Wedding vows traditionally account for the possibility of bumps in the road ahead. For richer for poorer is an admirable sentiment. But they rarely mention a couple’s changing fortunes when it comes to body mass index (BMI). It is not unusual for newlyweds to notice their waistlines expanding. And my research has shown that marriage does indeed make men fatter.

The study found a link between being married and men gaining weight – with the early days of fatherhood adding to the problem. On average, married men had a higher BMI than their non-married counterparts, which equated to an extra 1.4kg (3lbs) on the scales. There was no effect on male BMI if a wife became pregnant, but in the early years after childbirth, men gained weight. It took the period just before and after divorce to register a dip in male BMI.

The findings finally clear up the confusion over a possible link between BMI and men’s marital status. There are a number of competing theories about what happens to weight when people get married. The findings would support the idea – the so-called “marriage market theory” that people who are single but seeking marriage have more incentive to stay fit – that they appear to make more effort than men who are married. It also supports another idea, the “social obligation theory” that being married leads to more men eating more regular meals and attending more social occasions where richer foods are served.

Competing theories

Given the public health concerns about obesity, understanding more about the social factors that can cause weight fluctuation is important. The link between marriage and BMI has been much debated, with different theories offering conflicting predictions.

For example, there is some evidence that married people are generally healthier, as they benefit from family support and are less likely to engage in risky behaviour. Known as “marriage protection theory”, it would normally predict a lower BMI among married men.

Married individuals could also be expected to have a lower BMI due to “selection theory”. We all select a spouse based on a set of characteristics, including attractiveness. And fitter people are more likely to be chosen as spouses. According to this theory, marriage has no impact on individual BMI, but it tells us that people with lower BMI are more likely to become married.

Conversely, there are theories which tell us that married people do in fact “let themselves go” and gain a few pounds once that ring is on their finger. The marriage market theory, for example, treats the world of relationships a bit like a business – you’ve got to do a bit of advertising. It states that people who are single and on the look out for a future spouse have higher incentives – and make more effort – to stay fit than those who are already married. But once married, the pressures of the singles market are gone, which results in higher BMI in married people.

Traditions of social obligation also suggest that those in relationships eat more regular meals (and richer foods) due to an expanded social life which come with marriage.

With this ring I thee fed

To try and make sense of all these competing theories, I analysed information about 8,700 heterosexual males in the US from 1999 to 2013, using data from the Panel Study of Income Dynamics. In addition to standard socioeconomic variables such as education, income, employment status and age, I was able to factor in a person’s changing BMI over time.

I found that married men did have a higher BMI (half a point) than their non-married counterparts, which was largely driven by weight fluctuations before and after marriage. (Male BMI also decreases just before and after divorce, as they change their behaviour in line with the “marriage market theory” incentives to watch their weight again.)

My findings support the theory that marriage leads to more social occasions involving richer foods, or more regular meals for men.

As far as parenthood is concerned, in general fathers of children under 19 do not have a higher BMI than non-fathers or fathers of older children, although they do tend to have a higher BMI in the early periods following childbirth. New fathers may have less time to exercise. Having children can also decreases the chance of divorce, making the marriage market incentives to stay fit even less relevant.

The effects of marriage on BMI may not be large, but they are statistically significant. It is valuable to understand what social factors may cause weight fluctuations, especially common ones such as marriage and parenthood.

Being aware of potential risks allows us to make informed decisions about health. For married men who want to avoid BMI increases, that will mean being mindful of their own changing motivation, behaviour and eating habits.