Why do men with everything pay for sex? Is it some animal instinct? Kathy Marks thinks not
ROBERT De Niro, who has threatened never to set foot on French soil again after he was questioned by the Paris vice squad about alleged links to an international prostitution ring, says, categorically, that he has "never in his life paid a woman for sex".

If so, the Hollywood actor is a rare example of moral rectitude in a world where fame and wealth go hand in hand with sleaze. Take Michael Douglas, plagued by so insatiable a sexual appetite that he checked himself into an addiction clinic. Or our own Hugh Grant, who fell from grace after his adventures with a hooker on a seedy Los Angeles street.

Hollywood has always had its share of call-girl scandals. In 1921, the American actor Fatty Arbuckle was charged with crushing to death a starlet during an orgy in a San Fransisco hotel. Political history, too, is full of such stories - John Profumo, Lord Lambton and Allan Green, the former Director of Public Prosecutions, are among the men who have courted disgrace by using prostitutes.

There are many modish psychological theories about why rich and powerful men - men who have it all, including their pick of beautiful women -- choose to risk everything for the sake of a furtive encounter. Escapism, boredom, self-destructive urges, the sheer thrill of such flagrant sexual transgression, to name but a few. And the greater the risk, the bigger the thrill.

It is a conundrum which splits the population according to gender. The appeal of anonymous sex leaves most women mystified; the majority of men, on the other hand, understand it on some level and place it in the context of the promiscuity versus monogamy debate. It is not a question of morality and will-power, they explain, but of a biological imperative that obliges helpless, well-intentioned men to spread their genes as widely as possible. Or, as a London cab driver might put it, it's human nature, innit?

The notion that homo sapiens, in his sexual behaviour patterns, is driven by evolutionary survival strategies was given credence by the American geneticist, Stephen Pinker, in his recent book, How The Mind Works. Pinker wrote: "The reproductive success of males depends on how many females they mate with, but the reproductive success of females does not depend on how many males they mate with. That makes females more discriminating. Males woo females and mate with any female that lets them. Females scrutinise males and mate only with the best ones, the ones with the best genes."

Yet if it all came down to crude biology, the jungle should be full of furiously fornicating birds and beasts. But even primates, our closest relatives, vary in their sexual habits. While chimpanzees are renowned for their promiscuity, gorillas stick with the same harem, and mate only once every couple of years.

In any case, the behaviour of animals in the wild gives few clues as to why the male of the human species not only seeks multiple partners, but also goes to the lengths of paying for sex. Despite the recent discovery by scientists in Antarctica of penguins who apparently exchange sexual favours for precious rocks and stones to build nests, prostitution is not thought to be prevalent in the animal kingdom.

Perhaps the conduct of ancient tribal societies could prove illuminating. Do the chiefs of the primitive tribes of Amazonia, for instance, seek out the choicest young girls for their secret delectation? Absolutely not, says Stephen Hugh-Jones, a social anthropologist at the University of Cambridge. Polygamy is the norm among headmen in the Amazon, but clandestine fumblings are not. The Tukano tribes are particularly prudish in this respect, although the Inuits of Canada and elsewhere, intriguingly, lend each other their spouses as a matter of courtesy.

"There is no real parallel in tribal society," he says. "This kind of sexual thrill depends on entering an illicit and squalid nether world. But there are no seedy red light districts in the Amazon. I think tribal people would find this kind of behaviour rather odd."

The fact is that sexual sleaze, particularly involving prostitutes, is a feature of large, faceless cities. In order to flourish, it depends on the anonymity that does not exist in an intimate community like a primitive tribe or a small village.

The full story of the international vice ring that placed a brief question mark over Robert De Niro's probity has yet to be told. In the meantime, as long as the sun rises in the mornings, men will continue to pay women for sex. Which is fine, if they like that sort of thing. Just so long as they don't use anthropology or genetics to justify it.