Why Doesn't Parenting Affect Fertility?
By Bryan Caplan
Twin and kinship studies find that parenting has little influence on fertility.* While there is some family resemblance – big and small families run in families – heredity accounts for all or almost all of it.
This is a little counterintuitive even to me, because parents vary widely in their helpfulness. Some are willing to provide decades of free childcare to their grandchildren; others won’t lift a finger. It’s one thing to say that in the long-run, nurture effects fade out. It’s another to say that directly changing the cost of kids doesn’t make a difference.
Here’s my favorite way to resolve the paradox: Grandparents’ assistance and interference are highly correlated. Their assistance increases their kids’ fertility by lowering the cost. Unfortunately, their interference has exactly the opposite effect: Each kid becomes another flashpoint for unwanted advice and family strive.
As long as the assistance and interference are highly correlated and have roughly equal and opposite effects, twin and adoption studies will fail to detect a nurture effect on fertility. There just won’t be enough quietly helpful (or loudly lazy) grandparents in the data. This is very hard to test, but it makes sense to me. What do you think?
* Check out the following titles on google scholar: “Is Fertility Behavior
in Our Genes?,” “Behavior Genetic Modeling of Human Fertility,” “Genetic and Environmental Influences on Fertility
Expectations and Outcomes Using NLSY Kinship Data,” and “Natural Selection and Quantitative Genetics of
Life-History Traits in Western Women.”