In March 2007, Spain introduced a national policy granting most new fathers two weeks of fully paid paternity leave. The policy proved exceptionally popular, with 55% of men eligible in the first year opting to take the paid time. The amount of leave covered by the program was doubled in 2017 and expanded to five weeks in 2018, with additional increases expected between now and 2021.

Economists studying the effects of the original 2007 policy examined what happened to families that had children just before and just after the program began, and found differences in the outcomes. While the early cohort of men who were eligible for paternity leave were just as likely to stay in the workforce as the men who weren’t eligible, they remained more engaged with childcare after their return to work, and their partners were more likely to stay in the workforce as well. In that sense, the program seems to have done what policy makers would have hoped.

Unexpectedly, though, the researchers also found that families who were eligible for the paternity leave were less likely to have kids in the future. In a study published in the Journal of Public Economics (paywall), economists Lídia Farré of the University of Barcelona and Libertad González of University of Pompeu Fabra estimate that two years on, parents who had been eligible for the newly introduced program were 7% to 15% less likely to have another kid than parents who just missed the eligibility cutoff. While the difference dissipated further into the future, even after six years, parents who had been eligible for the leave were still less likely to have a child again.

The researchers suggest an intriguing reason why.

After paternity leave was instituted, surveys of Spanish men ages 21 to 40 showed they desired fewer children than before. Farré and González think that spending more time with their children—or the prospect of having to do so—may have made men more acutely aware of the effort and costs associated with childrearing, and, as the researchers put it, “shifted their preferences from child quantity to quality.”

At the same time, women started showing preferences for slightly larger families—perhaps a sign that having more children seemed more desirable with a slightly more equitable balance of labor at home.

As the authors point out, it’s impossible to draw sweeping conclusions from this observation of a single data point in a single country. Correlation isn’t causation, and it’s possible that other factors weighed more heavily than paternity leave on men’s family preferences. (The global financial crisis, for example, hit Spain in full force about a year after the leave policy was introduced.)

“There are a couple of reasons that I’d be hesitant to believe that these same impacts would apply elsewhere,” said David Evans, an economist at the Center for Global Development. “In Spain, almost no men were taking paternity leave before the policy, and that jumped to more than half of men after the reform. At the same time, men in Spain wanted more children than women did. That wasn’t the case in a number of other European countries.”