Why is Sex Fun? The Evolution of Human Sexuality

  • Jared Diamond
BasicBooks/Weidenfeld and Nicolson: 1997. Pp.168 $20, £11.99<)

“Recreational sex and menopause were as important for our development of fire, language, art and writing as were our upright posture and large brains.” Jared Diamond sets up a major claim at the start of Why is Sex Fun?. He then discusses not only recreational sex, but the evolutionary reasons why men philander more than women, whom it pays to desert first, why men don't breastfeed even though they could (and if in vitrofertilization keeps producing twins, perhaps will yet do so), and many other intriguing thoughts, ending with the overgrown human penis.

The author assumes no prior knowledge. Little of the story will surprise readers of the literature, or of Diamond's wonderful Third Chimpanzee (HarperCollins, 1992). But it is beautifully told, with male breast-feeding introducing a primer on embryonic sex differentiation, menopause one on life history strategy, and concealed ovulation the principle that evolution works on what is there.

Concealed ovulation and sex for fun may well have changed over time from the ‘many-fathers’ function, with troop males bamboozled into tolerating every infant as their own, to the opposite ‘daddy-at-home’ function of tribal males' paternal confidence, directed child-care and frequent sex with a loving — maybe even monogamous — wife.

Diamond's recurrent ploy is to present his case, then add, “By now you are probably objecting that…”. This lets him deal with counter-arguments while flattering the reader, who actually had only a dim feeling that there must be a catch somewhere. This book is simply written, but not simply thought out — it is a good book to leave around, half-hidden, to tempt teenage offspring to become biologists.

But what of Diamond's first big claim? He makes a clear case that monogamy within a community group, sex in private, menopause and our high parental investment are unique among our close primate kin. He does not quite sum up: there should be two pages more to pull the scenario together. Does he think that our unique sexual behaviour is itself what needs explaining, or is it an explanation?

Does he see a bipedal male australopithecine striding along like one of his New Guinea trackers, head high and hands free for defence against the genocidal males of the next group? And the female finding safety and paternal care more and more from a single mate as she evolves into early Homo, and focuses her extended sexual attractiveness from the many to the few, in private?

If this mating system accompanied the earliest expansion of the hominid brain, it would have provided a way for extended parental care to foster the slow growth of big-brained children (a familiar thought), while allowing an almost indefinitely expanded social group, which in turn would have demanded the brainpower of an expanding neocortex.

Diamond adds a factor much less often considered than the needs of children: the value of the old. A toothless crone who still recalls the famine food she ate after the hurricane of 1910 becomes both the tribal university library and their spaceship survival manual. Are longevity and menopause not just the result of bigger brains? Perhaps they evolved simultaneously or were even prerequisites? And how does this accord with Diamond's own preference for a ‘big bang’ emergence of language, art and religion a mere 30,000 years ago? Perhaps he should now write his missing two-page scenario.