Women make less money than men, even though they work 39 more days per year, according to a global report on gender equality the World Economic Forum released Wednesday.

The analysis, which uses data from 33 Organization for Economic Co-operation and Development countries ― including the United States, much of Europe, Nordic countries and nations in South America and Asia ― might seem odd at first.

You’d think that women would work less than men. After all, more men than women are employed around the world. Eighty-eight percent of men ages 25 to 54 are part of the labor force in the U.S., versus 73 percent of women. And in our prime working years, when incomes really start to peak between age 35 to 44, the numbers spread out even further: 90 percent of men, compared to 74 percent of women.

Men also outearn women. Women in the U.S. earn 80 cents for every dollar a man makes. Women around the world earn an average of $11,000 a year, compared to $20,000 for men, according to the new report.

The thing is, the OECD data considers unpaid work ― the truly critical labor that must happen in order for societies to function: child-rearing, cleaning, cooking, caring for the elderly.

“It is simply valuable work because it is a lot of what it means to be human,” Saadia Zahidi, the head of employment and gender initiatives at the World Economic Forum.

Women do an outsized share of this work ― and it’s holding back gender equality and economies around the world.