According to the writer Malcolm Gladwell, 10,000 hours of practice is what it takes to become world class at most things. Part of the reason the Beatles were the Beatles, he says, is that they spent so many hours playing dingy rock clubs first.

Gladwell’s principle comes to mind when I think about another statistic: Over a lifetime, the average American woman will spend about 40,000 more hours cleaning, cooking, and caring for children than the average American man. If Gladwell is correct, that gives every man enough extra time to become John, Paul, George, and Ringo. I wonder what amazing things women could accomplish if they had their choice of how to spend those 40,000 hours.

One of the most surprising things about this time gap is that it seems to fit the America of 50 years ago better than the America of today. Back then, the explicit ideal of family life was a mother and a father in separate spheres, one domestic and the other professional. But things have changed a lot since then. I can see that very clearly when I compare my mother’s life and my daughters’ lives. Now, women can be anything they want to be, and a lot of men are unapologetically enthusiastic about cooking elaborate meals and cooing back and forth with their babies.

The problem is that while the written rules are different, the unwritten rules haven’t changed as much as we might like. Girls my daughters’ ages — they are 13 and 20 — might not have quite as much freedom as they think they have — or as much as they deserve.

The explicit norms that organized people’s time 50 years ago have become implicit assumptions that run our lives today.

Every day, American women spend 4 hours on unpaid work, while American men spend 2.5. The explicit norms that organized people’s time 50 years ago have become implicit assumptions that run our lives today, even if we don’t recognize it — and they are ingrained from an early age. Boys are less likely than girls to be assigned chores, but more likely to be paid for them — as well as more likely to get an allowance to begin with. Even the chores themselves are unevenly divided: boys’ chores are more likely to be outside, while girls are more likely to be stuck inside. Sheryl Sandberg has called this the “toddler wage gap.”

I first came to this issue, which experts call time poverty, through my work in global development. Around the world, the time gap is even more skewed: In India, to take one example, women spend 6 hours every day on unpaid work, compared to less than 1 hour for men.