A"social atom bomb" - Look magazine's response to the publication of Sexual Behaviour in the Human Male 50 years ago - got it spot on. For the Kinsey Report blew sky-high America's cherished myths about its own sexual morality. A respected field biologist turned sex-researcher, Alfred C. Kinsey blasted readers with facts and figures to show that sexual acts standardly branded as wrong and unnatural were precisely what most men (and, it followed, many women) actually did.

In the course of an 800-page book derived from thousands of case histories, Kinsey revealed that over 90 per cent of the men interviewed had masturbated, 85 per cent had had premarital coitus, around 40 per cent had engaged in extramarital affairs, almost as many had had some homosexual experience - and so on, down to the 17 per cent of farm-hands who had buggered livestock. More than 95 per cent had had sexual "outlets" which were immoral or illegal.

Kinsey's revelation that American men were so encyclopedically defying conventional morality set off a shock wave, especially as his findings were not to be dismissed out of hand. After all, the Kinsey Report seemed a model of empirical inquiry. The Indiana University professor and his tireless team had clocked up some 18,000 face-to-face interviews with Americans of all sorts - black and white, college students and convicts - processing it all via the best punch-card system of the day. His was, beyond dispute, by far the largest empirical survey on the sexual conduct of any nation ever. America was sold on science, and the Dr Gallup of the orgasm had bared the secrets of its bedrooms.

Pillars of orthodoxy could of course respond that they had known it all along: in an era of movies and Mae West, wasn't it obvious that sin was rampant? The trouble was that instead of condemning this deluge of depravity, Kinsey seemed to be endorsing it. Indeed, despite the sober presentation of all those statistics, Kinsey clearly had a moral agenda of his own. The man in the street was doing what came naturally, and Kinsey was apparently giving science's blessing, no matter what form it took. There was nothing wrong with masturbation, and "it is my conviction", he wrote, with commendable courage, "that the homosexual is biologically as normal as the heterosexual". So much pain, shame and harm had been caused by the Christian gospel of sex hatred and denial; now the libido was to be liberated.

In particular, the Kinsey Report condoned what many considered its most shocking finding. Far from there being clear blue water dividing "straights" (those engaging in heterosexual genital intercourse) and "perverts", there was a continuum. Some or all of the time, most men performed sexual acts that transgressed what was authorised by the pulpit, by physicians or by the "hygiene" courses springing up on campuses. What was truly normal was diversity - a conclusion confirmed by the parallel study of female sexuality five years later. In exposing the yawning chasm between what was preached and what was practised, Kinsey trusted that scientific knowledge would breed tolerance and humanity. It was for this that he would later be damned by Billy Graham as the man responsible, more than any other, for the demoralisation of America.

Previous biographies have spun a familiar narrative of how the Indiana entomologist almost by accident turned into the great American sexual pioneer. Trained in biology, Kinsey became a specialist in Cynipidae, an insect which nests in oak galls. For 20 years, he had tramped the length and breadth of the country, collecting 52,000 such gall wasps, writing the definitive monographs in hopes of becoming "a second Darwin". Then, seeing that he knew all about the birds and the wasps, he was recruited to help teach the campus "marriage course". Shocked by the shortcomings in sexological science, he shifted his research interests; moving from wasps to Wasps, he trained his entomological fieldwork methods on to human biology: collect, measure, count. Unlike psychoanalysts and doctors who interpreted sexual variations as symptomatic of disease, Kinsey thus approached sex as a naturalist. His uniqueness lay in bringing scientific detachment to sexual research.

That conventional telling never sounded fully convincing, and James Jones's new biography shows it concealed more than it revealed. As Jones now exhaustively documents, Kinsey took to sexual research because he was driven by inner demons: alongside doing science, doing sex became his raison d'etre; probably the two became indistinguishable.

A Hoboken boy like Frank Sinatra, Kinsey grew up amid a rigid Protestant background. He was long "repressed"; he dated his first girl at 26, and went on to wed her, though the marriage was not consummated until months afterwards. He thus had powerful personal reasons to feel that a conspiracy of silence and repression had threatened to reduce him to a sexual cripple.

Not least, he had erotic secrets. Jones has discovered that in his teens Kinsey had developed masochistic masturbatory habits, while his involvement with the Eagle Scouts and the YMCA permitted some expression of his hankerings after adolescents. His feelings of being damaged and different, and his rage at the guilt he had suffered - all this fired him up to become a sexual crusader.

Kinsey's sexual persona had many strands. He battled against public sexual hypocrisy with the weapon he knew best: hard science. He needed to prove to himself that he was not "deviant" but "normal" - that is, like the two-in-five other American males with at least a gay streak. He also demanded what he came to regard, in an almost Benthamite way, as a basic need: sexual fulfilment. Not least, sexual investigation became his great turn-on. Kinsey - so Jones was told by many informants - got a kick out of delving into everyone else's sex lives. From interviewing students on campus he proceeded to slumming in the gay zones of Chicago and New York, seeking contacts; one voyeuristic thing led to another, and in time Kinsey was gawping at volunteers copulating or masturbating. (Did sperm spurt or dribble? Two thousand onanists had to be filmed to find out.) Finally he became involved as a participant observer, satisfying his homosexual and exhibitionist tastes. Kinsey particularly enjoyed exposing himself and even masturbating before his colleagues. He proceeded to bed almost all his own staff, male and female alike, and encouraged consciousness-raising office bed-hopping at what became the Kinsey Institute (of his right-hand man, Wardell B. Pomeroy, it was said: "he just ****s everybody"). It was anything goes, the only proviso being that the Chief had to give his personal ok. Some of this was immortalised on celluloid, for research purposes.

Kinsey displayed all the crazy frenzy of a secular evangelist. It was as though the last taboo had to be destroyed, the demons of repression finally exorcised. It was also, Jones shows, a rather desperate attempt, as in time Kinsey grew more embattled, more tormented by critics, more bemired in academic politics, more fatigued, irritable and insomniac, to rejuvenate his capacity for sexual release. His masochism had started in a fairly mild way - for instance, as a teenager, inserting a straw into his penis. Pipe-cleaners were to follow, and in the end he would truss up his scrotum with cord and ram a toothbrush up his urethra, bristles first, to provoke ejaculation. It is to Jones's great credit that he has elicited all this information, hitherto undisclosed.

Kinsey's sexual cavortings on and off campus make John F. Kennedy, Bill Clinton and Piers Merchant seem like paragons of discretion. Had the public got wind of it all, his work and name would have been vilified. So why did he run such risks? Perhaps it gave him a thrill, and evidently he convinced himself that nothing done in the name of science and liberation could be wrong. In any case, Jones suggests he was able to dismiss any fears from his mind, since his was a personality which was absolutely controlling. Whatever he was doing, be it counting insects or orgasms, he had an obsessive need to establish mastery; an autocratic workaholic, he regulated every detail of his research, career and personal relations. He had hand-picked a staff who became willing slaves. As a result, on a good day he could feel in charge.

Perhaps like an abused child or a partner in a masochistic relationship, his wife Clara idolised her "Prok" (Professor Kinsey) or at least was wholly compliant in the complex sexual games he played, sleeping with his lovers and serving persimmon pie at his orgies. To cap it all, Kinsey hit upon a brilliant wheeze for cowing outsiders and the press: he administered to them his sexual questionnaire and so became privy to their secrets.

Kinsey insisted that an individual's sexuality was not genetically or oedipally moulded, but was a chance product of habits formed in youth; whatever happened to give pleasure or pain then stuck. Jones takes over this model and applies it to Kinsey's own personal development from his New Jersey childhood onwards. His early years established a puritan work ethic, faith in science, a collecting mania, and a compulsion to impose the order which alone would distance and protect himself from his bullying, denying father. These habits outlasted the switch from scouting to sex missionary. Kinsey was no doubt a despot, but had he not been so domineering and megalomaniacal, would his audacious project ever have seen the light of day? Only an intense, uptight, armour-plated zealot could have withstood all the brickbats - from evangelists, from scholars who condemned the crudeness of his statistical methods, and not least from McCarthyites who accused him of launching a communist plot to destroy the family. A more open man might have devised subtler research than orgasm-counting, but such a person would probably have produced nothing at all.

It is tempting to see some similarities between Kinsey's labours and those of his biographer. Jones tells us that he has been working for years on this book, revealing a truly Kinseyan irrepressibility in tracking papers and questioning survivors. While with interviewees Kinsey never took no for an answer, his biographer, regrettably, has not been quite so lucky. He has been denied access to the Alfred-Clara correspondence, and most of Kinsey's children refused to cooperate - one would have loved to know if they hated their father as much as he hated his. (And did they know - did they actually witness - the orgies in the attic?) Kinsey was a non-stop talker, and one sometimes wishes Jones had had more of a mind to concision. Kinsey's labyrinthine dealings with the National Research Council and the Rockefeller Foundation are doubtless momentous but Jones's day-by-day account makes for fatiguing reading. And though Jones has many a neat turn of phrase, he can also be wordy and labour the obvious. Do we really need to have it explained to us, at some length, how at scout camp "boys told dirty jokes, recited risque poems, composed lewd limericks, made up obscene verses to songs, and bragged about their sexual experiences"? Do we then further need to hear that in such environments "middle-class boys of Kinsey's generation masturbated because their compulsion to do so was stronger than their will to abstain"?

The demands of brevity might have eliminated occasional sloppiness. "Kinsey wanted his marriage to be different from the one he had witnessed growing up, as he was determined not to behave like his father," Jones tells us; but then a couple of pages later we get: "Kinsey's views on marriage bore a remarkable resemblance to his parents' relationship". So which is it?

Occasionally one wishes Jones had been more of a Kinseyan in interpreting sex. Whereas "Dr Sex" always stressed the protean nature of the libido, Jones repeatedly tells us point blank that Kinsey was "a homosexual", and that "by late adolescence, if not before, Kinsey's behaviour was clearly pathological". These are far from adequate designations for a man who had a protracted relationship with his wife, siring four children, who bedded other women, and who seems to have embodied his own notions of pansexuality.

Despite its vastness, Jones's book also has some unfortunate omissions, especially regarding the big picture of sexological research. We are assured that Kinsey far outstripped the other contemporary American sex researchers, yet little information is given to support this judgement. Just how different, just how original was Kinsey's own work? Equally absent is any engagement with recent scholarly thinking (Foucault et al) respecting Kinsey's historical place in sexual science. For this, the reader must turn elsewhere, to Vern Bullough's Science in the Bedroom (1994), and Paul Robinson's The Modernisation of Sex (1977), still a gem.

Yet it would be mighty mean to end on a negative note. Over the years gossip has had it that there was more to Kinsey than the self-promoted image of the dispassionate scientist. Such rumours have now been substantiated down to the last toothbrush. As opposed to the earlier anodyne portraits, Jones's biography - forthright, scholarly, original, magisterial - constitutes an "atom bomb" of its own. Its fall-out will be enduring.

Roy Porter is professor in the social history of medicine, Wellcome Institute for the History of Medicine, London.