We've heard from the sex workers about MAR 16 2008
Psychologist Christopher Ryan, author of "Sex in Prehistory," says the desire for sex with more than one person has always been there -- for leaders and followers alike. "The desire is not a function of status or power -- it's a question of availability."
What's relatively new to the human race, he said, is the ability to exercise power and the connection between power and sex.
That's because, for most of human existence, there was only so far a man could coerce others when food was essentially free and hard to hoard. And until relatively recently, sex with multiple partners was the norm. "It would have been very unusual 100,000 years ago for a person to have one sexual partner for 30 years," said Ryan in an interview from Barcelona.
She points out that, while powerful men throughout western history have married monogamously (they had only one legal wife at a time), they have always mated polygynously (they had lovers, concubines, and female slaves). Many had harems, consisting of hundreds and even thousands of virgins. With their wives, they produced legitimate heirs; with the others, they produced bastards (Betzig's term). Genes and inclusive fitness make no distinction between the two categories of children. While the legitimate heirs, unlike the bastards, inherited their fathers' power and status and often went on to have their own harems, powerful men sometimes invested in their bastards as well.
As a result, powerful men of high status throughout human history attained very high reproductive success, leaving a large number of offspring (legitimate or otherwise), while countless poor men in the countryside died mateless and childless.
Update: And one more from Natalie Angier:
Yet as biologists have discovered through the application of DNA paternity tests to the offspring of these bonded pairs, social monogamy is very rarely accompanied by sexual, or genetic, monogamy. Assay the kids in a given brood, whether of birds, voles, lesser apes, foxes or any other pair-bonding species, and anywhere from 10 to 70 percent will prove to have been sired by somebody other than the resident male.