Behavior genetic modeling of human fertility: Findings from a contemporaryDanish twin study

Behavior genetic designs and analyses can be used to address issues of cent ral importance to demography. We use this methodology to document genetic i nfluence on human fertility. Our data come from Danish twin pairs born from 1953 to 1959, measured on age at first attempt to get pregnant (FirstTry) and number of children (NumCh). Behavior genetic models were fitted using s tructural equation modeling and DF analysis. A consistent medium-level addi tive genetic influence was found for NumCh, equal across genders; a stronge r genetic influence was identified for FirstTry, greater for females than f or males. A bivariate analysis indicated significant shared genetic varianc e between NumCh and FirstTry.