Individuals in couples—whether married or not—tend to weigh more than singles. But contrary to what has often been assumed, it is not so much marriage that leads to weight gain as moving in together. In fact, couples who move in together gain about twice as much weight in the first four years as do couples in the first four years of marriage. This effect persists even when important factors such as age, childbearing, exercise, smoking, health, and stress are excluded from the equation. „In other words, this weight gain is primarily due to the change in relationship status. Living together often brings about changes in people’s day-to-day eating habits – for example, someone who skipped breakfast or just grabbed a piece of fruit when living alone might start eating a more substantial breakfast after moving in with a partner. People tend to eat more and to consume more calories in company,“ says Ralph Hertwig, co-author of the study and Director at the Max Planck Institute for Human Development.
When couples separate, their body mass index (BMI), which describes the ratio of weight to height, tends to drop back to its pre-cohabitation level in both men and women. This finding is in line with the so-called marriage market hypothesis, which predicts that people who are looking for a partner try to reach a lower body weight, which is perceived to be more attractive. Interestingly, though, both sexes gain most weight after a divorce following a separation. One possible explanation for this finding is that many of them – especially the men – were back in a new relationship by the time they divorced.
„Moving in together and getting divorced are important windows of opportunity for targeted measures to prevent weight gain. Little attention has yet been paid to social factors – such as changes in relationship status – that influence the development of overweight and obesity. Instead, the focus has been on individual factors, such as knowledge and willpower. Our results show that an unmarried man who is slightly overweight before moving in with a partner will gain an average of about 7.5 kilograms over four years each of cohabitation, marriage, separation, and divorce. That increases his general mortality risk by up to 13 percent,“ says Jutta Mata, Professor of Health Psychology at the University of Mannheim and Adjunct Researcher at the Max Planck Institute for Human Development.
This study was the first to investigate the long-term effects of change in relationship status on BMI. The researchers compared data collected from 20,950 individuals aged between 19 and 100 years in Germany over a 16-year period. The data came from the longitudinal Socio-Economic Panel study (SOEP) run by DIW Berlin, which collects representative data on the population in Germany. Demographic data such as age, gender, and marital status were surveyed along with participants’ height and weight. Data on important life events such as having children or changes in employment status were also assessed, as were perceived stress, subjective health, eating habits, exercise, and smoking.
The Max Planck Institute for Human Development in Berlin was founded in 1963. It is an interdisciplinary research institution dedicated to the study of human development and education. The Institute belongs to the Max Planck Society for the Advancement of Science, one of the leading organizations for basic research in Europe.
Mata, J., Richter, D., Schneider, T., & Hertwig, R. (2018). How cohabitation, marriage, separation, and divorce influence BMI: A prospective panel study. Health Psychology, 37(10), 948-958. dx.doi.org/10.1037/hea0000654