Published in December in the Journal of Happiness Studies, the study analyzed people’s responses to two huge British surveys about life satisfaction. The study’s authors— John Helliwell, an economist at the University of British Columbia, and his former graduate student, Shawn Grover —hoped to answer two questions: Is marriage’s effect on happiness short-lived? And does marriage cause happiness, or is it that happy people are more likely to get and stay married in the first place?

To isolate these different variables, the researchers turned first to a longitudinal survey project by a British institute. From 1991 to 2009, demographers asked 30,000 adults of varying ages about their lifestyles and moods, posing the same questions year after year. This allowed the Helliwell research team to see how happy people were long before—as well as a decade after—they met their partners, and also to assess how their levels of happiness changed over the long run. Whether they ultimately married, divorced, stayed single or were living together, the researchers thus had a snapshot in hand of their pre-relationship sense of fulfillment.

This is crucial, because happiness is U-shaped across the adult life-span, meaning that it normally rises when we’re young adults, drops during middle age when life’s stresses and existential questions loom large, and then rises again as older adults regain their equilibrium. To bolster their findings, the researchers also used a second survey, 10 times as large as the first and based on U.K. census data. This Annual Population Survey by the national statistics office queried more than 300,000 people from 2011 to 2013 about their anxieties, social lives and happiness.

The researchers’ answer to their first question—whether marriage had a merely short-lived effect on happiness—was definitively no. When they controlled for their pre-marriage status, married people were 10% more satisfied than people who were single—and were more likely to stay that way. While cohabiting couples were happier than single folks, they were only three-quarters as happy as marrieds. “Marriage seems to be most important in middle age, when people of every marital status experience a dip in well-being,” the economists wrote.

Not all marriages are created equal, of course. At least a dozen studies show that marriages characterized by stonewalling, contempt or conflict are bad for us, undermining our sleep, immunity and cardiovascular health. Conversely, one of the most telling findings of the Helliwell study is that a close marital bond spurs long-term happiness. People who named their spouse as their best friend were twice as happy as those who didn’t. “The more likely you are to regard someone as your friend, the more likely you’ll think the best of them, and not take them for granted. If that’s true, it’s a very successful marriage,” Prof. Helliwell said.

It’s good to know that someone has your back. That level of commitment, formalized by a ritual and a legal document, may be one reason why the advantages of marriage trump those of just living together. Along with chocolate, it’s all food for thought as we approach Valentine’s Day.