Last updated: May 31, 2016

In a study presented at the annual meeting of the Gerontological Society of America, University of Michigan researcher Shannon Mejia and her team looked at health indicators from 1,568 married couples across the United States. The couples were separated into two groups: those who had been married for about 20 years, and those who had been married for about 50 years. Overall, Mejia found that the couples had striking similarities in kidney function, total cholesterol, and grip strength.

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In the psych literature, this phenomenon has the surprisingly poetic phrasing of “couples concordance in health.” There are two hypotheses as to why: It could be due to mate selection—which tends toward homogeneity in race, education, and age—or it could be due to shared experiences, where your health is the outcome of living your life together.

Mejia and her fellow researchers found that there was similarity in the biomarkers beyond the race, education, and age factors that they statistically accounted for. The strongest example was in total cholesterol: The math says that 20 percent of the outcome for total cholesterol is attributable to couple membership.

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But contrary to what you might think, couples in the longer-marriage group weren’t more similar than those in the group with shorter marriages. Mejia suspects this has to do with the limits of marriage duration as a metric: as is, the 20-year group includes couples that got hitched at 25 as well as 45. If you’re wed in middle age, she reasons, your patterns of health are already pretty well established. That’s a nuance that’ll be addressed as the research heads toward publication, she says.

The similarity between members of couples goes against what Mejia calls the “independence assumption” in the United States: Your health is thought to be individualistic. After all, it’s your body that the doctor investigates, not your partner’s. But as Mejia’s work indicates, environments matter.

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“It’s something that researchers have learned to control for, because it’s known that people in groups are more similar to one another than a random person on the other side of the world,” Mejia tells Science of Us. “In our case, we’re looking at couples. We’re taking what used to be taken as a nuisance—the non-independence of the data—[and it] becomes our outcome of interest.”

Because of the nature of the data she’s working with—a large-scale longitudinal study—Mejia can’t really isolate the mechanisms of couple health concordance. She points to the work of University of British Columbia psychologist Christiane Hoppmann, who takes a more granular approach. Hoppmann zooms in on the mechanics of coupledom, finding, for instance, that members of couples who share greater intimacy have lower levels of cortisol, a hormone associated with stress.

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All this research adds to the profundity of a wedding vow: Through sickness and in health, it’s a declaration of interdependence.

This article originally appeared on Science of Us.