Ever wonder why women, on average, make less money than men? For years, feminists have argued that discrimination is to blame. But most careful studies show that once you take into account differences in the hours worked, years of experience, and the actual occupational or professional category in which women work, the gap narrows considerably.

If you add marital status to the mix (married men earn the highest salaries, married women the lowest), the differences virtually disappear among the youngest groups of working men and women.

Now, a group of economists has come up with a different explanation of the pay gap -- one that adds a surprising bit of nuance to the "discrimination" thesis. According to a study by economics professor Linda C. Babcock of Carnegie Mellon University, reported recently in the Washington Post, women may not actually ask for as much money as men. And their reticence costs them in both starting pay and in earning higher raises.

Babcock and her colleagues observed how men and women reacted when told they would be paid according to a sliding scale. Men were eight times more likely to ask for higher compensation than they were initially offered to participate in a simple experiment. Even when told that the payment was negotiable, only 58 percent of the women, but 83 percent of the men, asked for more money.

And other studies have examined whether this female acquiescence applied in the real world beyond a laboratory experiment. A survey of students who received job offers after earning master's degrees demonstrated that more than half the men, but only 12.5 percent of women, negotiated for higher salaries than they were initially offered.

Babcock and her co-authors argue that this behavior may not be entirely self-defeating, however, since women may be penalized subtly if they are too aggressive. Their studies show that both male and female supervisors react more negatively to women who try to negotiate higher pay. But male supervisors don't penalize men who do so, while female supervisors prefer both men and women who aren't pushy when it comes to salary, accepting whatever they are offered.

Apparently it took years of study and, no doubt, substantial research grants, to determine something most of us realized all along: Men and women behave differently. In the past, feminists maintained, against all evidence to the contrary, that there were no differences between the two sexes, and any that might seem to exist were just a matter of socialization.

The first efforts to narrow the pay gap aimed at encouraging women to go into previously all-male jobs. Women should be just as eager to become plumbers, lumberjacks and long-haul truck drivers as men, they argued. When women didn't flock to most male-dominated jobs, the feminists then urged that we raise the wages in female-dominated job categories to close the pay gap. Nurses should make more money than electricians; child-care workers should earn more than tree trimmers; and librarians should earn more than garbage collectors.

But there is no central wage-setting mechanism in this country that could enforce such arbitrary efforts to increase pay in female-dominated jobs, thankfully. So these efforts to eliminate the wage gap also failed. And if these recent studies are correct, even if starting wages for women were higher, many women's salaries would eventually fall behind their male co-workers' because they failed to demand raises. But discrimination isn't the cause.

Discrimination -- against women, minorities or white men, for that matter -- still occurs on an individual basis. Even with harsh penalties and aggressive enforcement of anti-discrimination laws, some employers will choose to hire or promote based on their own prejudices. But employers who make a habit of rewarding less qualified individuals over better qualified ones will pay dearly for those prejudices in lower productivity -- and lower profits -- over time. And those employers who reward merit and effort will benefit by being able to attract the best workers -- that is, unless the workers themselves give up.

Marketplace economics works to reward talented individuals, but only if those individuals are willing to take risks on their own behalf. Women need to learn to play the game -- or quit complaining that they're underpaid.