A few years back the phrase “dad bod” emerged to describe men, especially fathers, who have hints of lean muscle lurking beneath noticeable body fat, perhaps particularly around their bellies. There’s increasing evidence that men in industrialized countries like the United States tend to gain weight after they move in with a partner, marry, or become parents, lending some credence to the “dad” in dad bod. Explanations for the dad bod phenomenon seem to focus around what men do, for example, indulging in pizza or beer and too often skipping the gym. As far as we know, these pop culture and casual conversations about dad bod have not considered the biological changes that men can experience as they get married or become fathers, but maybe they should.
When women become pregnant, give birth, and breastfeed there are clear physical signs and behaviors associated with those biological experiences of parenthood. Only over the last two decades have we learned that men’s bodies also respond to parenthood. Investigations into the “biology of bonding and parenting” reveal that women and men physiologically change as their commitment deepens in romantic and family relationships. Large long-running studies of men’s health and physiology demonstrate that the hormone testosterone typically declines when men become committed partners and parents.
The low down on partnering, parenting, and low(er) testosterone
Decades of research in birds and other (non-human) mammals identified the testosterone decline when males went from beefcake bachelors to dedicated dads and inspired our work in humans. Along these same lines, human fathers are thought to have evolved the capacity to reduce testosterone when they were committed partners and fathers because lower testosterone promotes empathy and relationship commitment and possibly attunes men to the needs and cues of their babies. Partnered parents with reduced testosterone may also be less reactively aggressive and competitive, enabling them to focus their time and energy on the demands of being sensitive and attentive in family relationships.
Life history status, physiology, and health for American men
Studies of the biology of partnering and parenting in humans often focus on these types of behaviors. But, hormones such as testosterone have many other roles in our bodies that can affect our physical and mental well-being and be shaped by our health behaviors, such as sleep. In societies such as the United States, people often eat lots of calories and tend to be sedentary and levels of infectious disease are low. Under those circumstances, men with higher testosterone generally have less body fat. Men with reduced testosterone have higher risk of cardiovascular disease (CVD) and are at greater risk of dying (overall), particularly as they age.
Our new insights on these questions
After bringing all of this information together, a clear set of questions emerged about American men’s testosterone, partnering and parenting status, and their risk factors for CVD. Against the backdrop of evolutionary medicine, which draws on evolutionary perspectives to understand contemporary human health and biology, our team has proposed that there is a potential mismatch between the biology of partnering and parenting and contemporary lifestyles and social demands for men in some societies.
We tested the relationships among testosterone, men’s partnering status and residence with children, risk factors for CVD, and their health behaviors using a large, nationally-representative sample of young to middle-aged men in the United States. We found that partnered men living with children had lower testosterone, higher abdominal fat, and lower “good” cholesterol compared to never-married men who did not live with kids. Those body fat and biological marker differences are all risk factors for the long-term development of CVD. As we predicted, the higher levels of abdominal fat among partnered men residing with kids was linked to their lower levels of testosterone. In other words, if we removed (or controlled for) the testosterone differences between partnered men living with children and never-married men who did not live with children, the groups had similar body fat.
We remain cautious about our findings because data in this study are cross-sectional, reflecting a snapshot in time. These findings, however, indicate that a biological change (reduced testosterone) that likely helps promote men’s sensitivity and focus within their families also contributes to them gaining weight, especially around their midsection. Also important, we studied these dynamics in the context of a US lifestyle, which, on average, involves the consumption of abundant calories, low infectious disease risk, and sedentary lifestyles. Combined, these factors make it relatively easy to accumulate excess body fat. We think that the lower testosterone associated with the “biology of partnering and parenting” in this “WEIRD” context may particularly heighten these changes in men’s bodies, perhaps contributing to the slightly chubby look we know as the “dad bod.” As scientists are prone to say, “future studies are needed,” but we think it’s possible that healthier lifestyles and behaviors among partnered men and fathers may diminish these effects. So, new dads, do not go running for a prescription to boost your testosterone. Maybe just go running.