In the old vaudeville joke, Moses descends from Mount Sinai with a stone tablet under each arm. He has good news and bad news: "The good news is, I got Him down to 10. The bad news is, adultery is still on it."
Adultery is only a joke for those not devastated by it, of course, and the assumption for thousands of years has been that it's the man who is more prone to be prone in an improper liaison with a woman not his spouse.
There are lots of reasons for this: women risk pregnancy (even in the age of the Pill); men find it easier to separate sex from love; men (usually) have the power and money and women (usually) take care of the children.
But major shifts in the power balance in marriage have altered adultery, too. The incredible shrinking double standard in matters of sex (if not always in matters of the heart) has created a shift of epic proportions in the nature of sexual infidelity.
"The sex differences in infidelity are disappearing," psychologist Shirley Glass, a Baltimore psychologist who has studied adultery for two decades, tells Psychology Today magazine. While this suggests that adultery is an equal-opportunity opportunity, what's startling about the new research is that men are getting more emotionally involved with their extracurricular partners.
In the original 1980 study, most male adulterers did not want to disrupt their marriage and the sexual affair usually did not make much impact on the husband's satisfaction with his marriage: "You could be in a good marriage and still cheat."
This anomaly was built mostly on the wife's sense of powerlessness, and the latest finding suggests that female emancipation has changed perceptions. Men have become more sensitive - just as women say they wanted him to be - but the sensitivity usually rewards the working woman, not the wife.
Today's adulterers are more likely to find fulfillment with a workmate. "The work relationship becomes so rich and the stuff at home is pressurized and child-centered," Shirley Glass says. "People get involved insidiously, without planning to betray."
As a result, adultery is more threatening and disruptive to the marriage and more likely to lead to divorce. Working wives have more sexual opportunities, too. (The milkman has disappeared, along with the cliché and the glass bottle.)
The sexual revolution, like all revolutions, has wrought unintended consequences. While cultural changes are not thought to influence biology, it's clear that the messages exchanged between men and women have changed behavior.