Laboratory

Fertility and Well-Being

Project

The Influence of Health and Cognition on Fertility

Susie Lee, Kieron Barclay, Natalie Nitsche; in Collaboration with Martin Kolk (Stockholm University, Sweden), Eugene Gardner, Matthew Hurles (both: University of Cambridge, United Kingdom)

Detailed Description

This project focuses on the association between several dimensions of health and cognition and how these are associated with fertility outcomes in men and women. For men, we have expanded our research based on military conscription data from the Swedish population registers to examine how body mass index (BMI), height, cardiovascular fitness, and cognitive ability scores measured at ages 17 to 20 are associated with long-term fertility outcomes. For women, we have expanded the project by examining how the health–fertility nexus compares between the two sexes.

Our new works on men continue to fill data gaps in studying the health–fertility nexus in men: In a new study using Swedish population-level taxation and conscription registers, we have found that men with higher cognitive ability tend to have higher fertility. Low income and low cognitive ability independently predict low fertility and high childlessness. The positive association between cognitive ability and fertility is even stronger when comparing brothers in the same sibling group or examining the association between fertility and cognitive ability within different levels of completed education. We have also identified a strong association between BMI during early adulthood and completed fertility among Swedish men. For example, men who are obese have a relative probability of childlessness twice as high as men with a normal BMI. These patterns persist at different levels of educational attainment, within every decile of cumulative income between ages 18 and 40, and even among men who are married. However, we have found that health is also a very strong predictor of partnership formation. These results suggest that the links between fertility, BMI, and fitness are explained by partnership formation processes as well as by the link between health and underlying fecundity.

We have also expanded the above-mentioned research on Swedish men by examining sex and ethnic differences in the relationship between BMI and fertility. Using data from the US National Longitudinal Survey (NLSY79, i.e., the cohort born during the years 1957 through 1964), we replicated the general patterns of higher childlessness among those who were obese during early adulthood – now in both sexes – and across all three ethnic groups studied: Blacks, Hispanics, and whites. Our results have shown that obesity is linked to women’s likelihood of remaining childless more than that of men. We have also seen another difference by ethnicity, differences that are more pronounced among women: For Black women, being underweight was associated with a lower chance of remaining childless later in life. This means that Black women who were underweight as teenagers have a higher chance of becoming a mother than Black women who were within the healthy range of BMI. This observation is contrary to what would be expected under the "buffering" hypothesis, which states that the sociocultural pressure to conform to a slender body type is not seen, or is seen less so, in ethnic minorities. Our study’s finding suggests that norms around slender body type may affect ethnic minorities, too.

Research Keywords:

Fertility Development, Health Care, Public Health, Medicine, and Epidemiology