers of gay men were more likely to be gay than are males in the general population. This does not in itself say much, as brothers share an environment as well as genes. However, gay men's relatives on the mother's side were more liable to be gay than were those on the father's, suggesting that the trait is passed through females. Again, this is not proof of an innate predisposition (even if it implies a possible gene on the X chromosome). The best evidence seemed to come from the X chromosomes of pairs of homosexual brothers. Most who took part in the study shared a particular segment of DNA towards one end of that chromosome. Somewhere in its hundred or more genes may, it was suggested, be one that inclines some carriers to that form of sexual behaviour.

After an initial burst of publicity, the result proved hard to replicate (as is often the case for such characters, in which different genes might be involved in different families) and the simple idea of a 'gay gene' is now dismissed. Whatever the science, the main interest lay in the response by some — but not all — of the gay community. Many, it transpired, were happy to use biology as a justification for their way of life. The idea that sexual preference was inherited meant, they concluded, that sexuality was not contagious and that battles by bigots to dismiss homosexual teachers were not justified. More important, it gave a welcome sense of separation: of a shared difference that was present for reasons beyond individual control. All this disconcerted the many biologists who had spent years fighting the idea that sexual preference, crime or poverty are inborn and cannot be altered by social means.

This new hereditarian orthodoxy, like the old liberalism, asks too much of biology. It echoes a forgotten dispute of the 1930s. The German geneticist Theobald Lang claimed to have found that the sisters of gay men had somewhat masculine characters, and that male homosexuality might therefore be inherited. Whatever the accuracy of his claim that hint of a 'gay gene' gave rise to two quite opposed (albeit equally logical) responses. The Nazis, needless to say, took the brutal view: 'they are not poor sick people to be treated; they are enemies of the state to be eliminated!' In contrast — and faced with the same information — the socialist medical association (then in exile) wrote that 'Homosexuality is inborn and not subject to the free will of the individuals who come into the world with this inversion. The laws against it should be abolished.'

Like some members of today's gay community, both left and right felt that if that behaviour was innate it must be outside the control of those who display it. Each political group saw its response — eugenic sterilisation versus liberal legalisation — as consistent. Neither asked what is meant by a gene 'for' something, homosexuality included. The story of the German 'gay gene' points up the irrelevance of genetics to political opinion. Whatever inherited basis a character may have, preconceived views about its merits will not be changed by science.