It’s been illegal to pay women less than men with comparable experience working the same job ever since the Equal Pay Act of 1963. You wouldn’t know that from the way Democrats are pushing a bill they call the “Paycheck Fairness Act.” It’s a cynical exercise in political posturing that would result in downward pressure on wages for all Americans.

The bill’s supporters cite a statistic to prove that pay discrimination still exists: They say women earn 77 cents for every dollar a man earns. The problem is that the statistic is misleading: It compares pay for all men and all women working in all jobs. Economists Mark Perry and Andrew Biggs take apart the myth in Tuesday’s Wall Street Journal. For one thing:

“Men were almost twice as likely as women to work more than 40 hours a week, and women almost twice as likely to work only 35 to 39 hours per week. Once that is taken into consideration, the pay gap begins to shrink. Women who worked a 40-hour week earned 88% of male earnings.”

Other trade-offs make a huge difference:

“Child care takes mothers out of the labor market, so when they return they have less work experience than similarly-aged males. Many working mothers seek jobs that provide greater flexibility, such as telecommuting or flexible hours. Not all jobs can be flexible, and all other things being equal, those which are will pay less than those that do not.”

Not all bachelor’s degrees are the same, either. Engineering often pays more than sociology, for example. Men are more likely to take jobs with physical risk or volatile earnings, the government reports.

The 77-cent story, Perry and Biggs write, is based on believing that employers are acting against their own interests:

“These gender-disparity claims are also economically illogical. If women were paid 77 cents on the dollar, a profit-oriented firm could dramatically cut labor costs by replacing male employees with females. Progressives assume that businesses nickel-and-dime suppliers, customers, consultants, anyone with whom they come into contact—yet ignore a great opportunity to reduce wages costs by 23%. They don't ignore the opportunity because it doesn't exist. Women are not in fact paid 77 cents on the dollar for doing the same work as men.”

Even the White House-friendly Washington Post has examined the Democrats 77 cent claim and declared it misleading – giving the claim "two Pinocchios."

This does not mean that there isn’t some gap between the pay of men and women, especially at some employers. It is interesting to note that, using that same misleading statistical method, the White House pays women on its staff only 88 cents for every dollar it pays its male staff, and a number of Senate Democrats pay women on their staffs far less than they pay male staffers. But it no more proves anti-woman discrimination than does the debunked 77-cent figure.

It certainly doesn’t justify passing a bill that essentially eliminates merit pay and will disadvantage women who work hard and deserve bonuses. It already is illegal to unfairly pay women less. This bill will do more harm than good.

More: Details on my pay policies.