To paraphrase the line often attributed to Mark Twain, there are lies, damn lies and the “equal pay” statistic.

The factoid that women earn only 77 cents of every dollar earned by men is the focal point of a feminist cargo cult. It has its own movement and its own quasi-holiday, the so-called Equal Pay Day, marking how far into a new year women supposedly have to work to match what men made the prior year.

The figure is presumed to clinch any debate over the continued existence of massive discrimination against women in the workforce. And so the factoid has predictably featured heavily in the latest push by Democrats on the alleged “war on women.”

Drawn from Census Bureau data, the 77-cent stat is a comparison of the earnings of women working full time to men working full time. Its fatal flaw is that it accounts for none of the important factors that play into the disparity, such as hours worked.

Mark Perry and Andrew Biggs of the American Enterprise Institute note that men are twice as likely to work more than 40 hours per week as women. Then there are differences in choice of occupation, in education and in uninterrupted years of work. Once such factors are taken into account, there is about a 5 percent differential in the earnings of women and men, about which various theories are plausible, including the effect of residual discrimination.

What is clear is that the wage gap is largely an artifact of the fact that women devote more time to caring for children than do men. Harvard economist Claudia Goldin points out that the earnings of women without children are almost equal to those of comparable men. Feminists are mistaking a byproduct of the laudable desire of mothers to spend time with their kids for a depredation of The Man.

When asked in an MSNBC interview about the reliability of the pay-gap number, White House economist Betsey Stevenson confessed: “I agree that the 77 cents on the dollar is not all due to discrimination. No one is trying to say that it is. But you have to point to some number in order for people to understand the facts.”

There you have it: For people to understand the facts, you have to give them an easily misunderstood statistic, usually without necessary context and spun in the most inflammatory fashion possible. Enter President Barack Obama. He wrings every bit of dishonesty he can out of the number.

At the Equal Pay Day event at the White House, he marveled at the simplicity of it all: “A woman has got to work about three more months in order to get what a man got because she’s paid less. That’s not fair. That’s like adding an extra six miles to a marathon. It’s not right.”

Such is his subtle rendering of a number that even his own economic adviser admits — at least under questioning — must be handled with care.

A cottage industry has now sprung up around hoisting Democrats with their own canard. Perry, of AEI, calculated that women at the White House make a median salary of $65,000 and men make a median salary of roughly $74,000, so female White House staffers make only 88 cents for every dollar earned by male staffers. The National Republican Senatorial Committee crunched the numbers and found a similar “gap” for the offices of certain Democratic senators. By the crude logic of the promoters of the 77-cent figure, every one of these offices is guilty of rank discrimination against women.

No matter. Hillary Clinton, whose prospective presidential campaign will be predicated on every feminist cliche her supporters can muster, tweeted on Equal Pay Day, “20 years ago, women made 72 cents on the dollar to men. Today it’s still just 77 cents. More work to do.”

Yes, never tire or relent. The flogging of the bogus statistic can never end.

Rich Lowry can be reached via e-mail: comments.lowry@nationalreview.com.