Abstract: Despite the large literature examining predictors of fertility, previous research has not offered a population‐level perspective on how health in early adulthood is related to male fertility. Using Swedish population and military conscription registers, we study how body mass index (BMI), physical fitness, and height are associated with total fertility and parity transitions by 2012 among 405,427 Swedish men born 1965–1972, meaning we observe fertility up to age 40 or older. Applying linear regression and sibling fixed effects, we find that these anthropometric measures are strong predictors of fertility, even after accounting for education and cumulative income. Men with a “normal” BMI and in the highest decile of physical fitness have the most children. Men who were obese at ages 17–20 had a relative probability of childlessness almost twice as high as men who had a “normal” BMI, and men in the bottom decile of physical fitness had a relatively probability of childlessness more than 50 percent higher than men in the top decile. In sibling comparison models the tallest men have the most children and men in the lowest two deciles of height have significantly lower fertility. Further analyses show that the strong associations persist even among men who married.
Discussion
Using population register data, we have examined how several anthropometric measures are associated with fertility for men in Sweden. We find remarkably strong patterns in our data. We observe a clear monotonic pattern where men who were less physically fit have substantially lower fertility, with the least fit men having 0.31 fewer children and a relative probability to be childless over 50 percent higher than the most fit men. The results for BMI were even more striking: those underweight, overweight, or obese at ages 17–20 also have substantially lower fertility, and were more likely to be childless, with men who were obese having more than 0.5 fewer children and an estimated probability to be childless 86 percent higher than men with a “normal” BMI, even after adjusting for educational attainment and cumulative income. In the full population of Swedish men born 1965–1972, the results for the relationship between height and later fertility show a curvilinear pattern where both the tallest and shortest men have lower fertility, consistent with previous research (Stulp et al. 2012), though in our sibling comparison analyses only the shortest men have lower fertility.
We suggested that there are two primary channels by which height, physical fitness, and BMI should influence later fertility, which were fecundity and desirability as a potential partner, with the latter channel also allowing for indirect pathways such as the effects of health on socioeconomic attainment, which is itself strongly associated with fertility. To test whether the association was mediated by socioeconomic attainment, we both adjusted for educational attainment and cumulative income by age 40 and examined interactions, but this made very little difference to the results, despite the fact that educational attainment and cumulative income were independently strongly associated with the fertility outcomes in our results. Although height, physical fitness, and BMI have been shown to influence socioeconomic attainment, which is itself strongly associated with fertility (Jalovaara et al. 2019), our results suggest that our anthropometric measures influence fertility by a channel other than socioeconomic attainment, such as desirability for a healthy partner. This is particularly clear in our interaction analyses and sibling comparison analyses: even after comprehensively adjusting for all early life factors shared by brothers, and looking within levels of attained education and cumulative income, the relationship between our anthropometric measures and fertility persists in both direction and magnitude.
As an indirect way of examining whether the association between height, physical fitness, BMI, and fertility is related to how these anthropometric factors affect finding a stable romantic partner, we examined the associations between the anthropometric measures and fertility among men who had ever‐married. Although our anthropometric measures are strongly associated with entrance into marriage, we also find that the relationship between physical fitness, BMI, and fertility persists even among ever‐married men. These findings suggest that the observed relationship between BMI, physical fitness, height, and fertility is not simply attributable to never‐partnering. Our findings indicate that height, physical fitness, and BMI do influence desirability as a potential partner, but they also suggest that BMI and physical fitness influence fecundity because the probability of childlessness was much higher among those with worse health, even among the men who had ever‐married.
Although the strong associations between these anthropometric measures and fertility among ever‐married men are striking, we want to highlight several important limitations of these analyses. First, nonmarital fertility in Sweden has accounted for over 50 percent of childbearing since the 1990s, and most of this nonmarital fertility occurs in stable cohabiting relationships. Therefore, most childbearing in Sweden occurs outside of marriage today. Second, our analyses of fertility among men who ever‐married do not condition on childbearing within marriage, they only condition on the men having become married at some point by age 40, and the childbirths could have occurred before or after marriage, or even after a subsequent divorce. Nevertheless, men who have ever‐married have in some fundamental way demonstrated that they can develop a long‐term relationship. Never partnering is the dominant pathway to childlessness in the Nordic region (Jalovaara and Fasang 2017; Saarela and Skirbekk 2020), and over 93 percent of men who ever‐married in the cohorts that we study did have children at some point. We therefore believe that the results from these analyses of ever‐married men allow some insights into the extent to which the relationship between the anthropometric measures that we study and fertility are attributable to never‐partnering, and the extent to which they are attributable to physiological aspects of fecundity.
We believe that the results from this study may have important implications for understanding a large related literature examining how reproductive history affects the postreproductive health of mothers and fathers. Previous research has shown that childless men and women, as well as those with many children, tend to have higher mortality (see Högnäs et al. 2017, for a review and meta‐analysis). Although previous research on the relationship between reproductive history and postreproductive health has included careful adjustment for socioeconomic confounding (Barclay et al. 2016), research on this topic has generally not controlled for health in early adulthood. Given the strong association between physical fitness, obesity, and mortality (Blair et al. 1995; Stokes and Preston 2016), our results suggest that health in early adulthood may be an important explanatory factor that explains why childless men and women, as well as those with many children, have higher postreproductive mortality. Indeed, in this study we observe that obese and overweight men, and men with the lowest aerobic fitness, are overrepresented both among the childless and those who have four or five children.
Although this study has many strengths, there are certainly limitations. First, it must be highlighted that we have measures of BMI, physical fitness, and height from ages 17 to 20, and we do not have dynamic information on changes to these anthropometric measures over time. Although this does not matter for height, research shows that people tend to gain weight and to become less physically active as they age (Seefeldt, Malina, and Clark 2002; Malhotra et al. 2013). As such, we do not know the BMI or physical fitness of the men that we study at the time of partnership formation or childbearing, unless these transitions occur at a similar time to our measurements. Previous research indicates that although people tend to gain weight as they age, this is largely an additive effect of age where individuals stay in roughly the same rank order on BMI within their cohort (e.g., see figure 2 in Malhotra et al. 2013). In terms of physical fitness, interage correlations in dimensions of physical fitness tend to range from 0.3 to 0.6 (Seefeldt, Malina, and Clark. 2002). Although it would be very useful to have measures of BMI and physical fitness over the life course, a strength of having these measures at ages 17–20 is that we generally avoid potential concerns about reverse causality in the relationship between BMI, physical fitness, and fertility.
Another important limitation is that we only had data on height, physical fitness, and BMI for men, and it is difficult to know the extent to which these results could be generalized to women. The relationship between height and fertility would almost certainly be different for women, but it is possible that the patterns for physical fitness and BMI might be similar. A related limitation is that we did not have information on the anthropometric characteristics of the female partner of the men that we study. Due to assortative mating, it is very possible that part of the lower fertility of men who are less physically fit or who are overweight or obese could be attributable to having a partner with similar characteristics. As such, the lower fertility of these men might be attributable to having a partner with lower fecundity (Ramlau‐Hansen et al. 2007), which we also know is more common than would be expected by chance due to assortative mating (Chen, Liu, and Wang 2014).
In this study we examine men born 1965–1972. The prevalence of being overweight or obese in childhood or early adulthood, and sedentary behavior, has become much more common in more recent birth cohorts in most high‐income countries, and it is well established that the prevalence of obesity has increased dramatically across the world over the past several decades. Indeed, global obesity is estimated to have tripled between 1975 and 2016 (Jaacks et al. 2019). Given the research that demonstrates that being overweight or obese, or having a largely sedentary lifestyle, has a negative effect on fecundity (Hammoud et al. 2008a), it is plausible that rising obesity and decreasing fitness may depress fertility. Given secular trends in BMI and sedentary behavior, further research is needed to better understand how these developments are influencing fertility, and particularly childlessness, in Sweden as well as other countries.
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