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There are upsides and downsides to getting married, but at least one of the perks has remained pretty consistent over time: People who tie the knot tend to be healthier than their unmarried counterparts. As recently as last month, research presented at the British Cardiovascular Society conference reported that single people with “modifiable risk factors” like type 2 diabetes and high blood pressure have significantly higher mortality rates than married people with the same conditions. Marriage has been linked to a longer life span, fewer heart attacks and strokes, and a lower risk of depression.

Of course, the stats aren’t 100 percent positive: Marriage has also been linked to an increased risk of weight gain. And not all studies have come to the same conclusions, especially those where participants self-report on their own health. While older research in this vein has generally shown a strong association between good health and marriage, more recent work has suggests that this protective effect is weakening — and a new study published earlier this month in the journal Social Science Quarterly suggests that it no longer exists at all.

The study, a comparison of married people born between 1955 and 1984, shows that while older generations see improved overall health with marriage, the effect has deteriorated over time. Married people only had the edge in relationships that had lasted ten years or more, and only among women — an effect that “was completely attenuated among women in the youngest birth cohort,” wrote study author Dmitry Tumin, a sociology researcher at the Ohio State University. Compared to their never-married counterparts, the youngest cohort didn’t experience any protective effect with marriage.

Why this is happening, though, remains unclear. Tumin’s research is based on self-reported health information; the problem with data like this, he says, is that it doesn’t shed any light on which specific aspects of health are improving or worsening after marriage.

A few possibilities: It’s possible that that the biggest change has actually been in the way the research itself is conducted, with scholars using increasingly sophisticated methods to weed out potential confounding factors. Alternatively, Tumin says, the recent evidence showing that marriage only weakly influences health “may reflect demographic and cultural trends that have undermined the protective effects of marriage.” For one thing, fewer Americans than ever are getting married, and the age at first marriage is steadily rising. There has also been a rise in people seeking social and economic support via sources other than a spouse, such as living with parents longer, or in long-term roommate situations. Meanwhile, the stigma in remaining unmarried is declining, and single people — especially women — have experienced increasing economic freedom over the past few decades.

In other words, women may indeed have seen health improvements with marriage in the past because it provided a level of economic security that many women are now able to access on their own. But it’s not just that singledom is healthier than it used to be (especially for women) — maybe it’s that marriage has become more stressful.

“Work-family conflict has increased in the closing decades of the 20th century, and spouses’ actual time spent together has decreased over this period,” Tumin wrote. “Against a backdrop of greater demands at home and at work, and less time spent together, today’s married couples may indeed experience marriage more as a source of conflict and stress than as a resource that safeguards their health.”

Of course, lumping all married people into one category is a pretty broad approach, and the quality of the marriage may be a more important predictor of health than the mere fact of its existence. Some studies comparing the effects of “happy” marriages with “unhappy” ones have found that people in an unsatisfying union have an increased risk of heart attack or stroke when compared with those living in wedded bliss.

Tumin’s study also pointed out a rather disturbing trend as another potential explanation: that marriage rates are on the decline among people of lower socioeconomic status. This might be skewing the protective effects of marriage across time, since wealthier people also tend to be healthier.

“It may be the case that in the most recent birth cohorts, when only the most affluent people marry, there could be little change in health after getting married because the health of people who marry is already very good before marriage,” Tumin said. (To account for this so-called “marriage selection,” Tumin estimated how each person’s health changed as they racked up more years in a marriage.)

Another caveat is that the study doesn’t include any data gathered from same-sex marriages. But based on the current data, Tumin said, “It seems unlikely that marriage of any kind would directly cause large improvements in health in recent birth cohorts.”

As sociologist Bella DePaulo, who studies single people, has previously put it to Science of Us, “It’s just ridiculous to think that single life is … a life of sadness and bad outcomes.” Here’s one more piece of evidence that that’s true.