Does Fertility Help Explain SES Gaps in Health and Aging? Evidence from Methylation Data

Lower SES adults have more children than higher SES adults. Meanwhile, low SES adults age faster than their high SES counterparts. Might these two empirical facts be connected in a causal way? Prior research has been mixed, mostly showing an association between higher fertility and longer life expectancy—despite evolutionary theory that suggests a tradeoff between organismal maintenance and reproduction. The limitation of prior studies is that they conflate the selection into fertility with the treatment effect of having children. The present study addresses this problem by examining the effect of the number of children on epigenetic (methylation) aging clocks in the Health and Retirement Study.

To separate out the true causal effect of the phenotype of high fertility from the underlying social and genetic attributes that select people into high or low fertility, we deploy an instrumental variable approach. Specifically, we deploy the sex-mix instrument as first developed by Angrist and Evans (1998) to study parental labor supply to study the effect of having three or more children as opposed to two children. We compare these instrumental estimates to the naïve OLS estimates and found that, for the most part, the pattern of results holds: Children seem to age their parents. This effect seems to be larger for low education individuals, suggesting that higher fertility may be a mechanism by which SES health gradients obtain.