Nicholas Bakalar reports: Study Says Women With Mate Get Heavier. The Australian study found that:
After adjusting for other variables, the 10-year weight gain for an average 140-pound woman was 20 pounds if she had a baby and a partner, 15 if she had a partner but no baby, and only 11 pounds if she was childless with no partner. The number of women with a baby but no partner was too small to draw statistically significant conclusions.
There is no reason to believe that having a partner causes metabolic changes, so the weight gain among childless women with partners was almost surely caused by altered behavior....
The only explanation offered in the article is pretty weak, women respond passively to their environment:
Maureen A. Murtaugh, an associate professor of epidemiology at the University of Utah who has published widely on weight gain in women. Perhaps, she suggested, a more active social life may help explain why women with partners gain more weight.
“Think of going to a restaurant,” Dr. Murtaugh said. “They serve a 6-foot man the same amount as they serve me, even though I’m 5 feet 5 inches and 60 pounds lighter.”
Uri Gneezy and Jason Shafrin have another explanation (Why Does Getting Married Make You Fat? Incentives and Appearance Maintenance):
Married individuals weigh more on average than non-married individuals. We suggest that exiting the dating market decreases one’s incentive to maintain their appearance and leads to an increase in body weight. We hypothesize that it is most difficult for individuals to exit a traditional marriage, and easiest for individuals to exit if the couple is cohabitating but not legally married....
So, people in a traditional marriage have more security in the relationship, and invest less in appearance as insurance against a return to the dating market.
Using a 14-year panel data set [from the Netherlands], we test whether or not the ease of exiting a domestic relationship affects weight gain. For men, we find that the type of domestic relationship has little impact on weight gain. For women, however, marriage leads to a 2.4 kg weight gain compared to cohabitating.
This is a weight gain of about five pounds. In this explanation, men and women are purposeful agents adjusting their behavior in response to changing opportunity costs.