Committing to a weight-loss program can have an unexpected benefit: Your significant other may shed some extra pounds, too, even if they aren’t trying. That’s what researchers found in a new study on the so-called weight-loss ripple effect, published today in the journal Obesity.

The study, which was sponsored by Weight Watchers, tracked the progress of 130 married or cohabitating couples for six months. In all of the couples, one partner had a desire to lose weight, and both partners agreed to weigh-ins after three and six months. Half of the partners followed Weight Watchers, while the others received a four-page handout with healthy-eating and exercise tips, and were told to try to lose weight on their own.

The researchers wanted to see if one approach would work better than the other. As it turns out, they were both effective, and everyone involved lost weight. After three months, those in the Weight Watchers group had lost more weight (about 7.4 pounds versus 4.3 pounds), but by the end of the trial, both groups had lost about 14 pounds.

But the partners who didn’t participate — most of whom were men — lost weight, too. On average, people whose partners joined Weight Watchers lost about 3.3 pounds in the first three months, and about 4.9 pounds over the full six-month study. Those whose partners were given a self-guided weight-loss approach had lost about 2.1 pounds at the three-month check-in, and 4.2 pounds at six months. Because of the study’s margin of error, the difference between the two groups was not statistically significant.

Lead author Amy Gorin, PhD, a professor of psychological sciences at the University of Connecticut, said in a press release that when one person changes their behavior, it’s not unusual for the people around them to change, as well. When one partner starts counting calories, weighing themselves regularly, or making healthier food choices, for example, their partners might emulate them.

Four or five pounds may not seem like a lot. But by the end of the study, about a third of the “untreated” partners in the study had lost more than 3% of their initial body weight, which the experts say has measurable health benefits.

Partners also tended to lose weight at similar rates: If one member of a couple lost weight at a steady pace, his or her partner did too, and the same goes for people who lost weight slowly.

“This data suggests that weight loss can spread within couples, and that widely available lifestyle programs have weight loss effects beyond the treated individual,” the authors wrote in their paper. They also suggest that programs like Weight Watchers could help more people by actively involving spouses and partners in the weight loss process.

Gorin says she and her colleagues next hope to look at whether the weight-loss ripple effect could go even further, to other family members who share a household, or to close friends or coworkers.