An entire movement has grown up around the factoid that American women make about 80 percent of the pay of men. It is a reliable talking point of Democrats who insist the country is racked by a "War on Women." A raft of proposed legislation purports to remedy the discrimination exposed by the damning number. It is the only bad statistic with a day devoted to it, "Equal Pay Day," which falls in April to signify how much longer women have to work into the new year to make what men earned in the previous year. Tradition says that the day must be marked with wailing and gnashing of teeth, and lots of press releases from advocacy organizations.

MSNBC host Rachel Maddow recently wielded the statistic on "Meet the Press," and reacted with shocked disbelief that anyone would question such a cold, hard fact, as if it were as incontestable as the circumference of the Earth.

Never mind that the figure is crude and misleading. The latest data from the Labor Department say that women made 82.2 percent of what men made in the first quarter of 2012. That's a considerable gap, but comparing all women versus all men is not particularly telling when all sorts of variables -- occupation, levels of experience, education, hours worked -- are in play.

"Women gravitate," Carrie Lukas of the Independent Women's Forum writes, "toward jobs with fewer risks, more comfortable conditions, regular hours, more personal fulfillment and greater flexibility. Simply put, many women -- not all, but enough to have a big impact on statistics -- trade higher pay for other desirable job characteristics."

The Institute for Women's Policy Research, a feminist outfit obsessed with the wage gap, published a study noting that twice as many women as men work in jobs with median earnings below the federal poverty line for a family of four. Unless all these women -- some 5.5 million -- were coerced into these positions, this fact alone shows how occupational choice influences the wage gap.

The slogan that invariably accompanies the 80 percent statistic is "equal pay for equal work." But men and women get paid differently for different work. Warren Farrell points out in his book "Why Men Earn More" that the 25 worst jobs in terms of stress and physical demands -- occupations like sheet-metal worker and firefighter -- are more than 90 percent male. In general, men who are employed full time work more hours a day than women employed full time (8.2 hours compared with 7.8, according to the Labor Department), and women are much more likely to interrupt their careers to have children, affecting their earning power over time.

All that notwithstanding, it is a strange time in history for self-appointed advocates for women to feel oppressed on their behalf. They must have missed the growing literature on "The End of Men" and similar themes. Women earn about 60 percent of bachelor's and master's degrees, and are reaching parity with men in medical and law schools. Their attitudes to work are changing. In a historic reversal, more young women ages 18 to 34 (66 percent) than young men (59 percent) say high-paying work is one of the most important things or very important, according to a new Pew survey.

In light of all this, it stands to reason that the wage gap will narrow, even if it doesn't disappear. A study by a research organization called Reach Advisors shows that single women in their 20s make 105 percent of single men in their 20s in urban areas, and 120 percent "in certain cities with a heavily knowledge-driven employment base." These women must not realize that they will never make their way in the workplace without Congress somehow acting to ensure "equal pay."

In the end, the reality doesn't matter. A bad statistic never dies.