The influence of health in early adulthood on male fertility

Despite the large literature examining socioeconomic predictors of fertility and the influence of reproductive history on post-reproductive mortality, previous research has not offered a population-level perspective on how male health in early adulthood is related to subsequent fertility. Using Swedish population registers and military conscription data, we study how body mass index (BMI), physical fitness and height are associated with total fertility and parity transitions by 2012 amongst 405,427 Swedish men born 1965-1972, meaning we observe fertility up to age 40 or older. Applying linear regression as well as sibling fixed effects, we find that our anthropometric measures are strong predictors of later fertility, even after accounting for educational attainment and cumulative income up to age 40. Men with a 'normal' BMI and in the highest decile of physical fitness have the most children by the end of our follow-up period. Men who were already obese at ages 17-20 had a relative probability of childlessness almost twice as high as men who had a 'normal' BMI at ages 17-20, and men in the bottom decile of physical fitness had a relatively probability of childlessness almost 50% higher than men in the top decile of physical fitness. These patterns were at least as strong in our models applying the sibling fixed effects design. We find that the association between male height and fertility is curvilinear in models estimated without sibling fixed effects, but only men in the lowest decile of height have lower fertility in sibling comparison models. Further analyses show that the strong associations between our anthropometric measures and male fertility persist even amongst men who married. We discuss the implications of our findings for fertility in high-income countries in light of secular increases in the prevalence of overweight/obesity.