Couples who work together to change their unhealthy habits appear to have more successful outcomes, a new study suggests.

Researchers in the U.K. found that men and women were more likely to stop smoking, exercise more or lose weight if their partner joined them in becoming healthier. For example, a smoker was more likely to quit if their partner also stopped smoking than if their partner had never smoked.

The study shows that "changing together" is associated with even better outcomes than even having a partner with a consistently healthy lifestyle, said study researcher Jane Wardle, a psychology professor and director of the health behavior research center at the University College London.

Exactly why couples who change their health behaviors together have better results is unclear, but it's probably because partners can influence each other's behavior in both positive and negative ways, the researchers said.

"Our study didn't address the reasons for success," Wardle told Live Science. But she suspects the explanation has to do with sharing the problem, providing mutual support and perhaps adding a touch of competition. [5 Wacky Things That Are Good for Your Health]

The findings are published online today (Jan. 19) in the journal JAMA Internal Medicine.

Changing as a couple

For the study, researchers looked at data collected from more than 3,700 middle-age or older couples in the United Kingdom who were married or living together. The men and women were participants in a long-term study on aging, and they regularly completed questionnaires about their health behaviors for up to a four-year period.

The researchers looked at the effect of one partner's decision to make a positive change in one of three health behaviors — smoking, exercising or losing 5 percent of their body weight — on the other partner's health habits.

The data showed that when one person changed an unhealthy behavior, having a mate who was also making efforts to become healthier in the same area over the same time period had a stronger influence than having a mate who already exhibited that healthy behavior and could serve as a role model.

Having a consistently healthy partner did make a difference when it came to quitting smoking or becoming more active. For example, having a partner who never smoked or liked to exercise appeared to increase the other mate's chances of changing their unhealthy ways in these two areas, compared with having a partner who was unhealthy in those areas. But it had no impact on weight loss success.

The researchers found that overweight people whose partners had no weight problem were not any more likely to lose weight than overweight people with partners who did have a weight problem. However, in couples where both the man and woman were too heavy, an overweight mate who lost weight served as a powerful motivator. The other partner in these pairs was three times more likely to also shed pounds, according to the findings.

Becoming healthier

The researchers suggest that behavior-change programs may be more successful if they were aimed at couples rather than individuals.

It's unclear what motivates couples to make them change together, whether it's a joint decision to become healthier, or observing one partner's attempts to modify an unhealthy habit encourages the other partner to also follow suit.

Regardless of the reason, the key to behavior change is making that change as easy as possible, Wardle said. To help a partner become healthier, she recommended being helpful about change rather than just reminding a person about the desired health behavior, or nagging that individual.

Another way to be supportive is by suggesting a reward for meeting a goal. For example, a couple might agree that if they stick a healthy diet for a whole week, they will treat themselves by going to a movie over the weekend, Wardle said.

Although the couples in this study were ages 50 and over, age may have little effect on whether twosomes change their health habits. Wardle speculated the results would likely be similar in younger pairs.

"It's nice to see that older couples can still influence one another," Wardle said. "They're not just set in their ways."

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