It’s a staple of feminist rhetoric: Women make less money than men because of discrimination.

“We’d all like to think, in 2012, that pay discrimination is a thing of the past,” the progressive activist Joy Lawson wrote in the Huffington Post recently. “But the pay gap still exists, and it’s big: women earn an average of 77 cents on a man’s dollar.”

April 17 has been designated Equal Pay Day: Supposedly that’s how long women have to work to catch up to men’s pay from the previous year.

President Obama has cited a similar statistic to promote legislation to make it easier to sue employers for discrimination. Democrats accuse Republicans who resist such laws of waging a “war on women.” Expect to hear more about the issue during the fall campaign.

Here’s the truth you won’t hear: The pay gap is exaggerated, discrimination doesn’t drive it and it’s not clear that government can eliminate it — or should even try.

Conservative economist Diana Furchtgott-Roth runs through the problems with the statistic in her book on the economic progress of women, Women’s Figures. She points out that part of the gap reflects the fact that women, on average, work fewer hours than men. Among people who work 40 hours a week, according to the Labor Department, women make 87 percent of what men do.

Furchtgott-Roth cites a 2005 study by economists June O’Neill and Dave O’Neill, which found that for the most part “the gender gap is attributable to choices made by women concerning the amount of time and energy to devote to a career.” They continue: “There is no gender gap in wages among men and women with similar family roles.”

In addition to being more likely to seek part-time work, women are also more likely to have gaps in their employment history and to enter lower-paying fields. The consulting company Consad, in a 2009 report for the Labor Department, found that these factors account for most of the pay gap. Correct for them, and men make only 5 percent to 7 percent more than women for the same work.

Even the American Association of University Women, in a recent report playing up the pay gap, conceded that 5 percent is a reasonable estimate of the difference between men’s and women’s wages that cannot be explained by choice of occupation, employment history and the like.

Not even that smaller gap can be attributed wholly to employer discrimination. Lawson, although she favors “legislative solutions,” also writes that women are less likely than men to drive hard bargains in salary negotiations. If true, that would explain part of the gap, as well.

To say that women’s choices result in their being paid less, on average, than men is not to deny that unfair social conditions may constrain those choices. Perhaps men should do more of the work of running households and raising children, and boys should be brought up with that expectation. Perhaps childcare should be made more affordable. Perhaps efforts should be made to make sure college women aren’t being steered toward majors that won’t prepare them for lucrative careers.

Carrie Lukas, who has written often about the pay gap for the conservative Independent Women’s Forum, says “it’s a mistake to default to the idea that it’s all discrimination.” She agrees with Lawson, however, that women should be less reticent about demanding higher pay: “As parents of daughters we can make a difference in that.”