As the American obesity rate keeps ticking upward, it masks other key trends. One is that, for a decent part of our population, weight actually isn’t going up. The slimmest segments remain just as thin as they were 40 years ago.

Instead, a lot of the weight gain has been concentrated in a smaller group of Americans. That’s created more variation in weight, a bigger gap between the lightest and heaviest Americans. Here’s what that looks like in graph form (for context, the National Institute of Health defines normal weight as a body mass index, or BMI, between 18.5 and 24.9. A BMI over 30 is considered obese):

Our increase in obesity is often explained, in large part, by a rise in the availability of cheap, unhealthy foods. But here’s what it doesn’t explain: Why are some Americans gaining a lot more weight, and others essentially staying the same? It’s not as if the slimmest individuals are living in isolation from McDonalds.

A new study suggests that impatience has a lot to do with it. Economists Charles J. Courtemanche, Garth Heutel and Patrick McAlvanah combed through historical data on body mass index. They also looked at indicators of impatience for the same population, factoring in preferences on whether they’d want a small prize now or bigger prize later. They controlled for other factors that might come into play, such as demographics and financial characteristics.

With everything else held constant, the researchers found that impatient individuals are more likely to be obese than people who are good at waiting. Here’s how the two BMI distributions match up:

“We controlled for basically every variable in the kitchen sink,” says Courtemanche, the lead author and a professor at the University of Louisville. “It seems if you genuinely hold all else constant, the more patient you are, the less you weigh.”

It didn’t used to be like this. If you go back to the 1950s, as these researchers did, patience — how good Americans were at waiting — didn’t really matter when it came to weight, probably because they didn’t have much in the way of shortcuts: no burgers at McDonalds or microwave meals, no drive-thrus. You may have been impatient, but paying more for food that took longer wasn’t really an option back then. It was a fact of life.

“You could think, ‘Maybe we can just afford to eat more food, and that’s why were gaining weight,’ ” says Courtemanche. “But it’s more complicated than that and not the end of the story. Foods getting cheaper, faster or whatever makes them more available only changes behaviors for a certain group of people.”

Our food environment has changed a lot over the past 50 years. As David Kessler, former FDA commissioner, often puts it, we live in a “food carnival” where just about any craving we have can be almost immediately satisfied. It’s not just that we have more food options, or that they’ve gotten cheaper, but also that a lot of the social norms around how we eat have broken down: Meals happen while we’re walking down the street or driving in our cars. We have more food shortcuts and ways to indulge our impatience that can become hard to resist.