The mental and physical health benefits of marriage have been noted in many scientific studies. But maintaining a healthy weight isn’t one of them, evidently.

A new multi-nation study published in the journal Social Science & Medicine has found that married couples have a higher body mass index, on average, than singles.

The findings, by researchers at the University of Basel and the Max Planck Institute for Human Development, are based on analysis of more than 10,000 men and women from nine countries — Austria, France, Germany, Italy, the Netherlands, Poland, Russia, Spain, and the United Kingdom..
Although the research found that married couples tend to eat healthier meals than singles, they also tend to weigh significantly more and engage in less exercise.

According to the World Health Organization, a normal body mass index is between 18.5 and 25. Overweight is defined as an index between 25 and 30, and obesity as above 30.

The average body mass index of the single men in the study was 25.7; that of the married men was 26.3. For women, the average index was 25.1 for singles and 25.6 for married women.

A high body mass index can be a risk factor for chronic illnesses such as diabetes or cardiovascular disease.

"Our findings show how social factors can impact health. In this case, that the institution of marriage and certain changes in behavior within that context are directly related to nutrition and body weight," said Ralph Hertwig, director of the Center for Adaptive Rationality at the Max Planck Institute in Berlin.