Marital bliss may bulk up your well-being, but it also tips the scales when it comes to weight, according to a new four-year study.

Reporting in the journal Health Psychology, researchers find that relationship satisfaction is linked with an increase in body mass index over time. By contrast, when couples are less satisfied in their marriage, or even contemplating separation, they're significantly less likely to incur the weight penalty of their happier counterparts.

"It's pretty well established that marriage is associated with weight gain, and divorce is associated with weight loss," said Andrea Meltzer, assistant professor of psychology at Southern Methodist University in Texas. "But the extent to which satisfaction plays a role hasn't been examined until now."

The outcome of the study was uncertain from the start.

Prior research has found that satisfying relationships are actually helpful in promoting good health practices. But Meltzer notes that those studies focused more on behaviours - such as taking medication on time or getting an annual physical - than weight.

Literature on mating, meanwhile, has shown that weight maintenance is motivated primarily by a desire to attract a partner. From this perspective, it makes sense that keeping svelte could be a function of dissatisfaction, and a desire to get back on the market.

To test which of these models held true, Meltzer and her co-authors tracked 169 newlyweds (married within the previous six months) for four years, checking in biannually to assess such measures as height, weight, marital satisfaction, stress, steps toward divorce and so on. Upon analyzing the results, they found that more satisfied couples gained more weight, even controlling for confounding factors such as pregnancy.

"It was a relatively small amount of weight," said Meltzer, who used changes in body mass index to assess this. "But we only looked at a snapshot of the first four years. If you take one of those happy marriages that go on for 20, 30, 40 years, it could potentially become unhealthy."

In fact, a 2007 study of nearly 8,000 people found that over a five-year period, married men gained six more pounds than their same-aged bachelor buddies, while wedded women gained nine more pounds than their single counterparts.

As for why a happy marriage is correlated with a heavier physique, researchers can only speculate.

"What I think is happening is that people are thinking about weight-maintenance in terms of appearance as opposed to health," Meltzer said. "The individuals who were buffered from weight-gain were the ones who were considering going back into the mating market and having to find a new partner, which suggests it has something to do with looks."

Meltzer suggests that happy couples who consider weight in terms of health, as opposed to appearance, may be able to avoid the costs at the scale. But she also cautions that happy couples whose pants size has crept up over the years can't discount the effects of, say, eating a quarter pounder every day.

"There's more than just relationship satisfaction accounting for increased weight over time," she said.