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In sexual reproduction, each new individual begins when a sperm and an egg unite at the moment of conception. In humans, this momentous cellular encounter takes place inside the female (save for a few very recent, laboratory-assisted exceptions). If the single new cell achieves full gestation, birth and growth to puberty, dividing enough times to create an organised assembly of perhaps 10,000 billion cells, it will have acquired a great many attributes not present at conception. Some, like the urge to contrive further unions between sperm and egg, will be common to other sexually reproducing species. Others, shared only with closer animal relatives, will include the ability to build a wide repertoire of behaviour around the actions which, biologically, originated in the requirement for such meetings to take place. And uniquely human will be the construction of a rich range of views on the merits, morality and meaning of such actions.

Hardly any of these views will be uncontentious. The first sentence of this review will pass, I think, as an incontrovertible biological fact. But being able to recognise it as such is a considerable, and contingent, historical and cultural achievement. And there are plenty of disciplinary standpoints from which the reductionist cast of this sketch misses the point, if not being downright objectionable.

So while these three books are all about knowledge of sex, they each mean something different by knowledge. The two behavioural ecologists, Robin Baker and Mark Bellis, give an exhaustive account of their attempts to extend one aspect of scientific knowledge, by asking why there is normally a single human egg available for fertilisation but the male furnishes several hundred million sperm at a time.

Their work is a new addition to the story Vern Bullough tells of the development of "sex research" from its origins in the 19th century up to the present. Here, knowledge is a gradual accumulation of the findings of the biological and behavioural sciences. Bullough is a social researcher on sex himself, a participant in his own story, which he tells with commendably few whiggish moments. Roy Porter and Lesley Hall, on the other hand, social historians, are less concerned with the acquisition of scientific knowledge than with the conditions under which ordinary people come to know what they think they know about sex. Unsurprisingly, this is a more complex picture, not least because they try to cover three centuries of sexual knowledge - or knowledges in the usage the text sometimes adopts.

It is also more complex because getting to know about sex has never been straightforward. Porter and Hall focus on what we now call sex manuals. They offer a very detailed and sophisticated elaboration of the obvious point that such texts are designed to constrain as well as enable. They are always prescriptive as well as descriptive, condemnatory as well as comforting. This is as true today, when every other magazine on the newsagent's shelf offers tips on improving sexual performance, as when Nicolas Venette's Tableau de l'amour conjugal - a bestseller in 18th-century England - was struggling "to square the physiology of sex with the narratives of love".

Together, the three books illuminate both the fragility of the conditions under which something approaching objective knowledge of sex can be developed by specialists, and the obstacles which impede wider discussion of their findings. The possible scope of sexual discourse is always filled with things which some decline to acknowledge, or actively repress. Suggest inquiring into any particular topic and the possible responses include at least four levels of refusal: to admit that it exists; to acknowledge that it might be a fit subject for investigation; to admit that the findings might be valid; and to permit their general publication, outside the circle of specialists.

All of these refusals can be seen in Bullough's account and, in less detail, in The Facts of Life. Bullough also illustrates the many other, technical difficulties in creating sexual knowledge. Science in the Bedroom is, of course, a contradiction, though not one he explicitly acknowledges. Sex in our culture normally happens in private spaces. The bedroom, by definition, is where science isn't. Turning individual experience of sex into reliable data - carnal knowledge into common knowledge - entails bringing sex into the laboratory.

Up to a point, there is a progressive story to tell about that, at any rate for the physiology of heterosexual intercourse. The half century from Havelock Ellis in Britain to Masters and Johnson in the United States saw a development from asking friends and acquaintances to write their sexual auto-biographies to inviting volunteers to be wired up for sex under the scientists' gaze.

Now that the results of the latter are widely familiar, it is more difficult to appreciate what a departure even Ellis's highly indirect methods were. As Porter and Hall put it: "The idea, naive though it might seem, that honest accounts by ordinary persons of their ideas and feelings about sex might contribute to discovering 'the truth' about the subject surely represented a democratisation of knowledge, and a move away from religious or medical prescription."

Establishing the same progression for behavioural data is harder. There is no reason to suppose that a woman who agrees to masturbate to orgasm with a transparent phallus so that a scientist can film her vaginal muscles has physiological responses any different from those who might balk at the suggestion. But it may be that when interviewed about her more private behaviour she is less representative.

One result is an abiding asymmetry in the quality of data. So far as the mechanics of the business go, there may not be much more to learn. Baker and Bellis's concern with sperm competition, the idea that at least some of the time sperm from more than one partner may have a chance of fertilising the same egg, means that they want to know how many sperm stay in place - not so much science in the bedroom as the science of the wet patch in the bed. And they apparently have little difficulty in persuading couples in Manchester to hold a beaker ready to catch any fluid leaving the vagina after intercourse - in pursuit of measurement of "flowback". This is one part of their comprehensive study of what happens to all those sperm inside the the egg-bearer.

But when it comes to general patterns of behaviour of people, rather than spermatozoa, they have to rely on responses to a survey published at their behest in Company magazine. They want to explain a wide range of human physiology and behaviour as adaptation to sperm competition, the selfish sperm as a successor to the selfish gene. But the data on which they base their estimate of the number of human conceptions which might arise in this way (around 4 per cent) are so much weaker than their strictly biological findings that they tend to undermine their sociobiological project.

Nevertheless, the beaker-bearing volunteers perhaps testify to a lasting effect of sexual science. One striking thread running through the narratives of all three books is a developing mutual aid between investigators of the sexual life and their subjects. It is not just that many researchers were also therapists and advice givers, though they often were. Marie Stopes is a key figure for both Bullough and Porter and Hall, as also for Cate Haste in her recent account of British history in Rules of Desire. And Stopes's vast correspondence contributed to her own knowledge at the same time as it helped her correspondents, and the many readers of her books.

The books in turn helped spread the idea that systematic study could yield information worth having for the many who had no direct contact with the researchers. One can only speculate about the extent to which this made succeeding generations more amenable to requests from researchers, but it seems plausible that as people have found more "scientific" accounts helpful in understanding their own sex lives, they may have been more willing to contribute to further scientific investigation.

At the very least, they may have become more interested in the results from new work. The distance between Ellis and Masters and Johnson is also one of acceptance of discussion of their findings. When Ellis's Sexual Inversion was seized by the authorities in 1898, an episode which both the historical books here relate, the Lancet was able to admit that the subject could be discussed, that a "book written solely in a spirit of scientific inquiry" could not be "indecent literature". But it made clear that such works should only be on sale to scientists and medical men, not the public at large.

Now, the public at large has ready access to academic findings, whether in reading about The Joy of Sex, as Alex Comfort has it, or The Magic of Sex, in Miriam Stoppard's version, complete with tastefully coloured graphs and diagrams. And the popular, though very sober, version of the recent Wellcome Trust-funded national behavioural survey in Britain was an immediate paperback publishing success.

None of this means that the acquisition of personal sexual knowledge is not still problematic. Each new generation starts from ignorance, and each male or female can only know the sexual experience of the other half of humanity at second hand. How far each of us may get toward sexual knowledge, and how much we actually need for "doing what comes naturally" are as hotly contested as ever, as even a casual acquaintance with the tabloid press in the age of Aids reveals.

Reading Porter and Hall is a good reminder that there have been times when the balance of social power was firmly behind the creation of sexual ignorance rather than knowledge. And they might have done well to stick with "knowledges" throughout, to acknowledge more openly that knowing what people know is even harder to investigate than what they do.

But these books, and other evidence like the willingness of people - contra Margaret Thatcher's reason for blocking funding from the Medical Research Council - to answer all the questions in the British survey, do suggest some real gains from sexual science. It would be idle to pretend that research has ushered in a new age of sexual enlightenment. But at least the suspension of disgust as a scientific position seems to have carried over to some extent into social life. If disgust is gone, we are left with mere disapproval to regulate other people who choose not to behave as we do. That can still be a powerful force, but it is surely easier to live with.

Jon Turney teaches science communication, University College, London.