Is marriage good or bad for the figure?

The researchers drew on representative cross-sectional data from 10,226 respondents in Austria, France, Germany, Italy, the Netherlands, Poland, Russia, Spain, and the United Kingdom. Their study is the first to compare the relationship between marital status and body mass index in nine European countries. Beyond their focus on married couples, the researchers conducted additional analyses with cohabiting couples. They also examined possible reasons for weight gain by asking respondents about their eating and exercise behaviors.

Findings from all nine countries showed that couples have a higher body mass index than singles -- whether men or women. The differences between countries were surprisingly small.

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.

Although these differences may seem small, they are meaningful. In an average-height woman of 165 cm or an average-height man of 180 cm, they represent a difference of about 2 kg. Importantly, the effects of socio-economic status, age, and nationality are already taken into account in these results. "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," says Ralph Hertwig, Director of the Center for Adaptive Rationality at the Max Planck Institute for Human Development in Berlin.

Survey findings on respondents' eating and exercise behaviors offered possible reasons for this trend. For example, couples reported buying more regional and unprocessed products and less convenience food. Moreover, married men were more likely than single men to buy organic and fair trade food. "That indicates that particularly men in long-term relationships are more likely to eat more consciously and, in turn, probably more healthily," says Jutta Mata, lead author of the study and Assistant Professor of Health Psychology at the University of Basel. But it does not mean that they are generally healthier: The study also shows that married men do less sport than singles. "Our findings indicate that couples are not healthier in every respect, as has previously been assumed," says Jutta Mata.

The respondents were asked about their eating and exercise behaviors and attitudes in face-to-face interviews. This approach ensures higher data quality: People's self-reports, for example of their weight, are more realistic if they are asked in person rather than, for example, by telephone.