Granny knows best

THE MORAL ANIMAL by Robert Wright, Little Brown pounds

WHEN I was growing up, I was occasionally taken to visit one of my two French grandmothers who lived in Paris and, perhaps for that reason, saw herself as something of an expert on men. Mami Simone, as she was known, liked to dispense advice over a small pastis.

"You talk too much," she'd say. "Don't. Men prefer women who are modest. Always have several admirers at once, and never sleep with a man until..." She would pause, and count on her fingers. "Until you've been on one, two, three dates."

It was the sort of admonition that made me want to thump her. I preferred Mami Giselle, my other grandmother who wore a turban and was a sculptress in Africa. Never mind that she married three times and her love life was miserable. Mami Simone, I thought, was a bit of a fruitcake. Like most teenagers, I simply knew I was right.

Turns out, it was probably Mami Simone, after all, who was right. Or something like Wright.

Robert Wright comes from an honourable tradition of scientific thinkers - which includes the Californian physiologist Jared Diamond and our own Richard Dawkins - who are adept at using words or television to popularise new theories that would otherwise remain dry and unreachable. In The Moral Animal, Wright takes a look at the fashionable science of evolutionary psychology, or the study of "why we are the way we are" (the subtitle of his book). Every aspect of human behaviour, he argues, has a genetic base, from a guilty conscience to our enduring belief in romantic love. Wright uses the life and work of Charles Darwin - his upbringing, marriage, children, his letters and scientific writings - to re-examine the work of the social scientists and evolutionary biologists.

Like them, he starts with sex, which leads him to some startling observations, many of which my grandmother would find quite sympathetic.

Between us and the australopithecines, who walked upright but had an ape-sized brain, stand a few million years, no more than 200,000 generations. Over that time, human sexual behaviour has evolved in remarkably similar fashion the world over. Yet we don't always understand why. One of our sexual characteristics, for example, is that we prefer to copulate at night and in private. Just imagine the confusion if, during a board meeting, two of the participants were suddenly to take off their clothes, climb on the table and have sex amid legal notepads and glasses of water. That is what most animals do in the wild. Why not us?

In the scale of primates, the mostly-faithful human race comes somewhere between the orang-utans, who are known occasionally to choose a mate by abducting and forcibly subduing her, and a species of gibbon whose sense of fidelity would make Mary Whitehouse proud. Both male and female start the day by singing a loud duet, pointedly advertising their familial stability to any would-be home wreckers.

If observing comparative behaviour is one way of assessing sexual fidelity, so is weighing testicles - or, more accurately, measuring the ratio of testes weight to body weight. Chimpanzees and other species with high relative testes weights are also the species in which females are most promiscuous. When females mate with many different males, competing hordes of sperm have to do battle; sheer volume may determine which male gets his DNA into the egg. Human relative testes weight falls between that of the chimpanzee and that of the gorilla, which means that we girls, while not nearly as wild as female chimpanzees, are still quite adventurous by nature.

Women are inherently more faithful than men because males, genetically at least, need to spread as much seed around as they can to give their genes the best possible chance of getting into the next generation. Men can reproduce hundreds of times a year, if they find enough women willing to help out. A woman can only reproduce once a year. For women, therefore, the be-all and end-all is not fertilisation but helping their precious eggs grow - hence a preference for nurturing over cuckoldry. But does that explain why, as my grandmother endlessly repeated to me, men are always eager that you should play hard to get? Absolutely.

Giving birth to a child involves huge commitment, not to mention energy, for a woman. And since nature has put a ceiling on how many such enterprises she can embark on each child is, from a genetic standpoint, an extremely precious gene machine. So it makes sense for a woman to take her time about selecting a man to help her build this gene machine.

Darwin - a true Victorian - believed that women were precious because they were coy. He was wrong. Females are inherently precious because of their slow rate of reproduction. Their coyness is the result of natural selection having understood that. The Moral Animal overturns some old ways of thinking and explains others. It is a book my grandmother would have crowed over with delight.