The sex discrimination case against Wal-Mart, in which the U.S. Supreme Court handed an important victory to the retail chain on June 20, revives a longstanding debate: are disparities in the workplace due primarily to gender bias or to deep-rooted gender differences? The answer is anything but simple—which is why the ruling was correct.

The lawsuit was filed 10 years ago by three female employees. However, the plaintiffs and their lawyers have sought to expand it into a class-action suit on behalf of every woman who has worked for Wal-Mart at any time since December 1998—as many as 1.5 million. While they collected statements from 120 women alleging discrimination, the main argument for the class-action suit relied on sociological and statistical analysis. Women make up nearly two-thirds of hourly workers at Wal-Mart but only one-third of management. Such disparities, the complaint argued, can be explained only by bias.

This recalls a notorious 30-year-old sex discrimination case: the suit brought by the Equal Employment Opportunity Commission against Sears, Roebuck & Co., charging that women had been kept out of commission sales and herded into lower-paying salesclerk jobs. There, too, statistical disparity was taken to prove bias. From 1973 to 1980, women accounted for 43 percent of all promotions from non-commission to commission sales at Sears; EEOC experts calculated that it should have been 68 percent.

In response, the company challenged the assumption that men and women were equally interested in and qualified for commission sales. In a controversial twist, a feminist historian, Rosalind Rosenberg of Barnard College, testified as an expert witness for Sears. Men and women, Rosenberg argued, generally have different expectations and preferences regarding work—and, however, desirable more equality in the workplace may be, it is "naïve" to see the disparities as proof of discrimination. (She was, of course, branded a traitor to the sisterhood.) Sears won the case in 1986.

For all the strides women have made since then, the issues of the Sears case remain relevant. Rosenberg noted that working women have tended to choose more family-friendly jobs over better-paying ones. Today, women are far more likely than 25 years ago to be the principal or equal earners in their families, and men are more involved in child care and housework; but the traditional pattern of the male primary breadwinner and the female primary homemaker is very much with us. For a man, family obligations are still likely to create pressure to work longer hours; for a woman, the opposite.

What implications does this have for the Wal-Mart lawsuit? One answer comes from left-wing journalist Liza Featherstone, whose 2004 book about the case, Selling Women Short: The Landmark Battle for Women's Rights at Wal-Mart, is strongly sympathetic to the plaintiffs. In an interview for the online magazine Stay Free! after the book's publication, Featherstone was asked about other suits against Wal-Mart, including one by the widow of a male manager who had died of a heart attack. Featherstone explained, "Her husband was incredibly overworked, as many Wal-Mart managers are … assistant mangers are forced to work 70-80 hours a week. In some sense, they are more exploited than hourly workers, because they are salaried, so they don't get overtime."

In another interview, in Salon, Featherstone noted that Wal-Mart expects managers to be available to work at any time and that the chief plaintiff in the women's case, Betty Dukes, felt her career had suffered because she refused to work Sundays.

All this lends credibility to Wal-Mart's assertion that far fewer women than men have been interested in management jobs. Does this mean that the individual claims of sex discrimination against the store are without merit? No, only that gender imbalances in such jobs do not automatically prove wrongdoing.

The High Court unanimously agreed that the class-action suit was green-lit under an improperly broad standard—one that would confer victimhood on any current or former female Wal-Mart employee within the given time frame, with or without her consent. On another issue, however, the court split 5-4. The majority nixed any kind of class-action suit based on the plaintiffs' claims: Justice Antonin Scalia reasoned that, since Wal-Mart left promotions and pay to the discretion of local stores, the allegations of bias involved so many varied individual decisions that they could not be lumped together as common practice.

The minority took the view that a collective lawsuit should have been allowed to proceed under a stricter standard of proof (and an opt-out provision for members of the presumably injured class). The dissent, penned by Justice Ruth Bader Ginsburg, argued that sexist stereotypes can create a pattern, with different managers making similarly biased decisions.

Does Scalia underestimate the pervasiveness of sexism? Does Ginsburg overestimate it? Like many other issues, the question of gender inequality in the workplace is often reduced to a simplistic either/or. Women's traditional preferences don't negate the existence of sexist barriers or subtle biases. Personal behavior and societal expectations reinforce each other: when most women with small children curtail their participation in the workforce, this creates assumptions that hurt more career-oriented women. Even a woman married to a stay-at-home dad may be unwillingly mommy-tracked. Conversely, women's—and men's—choices are not made in a vacuum; they too are influenced by societal expectations.

Yet legal action is far too blunt and heavy an instrument to deal with these issues. Sometimes, as with the ban on racial segregation or on overt sex discrimination in the workplace, law can change culture in the right direction. But for the law to intrude into a complex web of human relationships and attitudes is an overreach likely to cause more harm than good. For one, we live in a time when state intrusion into private actions is viewed with suspicion. To say that women's advancement requires the government and the courts to micromanage business decisions—to the point of telling a corporation that it cannot let local managers control promotions and pay—is to invite a backlash.

In the meantime, Wal-Mart now has a program to help boost the share of female managers—launched two years ago, perhaps in response to public opinion as much as legal action. The culture is changing. To try to force this change by massive litigation based on fuzzy logic is bad for the economy, bad for the law, and ultimately bad for women as well.

Cathy Young is a contributing editor at Reason magazine and a columnist at RealClearPolitics. This article originally appeared at RealClearPolitics.