As any debater knows, defining the issue is a major part of the battle. On Tuesday, Democrats failed to persuade the Senate to approve the Paycheck Fairness Act. What are we to conclude from that outcome? That paychecks will be unfair, to the detriment of America's working women.

That's the claim of those supporting the legislation. President Barack Obama said it would merely mandate "equal pay for equal work." Senate Democratic Leader Harry Reid of Nevada warned beforehand that failing to pass the bill would send "the message to little girls across the country that their work is less valuable because they happened to be born female."

On Rachel Maddow's blog, the complaint was that women are "still only making 77 cents for every dollar men earn in similar jobs," but Republicans "seem indifferent to the problem."

This is a myth resting on a deception. The Washington Post's official Fact Checker faulted Obama's claim, noting that "there is a wage gap, but it has declined over the decades—and depending on how the data are viewed, in some cases it barely exists."

A difference, in any event, does not prove discrimination. Most Broadway theatergoers are female, but not because playwrights have an animus toward males. The gap reflects many benign factors stemming from the choices voluntarily made by women and men. Same with the pay gap.

Women, on average, work fewer hours and are more likely than men to take time off for family duties. A 2009 report commissioned by the U.S. Labor Department concluded that such "factors account for a major portion and, possibly, almost all of the raw gender wage gap."

"The gender gap shrinks to between 8 percent and 0 percent when the study incorporates such measures as work experience, career breaks and part-time work," Baruch College economist June O'Neill has written.

Statistical analysis, however, cuts no ice with some. Sen. Charles Schumer, D-N.Y., ridicules those skeptics who cite such inconvenient evidence: "They're basically saying women choose to be paid less than men."

Not exactly. I didn't volunteer to be paid less than a big law firm partner or corporate CEO—but going into journalism assured that I would be. Women don't go to the boss and request a smaller salary, but many of them make choices that lead to that outcome.

A fact sheet from the American Association of University Women (which favors the bill) acknowledges that "10 years after graduation (from college), 23 percent of mothers in our sample were out of the workforce and 17 percent worked part time. Among fathers, only 1 percent were out of the workforce, and only 2 percent worked part time." It's safe to assume that men who make similar work decisions experience similar consequences.

You could argue that oppressive social conventions saddle mothers with the main responsibility for this task. But given the drastic changes in sex roles and expectations over the past half-century, why should we assume that this one is being forced on women? If they tend to place greater importance on child-rearing than men, they will be more inclined to interrupt their careers, even at a sacrifice in long-term earnings.

Pay differences stemming from factors within the control of females are a "problem" only if you define them as one. By that logic, we need a Higher Education Fairness Act because men earn only 43 percent of all bachelor's degrees and 40 percent of master's degrees.

If universities are taking steps to discourage guys from enrolling, it's a problem that may be amenable to government action. But if the imbalance is the result of males skipping college in favor of other options, there is no social injustice to undo.

What the alleged gender pay gap reflects is the interaction of supply and demand in a competitive labor market. Even in a slow economy, companies that mistreat women can expect to lose them to rival employers.

The Paycheck Fairness Act would upend these processes, with the government and courts assuming responsibility for what each worker should be paid, according to Harry Reid's standards of justice and fairness. Every salary decision would be fraught with the dread prospect of litigation—promoting rigid pay scales simply to minimize the liability risk.

The result would be a less nimble and efficient economy, which over time dampens productivity improvements and stifles wage growth. The effect on paychecks? Not fair, but foul.

Steve Chapman blogs daily at newsblogs.chicagotribune.com/steve_chapman.