Studies consistently find that people with more education tend to live longer. But scientists aren’t exactly sure why.

One explanation could be that they’re richer and thus have more money to invest in their health. Or maybe there’s just something intrinsically special about people who stay in school longer. Perhaps they’re smarter, more resilient. Those qualities could easily help them live longer, healthier lives.

But that isn’t a satisfying answer because we — mostly — can’t control our genes and the environments we’re born into.

There’s another hypothesis for why school may yield longer lives. It’s that education builds “human capital,” or a systematic way of thinking that benefits every decision. Those tiny good decisions add up to a protective factor that helps you live longer.

“Education is likely to provide general human capital that can be used to maintain and improve health in a wide range of circumstances,” David Cutler, Angus Deaton, and Adriana Lleras-Muney — Harvard and Princeton economists — write in a 2006 paper in the Journal of Economic Perspectives. This human capital, they say, will aid long-term survival “whenever there exists a mechanism or technology that more knowledgeable and educated people can use to improve their health.”

A new study from researchers in Sweden adds compelling evidence to the human capital argument. Its main conclusion: People with higher levels of schooling (more than 13 years) live around three years longer than people with less schooling (under 10 years), after controlling for biology and childhood environment.

Three years is a long time. It’s comparable to the average difference in longevity between men and women, the authors of the study, published recently in Demography, explain. It’s also the same as the gain in longevity from quitting smoking or getting a kidney transplant at age 60 or older.

What 50,000 Swedish twins can tell us about schooling and longevity

The researchers couldn’t run an experiment to determine if education matters for longevity. That would require taking identical groups of people, giving one group more years of schooling than the other, and then tracking outcomes for the rest of their lives.

Luckily, however, they weren’t completely helpless. Nature provides a natural experiment through which researchers can look for answers: twins. Studying identical twins allows researcher to more finely control for factors like IQ, personality, and home environment that can contribute to longevity. A big enough data set would allow the researchers to compare twin pairs and ask a simple question: If one twin gets more education than the other, does that twin live longer?

Doubly lucky was that in Sweden, there’s an absurdly dense treasure trove of twin data.

The Karolinska Institutet in Stockholm houses a registry of data on more than 85,000 twin pairs — the largest such repository in the world. The research team mined 50,000 of those twins (18,000 of which were identical) to help answer this question of school on education.

The study combined the twin data set with public records on mortality and educational attainment. The researchers even took extra steps to control for differences between the identical twins, adjusting for birth weight and height — which roughly correlate with childhood health and intelligence — in the analysis. (“By no means is this a perfect solution,” the authors note.)

Controlling as best they can for these variables, the researchers found a significant and persistent impact of education on longevity. This was true for both genders.

This sounds too good to be true. What’s the catch?

The data set used in this survey is huge and comprehensive, but it comes with a huge caveat.

The 50,000 twins were born a long, long time ago — between 1886 and 1958. Back in the early 20th century, schooling wasn’t as common as it is today, so it’s hard to know if the results can generalize into the modern world.

(That the data is so old contains an advantage too, as the researchers were able to track the impact of schooling over entire lifetimes. At the least, it’s interesting to learn that for the late 19th and early 20th centuries, schooling was important for lifelong wellbeing, at least in Sweden)

The researchers, however, suggest one tangible way the data can inform education policies today, particularly in the 136 countries whose residents get, on average, less than 10 years of schooling, according to the most recent data from the United Nations. (It’s a reminder most kids around the world don’t actually have a choice about how long they get to stay in school. They drop out because they have to work, or because there’s no affordable school nearby.)

But maybe the link between school and longevity can help persuade governments to invest more in education where it’s lacking.

“Our results therefore suggest that the longevity gains from extending levels of schooling in these countries may be substantial,” the study authors write. “Such gains and improved health may well constitute important mechanisms by which schooling also affects productivity and economic growth, which would further emphasize the crucial role of schooling in the development of human welfare.”

It’s also entirely possible that schooling matters more than ever for long-term well-being, as the authors note that the “socioeconomic gradient in mortality has grown stronger over time.”

These findings dovetail nicely with some other small but compelling evidence that finds school can increase a person’s IQ. Analyses have found evidence that each additional year in school adds 3.7 IQ points. And great IQ is also associated with longevity.

The gap between the rich and poor in longevity continues to widen. If school is the ticket to a higher social class, it may still be the ticket to a longer life.