THE ECONOMY IS thriving, unemployment rates are low, and companies that have to compete for quality employees are expanding benefits, including paid time off. That makes this an odd moment for conservatives to shift their position on whether the government should implement a family leave mandate.

A 2017 working group made up of representatives from the center-left Brookings Institution and the conservative American Enterprise Institute outlined the need for a federal paid family leave law. They point to Bureau of Labor Statistics data showing that only 13 percent of private sector workers receive paid leave.

This number ignores a multitude of paid leave options and other benefits that frequently are provided by employers, however. In a new report, the Cato Institute's Vanessa Brown Calder corrects the record. Citing more comprehensive government metrics, she finds that as of 2008 (the last year for which we have data), as many as 61 percent of women reported having access to some form of paid leave from their employers, up dramatically from 16 percent in the 1960s. Calder notes that even in the absence of a federal policy, the number of new moms who quit working declined "from over 60 percent in 1961 to just over 20 percent in 2008."

There's no reason to believe that trend won't continue, especially since many firms--encouraged by the prospect of economic growth following the tax reform law of 2017--have publicly announced a commitment to paid leave.

Left and right often disagree about how to get more businesses to do the same. Progressives prefer the European approach, in which government requires companies to provide paid leave to all employees; in some countries, the state even finances that leave through taxes. Conservatives, meanwhile, are more comfortable with less intrusive policies such as allowing people to borrow money from their own future Social Security checks--perhaps because they know mandates ultimately hurt employees.

Calder reviews the literature and shows that top-down paid leave requirements are likely to be offset by lower wages or fewer employee benefits of other types. They could also incentivize employers to discriminate in favor of older workers and against applicants of childbearing age.

Interestingly, when voters are presented with these costs, their support for a federal paid leave policy collapses. In an upcoming...

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