FEWER POLICIES, FEWER SIDE EFFECTS: Perfectly Nice Policies With Less Nice Side Effects.

What happens when you suddenly offer parents generous family leave benefits, paid at the expense of the government? You can probably think of dozens of outcomes. But here’s one you might not have been expecting: people die.

That’s the finding of Benjamin Friedrich and Martin Hackmann, in a new working paper at the National Bureau of Economic Research. The culprit? Nurses, who skew female, provide a lot of vital health care, and made heavy use of Denmark’s new paid family leave benefit when it passed in 1994. Since the supply of nurses was limited, and their skills could not easily be replaced, hospital readmissions went up, and more troublingly, mortality spiked among elderly patients in nursing homes.

Advocates of paid parental leave are no doubt bristling at the implication that their favorite benefit might kill people. But that’s not quite the right implication to take away from this paper. What it really highlights is how difficult it is to know how a given policy will turn out. Had officials understood that in advance, they might have taken steps to mitigate the effects — such as training extra nurses beforehand. The problem, in other words, wasn’t necessarily family leave policy, but the limited visibility policymakers have into the outcomes of their plans.

That’s the knowledge problem. Then there’s the second-order knowledge problem, where policymakers don’t even know what policies they enact.