More than a third of American firms offer at least some paid maternity leave, and an increasing number of them are extending such benefits to new dads as well. But paternity leave is useless if men don’t take it. A 2018 Deloitte survey of more than 1,000 men found that a third worried that taking a leave would hurt their careers, and more than half feared it would signal that they weren’t serious about their jobs. A 2017 Pew survey found that the median paternity leave was just one week. New moms, meanwhile, typically take whatever time they can get.

That’s why the “mandatory” piece for men is key. Most organizations that offer voluntary leave have a “use it or lose it” approach, which, it turns out, actually hurts women, since they are more likely than men to use it. In Denmark, for example, a generous leave policy offers families 52 weeks of paid time off to be split between both parents. Yet in practice, women end up taking 92.8% of the total time, according to the OECD. These women find it almost impossible to climb back on the career track afterward. As a result, even two decades after the birth of their first child, they face a 20% gender wage gap, a 2018 National Bureau of Economic Research working paper concluded.