BOOK REVIEW / Genes that are designed to turn you on: 'The Sexual Brain' - Simon LeVay: MIT Press, 14.95

IN THE perpetual state of frothing-at- the-mouth that afflicts some parts of the media, one cause of outrage is the moral, ethical and social fallout of the suggestion that homosexuality may have a biological cause (perhaps genetic) and is therefore a condition no different from being right-handed or having red hair. In the resulting frenzy of assertion and prejudice, scientific facts have been notably absent - and they are precisely what this book provides.

Simon LeVay's original interest was in how, neurobiologically speaking, the brain integrates input from the two eyes into a single view of the world. But work on the hypothalamus led him to publish a paper on the neuronal difference between the brains of the homosexual and heterosexual men he'd studied. That led to further experiment, and thence to this book.

The Sexual Brain sets out to understand sexuality in terms of the cellular processes which generate it. LeVay vehemently dismisses the way the subject has been 'studied' in the past - by talk alone, as if it were philosophy, not science. He is splendidly rude about Freud, whose self-proclaimed 'scientific' methods were flawed (LeVay says) by preconceptions and inadequate experimental technique, and led him, first, to claim that homosexuality was a pathological result of bad parenting, and thence to the notion that the condition was 'preventable', so that if it persisted someone, the child or the parents, must be 'guilty'.

In 13 crisp chapters LeVay sets out the current state of knowledge and research on the neurobiology of sexuality. He covers the evolution of the sex instinct, the regional specialisation of the brain, the neural functions which control sexual behaviour, the causes of the differences between women and men, the nature of gender identity, and - the heart of the whole book - the biological mechanisms which may contribute to sexual orientation.

LeVay's style is lucid, friendly and comprehensible. Its US origins show in occasional over-explicitness (explaining that a joiner is 'a maker of small wooden articles') or in Disneyish metaphors like the claim that, to most brain scientists, the hypothalamus 'houses some witches' brew of slimy, pulsating neurons adrift in a broth of mind-altering chemicals'. His arguments for and about homosexuality become a touch obsessive. But the topic of sexual orientation has been so consistently neglected by scientists, and is so infested with lay incomprehension and old wives' tales, that that any presentation of objective results is welcome. This book makes a useful start.