But why is it so much more harmful and insulting when aimed at women?
Some clues can come from looking at humans in an evolutionary context.
In most mammals, the male invests less in raising offspring. In mammals, it's the females who have to gestate them, suckle them, and in some cases raise them. In the Darwinian struggle, many female animals benefit from choosing the strongest, most attractive, richest, or otherwise most desirable males.
Most male animals benefit from mating early and often. It increases the odds their genes will survive to reproduce a new generation.
Humans are no exception. When psychologist David Buss surveyed his students at the University of Texas, he found that women said they wanted about one sex partner in the next two years and four or five in a lifetime. Men wanted two within the month, eight in the next two years, and 18 in a lifetime.
When asked how long they'd need to know someone before engaging in sex, women said about a year. Men said they'd probably have sex with a woman they had known for a week.
Introducing another asymmetry into sex is the fact that males of many species run the risk of being cuckolded, which is an evolutionary disaster - the male helps pass along another male's genes to the next generation. In most animals, the female knows which offspring are hers, but the male doesn't.
In humans, the accusation of promiscuity can cause harm to a woman by lowering what evolutionary psychologists call "mate value." That may stem from an instinctive fear that such a woman would cheat and dupe a man into investing resources in another male's offspring. If anything, a reputation for getting around may enhance a man's reputation by suggesting to women that he is desirable to others.
Famous men are occasionally tarred by their sexual foibles, but only if their sins go beyond promiscuity. Herman Cain was not just a slut. He was accused of cheating on his wife and taking advantage of his power. Newt Gingrich isn't criticized for being a slut so much as he is for being heartless enough to divorce a wife suffering from cancer.
Data show that across cultures, males value chastity in women much more than women value it in men, if they value it at all, said Shackelford.
In the realm of gossip, the University of Texas' David Buss and colleagues have found predictable sex differences in the way humans discredit each other. If a man is trying to put down a male rival, he might say the man is a loser, unable to keep a job or buy a car.
Women trying to discredit a female rival are much more likely to spread rumors that an enemy has slept around or slept with undesirable men.
These tactics just don't work as well if reversed, Shackelford said. If a man hears that a beautiful woman is short on marketable skills and doesn't own a car, she's unlikely to be diminished in his eyes. If a woman hears that a man is promiscuous, she might think he's harder to get, but no less worth getting.
Shackelford said there's one aspect of the Limbaugh incident that promises hope for the human race. "Fifty year ago, people would have guffawed along with it," he said. "But Rush has gotten an incredible backlash . . . it's an indication of moral progress."
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