MELISSA BLOCK: The economist John Maynard Keynes once wrote an essay titled "Economic Possibilities For Our Grandchildren." It was 1930. And in the essay, he made a startling prediction. Keynes figured that by the time his children had grown up, basically now, people might be working just 15 hours a week. David Kestenbaum with our PLANET MONEY team tried to sort out where Keynes went wrong.

DAVID KESTENBAUM: Since the essay was called "Economic Possibilities For Our Grandchildren," we thought we'd check in with Keynes's grandchildren all these years later and see how many hours they ended up working. It turns out Keynes did not have any kids, so we did the next best thing.

NICHOLAS HUMPHREY: My name is Nicholas Humphrey.

KESTENBAUM: And your relationship to John Maynard Keynes...

HUMPHREY: OK. Yes. I'm Maynard Keynes's sister's grandson.

KESTENBAUM: You're the closest thing we have to his grandkids.

HUMPHREY: Yes. We're kind of ghost grandchildren.

KESTENBAUM: Humphrey is a retired professor. I asked how many hours a week he used to work - not 15.

HUMPHREY: Oh, I probably worked about 15 a day, and so...

KESTENBAUM: Fifteen a day...

HUMPHREY: Oh, but...

KESTENBAUM: That's over a hundred hours a week.

HUMPHREY: Yeah. I mean, I worked till, like - from breakfast till I went to bed at night.

KESTENBAUM: We tried another person in the family tree.

SUSANNAH BURN: My name is Susannah Burn.

KESTENBAUM: And what's your relationship to John Maynard Keynes?

BURN: I'm his sister's granddaughter.

KESTENBAUM: Susannah Burn is a self-employed psychotherapist. She says she lives in a small, quiet town, and she works 50 hours a week - actually has a little trouble taking time off.

BURN: I used to write, not working in my diary to remind myself not to put in an appointment. And I discovered that I still worked.

KESTENBAUM: (Laughter).

BURN: So I now have to write, don't work on the page in order to frighten myself when I look at the page and realize that I mustn't.

KESTENBAUM: John Maynard Keynes's argument for the 15-hour work week was that over time, thanks to machines and technology and new ideas, people get more productive. An hour of labor produces more and more stuff. Keynes figured we'd just decide to work less. In some countries, the number of hours worked has dropped some. But take the United States. In 1950, people here worked, on average, about 38 hours a week. Today, six decades later, we work 34 hours a week - a bit less but not much. Richard Freeman, a Harvard economist, says one of the things Keynes underestimated was the human desire to compete.

RICHARD FREEMAN: Think about these basketball players who get $80 million, $100 million contracts. You might have said they'll just take one contract. That's enough money to live your entire life in wealth and just never work a day.

KESTENBAUM: Or just play, like, one year. Or just play, like, a - play a couple years, right? Yeah.

FREEMAN: Yeah, exactly right. But they don't, and they push themselves. They push themselves. Keynes would've had great trouble understanding...

KESTENBAUM: (Laughter). LeBron James - yeah.

FREEMAN: (Laughter). Yes, yes.

KESTENBAUM: In fact, he says, earning more money can make it harder to take time off. If someone is paid $200 an hour, do you really want to leave early and go to the beach? You'll be sitting there on your towel, reading a novel, thinking, is this really worth $200 an hour 'cause you could be back at the office. The better you are at your job, the harder it can be to not do it. It's worth pointing out that Keynes himself seemed to have trouble following his own advice.

HUMPHREY: Maynard, of course, died from working too hard.

KESTENBAUM: This is Nick Humphrey again.

HUMPHREY: You know, his heart ran out. He was just - he wasn't sleeping. He was overdoing it all the time.

KESTENBAUM: World War II had just ended. Keynes was trying to help put the global economy back together. His wife worried working would kill him. Here's Susannah Burn.

BURN: His wife was very angry about it.

KESTENBAUM: She was angry that he was working so hard at the end.

BURN: Yeah, yeah. She spent a lot of time trying to protect him from himself and would stop him doing things. She tried to help him say no, but in the end, he couldn't say no.

KESTENBAUM: Keynes once said that his only regret in life was that he hadn't drunk more champagne, but I don't think that's true. He liked working. David Kestenbaum, NPR News.