Lauren Sandler unpacks them:
Stunningly, the postponement of marriage and parenting — the factors that shrink the birth rate — is the very best predictor of a person’s politics in the United States, over even income and education levels, a Belgian demographer named Ron Lesthaeghe has discovered. Larger family size in America correlates to early marriage and childbirth, lower women’s employment, and opposition to gay rights — all social factors that lead voters to see red.
Amanda Marcotte adds two cents.
(Map: Fertility rates: CDC, "Births: Preliminary Data for 2011" (pdf))