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Why is Sex Fun?: the evolution of human sexuality by Jared Diamond, Weidenfeld, pounds 11.99

The title of this small but perfectly formed text is eyecatching, but a bit of a con. Jared Diamond doesn't even mention orgasms. You won't find anything here about the evolution of sexual pleasure, in all its extraordinary variety.

Like most sociobiologists, Diamond is chiefly interested in the "norm" - the middle-class American couple with two children and the occasional affair on the side. He starts from the perspective of the family dog: Diamond's mission is to explain why our species goes in for "recreational sex", when lots of other animals only spend any time on sex when there's a good chance of making babies.

His general argument will be familiar to a generation raised on Richard Dawkins' Selfish Gene. Men like sex because they are programmed to spread their genes around. Women are more wary, afraid of ending up pregnant and single. But women have recreational sex with their partners in an often vain attempt to try to hang on to them.

In Diamond's world view, the battle of the sexes is hard-wired; a conflict of genetic interests is the "cruel fact" of life. In this storyline, women get left holding the baby because their biology forces them to invest more in their offspring than the male does. While his partner is pregnant, a man can easily produce enough sperm to fertilise every one of the world's two billion reproductively mature women. "That's the evolutionary logic that induces so many men to desert a woman immediately after impregnating her and to move on to the next woman," concludes Diamond.

One-night stands would be the norm, Diamond opines, were human babies not such a handful. Women need help rearing children, as Diamond, father of twins, knows only too well. So women have evolved sexual strategies to keep the men interested. They give no overt sign of fertile periods - the phenomenon of "concealed ovulation".

But if men are evolutionarily primed to stick around, albeit reluctantly, why don't they do something useful, like helping with the breast-feeding? The recent discovery, in Malaysia, of male fruit bats nursing their offspring, shows that lactation is not a physiological impossibility for male mammals. Diamond raises the question, but doesn't really have an answer, other than to conclude limply that it must not be in a man's genetic interests to do so, given that the woman will do the job, and that the babies might not be his anyway. Bizarrely, he suggests that expectant fathers might contemplate having "some combination of manual nipple stimulation and hormone injections" to activate their latent ability to lactate, once they have had their "confidence in paternity buttressed by DNA testing".

At times, Diamond's arguments have more than a passing resemblance to just-so stories; it is possible to dream up an evolutionary explanation for just about anything. OK, so the human penis is several inches bigger than a gorilla's or an orang-utan's. But is the human appendage really so big as to constitute a handicap to its owner, comparable to a peacock's tail? Diamond argues that the penis is a costly and exaggerated "signal of virility" because it consumes valuable tissue that could otherwise form extra brain cells. "In effect, a man is boasting, 'I'm already so smart and superior that I don't need to devote more ounces of protoplasm to my brain, but I can instead afford the handicap of packing the ounces uselessly into my penis.'"

The only thing stopping its continued expansion is the unfortunate need to fit into a woman's vagina, says Diamond. You wonder why he doesn't propose a comparable drive towards ever bigger vaginas. After all, he tells us, "every woman knows" that she must "compete intensely" with other women to get "one of the few high-quality men", who will presumably be well-hung.

Why Is Sex Fun? is an excellent introduction to the conceptual world of contemporary sociobiology. With his usual verve and style, Diamond ransacks anthropology and field biology alike to find examples that suit his purposes.

He recounts the self-seeking exploits of big-game hunters among the Ache people of Paraguay, and delights in the massive penis sheaths of New Guinea tribesmen. He tells us about the philandering of the male pied flycatcher, and the sexual tyranny of female phalaropes - shore birds in which the larger female pursues the male, who ends up tending the eggs while she pisses off to look for another sucker.

After reading Diamond's book, I'm still not sure why sex is fun, though I can certainly see why sociobiology is. But whether this approach provides profound or even useful insights into the human condition is quite another matter.