Equal pay advocates insist we need a government fix for the sexism they say is causing wage inequality between men and women. Turns out the holes in that argument are so big you could drive a bus right through them—or an entire fleet like a pair of Harvard economists recently did.

In their new study authors Valentin Bolotnyy and Natalia Emanuel analyze several years of data about Massachusetts Bay Transportation Authority bus and train operators. They conclude, “Even in a unionized environment where work tasks are similar, hourly wages are identical, and tenure dictates promotions, female workers earn 89 cents on the male-worker dollar…”

This finding alone should give even the most diehard government fans pause.

After all, if the Commonwealth’s MBTA can’t achieve wage equality under such “ideal” conditions, what hope does government writ large have? Not much, as my IWF colleague Charlotte Hays aptly documents here and here.

The reason is that income inequality isn’t the result of some vast, sexist conspiracy. It “can be explained entirely by the workplace choices that women and men make,” according to Bolotnyy and Emanuel. Specifically, they find that compared to MBTA operators who are women, men:

-take 48 percent fewer unpaid hours off

-work 83 percent more overtime hours annually

-sign up for 7 percent more scheduled overtime, and

-take almost twice as much last-minute, unscheduled overtime

Bolotnyy and Emanuel find that women generally prioritize flexibility and time off to avoid unscheduled overtime or undesirable shifts, especially single mothers. Single fathers, however, prioritize earning more money. For married operators with children, the overtime rate for men and women is nearly identical. This parity suggests married parents find a balance between home caretaking and outside earning, with mothers working a bit more overtime and fathers a bit less than they would if they didn’t have children. Yet for married workers without children, men are far more likely to accept overtime than women—underscoring the premium most women place on flexibility and time over earnings.

Rather than build on those individual preferences by allowing operators to trade shifts with each other and coordinate with their supervisors, the MBTA enacted rigid policy changes during the study period that ultimately (albeit unintentionally) narrowed the wage gap by 5 cents.

However, the move proved to be pennywise and pound foolish because both women and men, not to mention commuters and taxpayers, paid a price.

One change to the Family Medical Leave Act made it harder to take last-minute unpaid time off. The other redefined overtime, complicating efforts to earn back wages. For men these changes made it much harder to take FMLA unpaid leave, and then make it up with less desirable but more lucrative overtime shifts paying up to 1.5 times the regular hourly rate. So, men lost their choice and flexibility to earn more. Meanwhile, women lost both flexibility and time—as well as earnings.

After the changes, women could no longer take unpaid days off as needed, then make up the difference through scheduled overtime, a practice that essentially allowed them to break even. Instead women took more unexcused leave, which resulted in disruptions to daily MBTA operations, more lost trips for commuters, fewer overtime opportunities to make up their wages, and less job security.

Bolotnyy and Emanuel conclude that as these “policies illustrate…not all ways of shrinking the gender earnings gap are created equal, and some may come with substantial negative consequences.”

As the Wall Street Journal Editorial Board puts it, “the common-sense observation that men and women often have different priorities, and the best way to accommodate them is through the marketplace, not the untender mercies of government.”