Not Bryan Caplan's Best Argument

The second chapter of Bryan Caplan's book Selfish Reasons To Have More Kids is dedicated to arguing for the importance of nature - heredity - for predicting the outcomes of children's lives. Nurture effects such as upbringing, environment, etc., hardly matter at all, according to Caplan's survey of twin and adoption studies over the last century.

He makes a strong case for his point, until he writes the following paragraph on pages 70-71:
Parents have little or no effect on childbearing. I often half jokingly tell my three sons that they're required to have three kids each, but twin studies say I'm wasting my breath. While fertility runs in families, the reason nowadays is almost entirely genetic. A major study of Danish twins born in 1870-1910 found moderate nurture effects on family size. Half a century later, though, these nurture effects had disappeared. Upbringing had a tiny influence on when Danes tried to start a family, but none on the total number of children produced by those thirty-five to forty-one years old. A different team of researchers looked at about 2,000 American tines, siblings, half siblings, and cousins born between 1958 and 1965 and found minimal nurture effects on family.
Caplan's point is clear: childbearing is predicted not by nurture effects, but by heredity.

How did the author of a book that aims to convince people - through non-hereditary argumentative reasoning - to bear more children allow himself to finish the book at all?