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Associate Professor of Sociology, University of Virginia 

BMI and Relationship Transitions in Germany

Bryce J. Christensen and Nicole M. King

It is commonly known (though not as well researched) that individuals tend to gain weight once they are in a committed relationship. The guard against gaining those extra pounds is let down a bit, once finding a mate is no longer a high priority. Now, researchers out of Germany attempt to study exactly how, when, and how much weight gain occurs upon entry into and exit from both cohabiting and married relationships.

The researchers open by explaining the reasons for their inquiry. “Most empirical studies,” they write, “on the relation between marital transitions and BMI support the marriage market hypothesis,” which suggests that “individuals who are no longer on the marriage market, and thus no longer concerned with attracting a mate, gain weight.” Findings for weight gain after divorce or cohabitation dissolution are mixed. Why might individuals either gain or lose weight upon relationship transitions? The researchers posit that negative-protection behaviors such as regular and more frequent meals, as well as more calorie consumption at meals shared with others, may be to blame for weight gain after marriage or the beginning of cohabtitation. Likewise, a marriage-protection hypothesis would suggest that marriage (or cohabitation) helps couples keep at a healthy weight because they can encourage each other to eat well and exercise, and also cook together, which is a healthier alternative to fast food or other convenient options. This study aims to discover whether either of these hypotheses is supported.

The data comes from the German Socio-Economic Panel, “an ongoing, nationally representative longitudinal study of private households in Germany initiated in 1984 with several refreshment samples thereafter.” The final sample size was 20,950 individuals with 81,926 observations. The measurements were: body weight and height, duration of cohabitation or marriage/time since separation or divorce, weight-related behaviors (exercise, healthy eating, and smoking), and also control variables (pregnancy, recent birth, having children, employment status, perceived stress, and subjective health). 

And indeed, the effects of both marriage and cohabitation upon weight gain were significant. The researchers report that only three percent of all observations “did not report any change in BMI after any relationship transition.” Indeed, “[i]n both men and women, body weight was higher for cohabiting and married respondents than for those without a partner.” Contrary to the researchers’ expectations, however, cohabitation proved much more deleterious to weight gain than marriage. They summarize, “These results are largely consistent with the marriage market hypothesis: Transitions into cohabitation and marriage were followed by weight gain. However, the weight gain after four or more years of cohabitation was much larger—in fact, double the size—of that occurring after four or more years of marriage.”  

Also interesting is the influence that relationship exits had on participants’ BMIs. The researchers find, “Partly consistent with the marriage market hypothesis and its implication that people lose weight again after separation or divorce, respondents who separated generally showed BMIs similar to those they had when living alone.” But, they continued, “the present findings showed weight gain in the wake of divorce. In fact, men gained more weight after their divorce than during their first marriage.” 

In the discussion section of their analysis, the researchers note that various controls, such as exercising, healthy eating, and smoking behavior, “did not account for changes in BMI after relationship transitions. Thus, the present findings contradict both the respective hypotheses formulated here and previous findings indicating that weight-related behaviors change after cohabitation or marriage. . . ; they do not support either the negative protection hypothesis or the marriage protection hypothesis.” The researchers suspect that their study set-up did not allow them to adequately measure these variables, however, and note that other studies have shown different results.

In closing, the researchers note that 1) cohabitation has a stronger effect on BMI than does marriage, and 2) that effect is longer-term. Their findings, they remark, are important for a number of reasons, among which is the increase in relative mortality risk caused by relationship transitions. “Summing up the observed relationship effects on BMI from four years each of cohabitation, marriage, separation, and divorce . . . reveals that men and women gained around 2.3 and 1.4 kg/m2”—an increase that “would increase [men’s] all-cause mortality risk by up to about 13%.” 

The researchers suggest that closer attention be paid to the effect that such transitions have upon BMI, as neither marriage nor cohabitation are unqualifiedly good for BMI, and “today’s population levels of obesity do not afford the luxury of ignoring any contributing factor.” 

(Jutta Mata et al., How cohabitation, marriage, separation and divorce influence BMI: A prospective panel study, SOEPpapers on Multidisciplinary Panel Data Research, No. 973 [2018]: 10.1037/hea000654.)