While numerous policies and financial incentives have been employed, few have been proven to reliably boost fertility. But a new study from Nobel Prize–winning economist Claudia Goldin points to the extra hours women spend on child-rearing and household chores compared with men.

She found that was a major difference between countries with low fertility and those with even lower rates, or the “lowest-low” nations.

In Sweden, for example, women spend just 0.8 hours more than men each day around the house, and the country’s fertility rate is 1.7 kids per family. And in Denmark, where women work 0.9 hours extra, the fertility rate is also 1.7.

To be sure, both rates are below the so-called replacement rate of 2.1 kids per family, which is needed for a country to maintain a stable population.

But they exceed countries with even lower fertility. In South Korea, where the fertility rate is 0.9 kids, women put in 2.8 hours more than men on the home and family each day. And in Italy, where the fertility rate is 1.3, women spend 2.9 extra hours.

The U.S., meanwhile, has a Scandinavian-like fertility rate of 1.7, even though women do 1.8 more hours of work at home.

In an interview with Washington Post columnist Heather Long, Goldin said men’s assumptions about what other fathers are doing are important.

“It won’t change unless the guy’s expectations about what he’s ‘supposed to do’ changes,” she explained. “There’s some evidence that individual men believe in couple equity more than they act on it, because they believe other people don’t feel this way.”

As economies developed rapidly or experienced steep postwar recoveries, women entered the workforce in droves, clashing with some cultural norms.

“When you have rapid growth, then you don’t give generations enough time to get used to modernity. You thrust them into modernity,” Goldin said.

While her paper doesn’t advocate for a particular policy solution, she told the Post that U.S. lawmakers could provide government-subsidized childcare, saying that’s more critical than parental leave.

The study comes as other data point to the crushing burden of caregiving. In Care.com’s latest Cost of Care report, the online marketplace for caregiving services found that 90% of parents lost sleep; 80% cried (with the number rising to 90% for mothers); 85% sacrificed other life goals; 73% lashed out at loved ones; 71% suffered health issues; and 29% considered suicide or self-harm.

Meanwhile, a recent study from the McKinsey Global Institute said much of the world is facing a “youth deficit” as people have fewer children, setting up top economies for population collapse.

Some of those economies are on track to see 20% to 50% population declines by 2100, requiring big changes in how societies and governments operate.

If these demographic trends continue, younger people will endure slower economic growth while supporting bigger cohorts of retirees, eroding the historical flows of generational wealth, the study warned. 

“The current calculus of economies cannot support existing income and retirement norms—something must give,” it said.