related research (Christensen et al., 2003) that genetic factors contributing to fecundity, as measured by the waiting time to pregnancy, occur primarily through relatively complicated gene interactions across different loci.
The main conclusion of our analyses that the patterns of genetic variance, whether measured as heritabilities or by comparing MZ and DZ twin correlations, are strongly socially conditioned and that contemporary societies might lead to a strengthening instead of a weakening role of genetic favors for variation of fertility outcomes is supported by recent investigations of the intergenerational transmission of fertility. Studies in a number of countries and time periods (e.g., Anderton et al., 1987; Berent, 1953; Johnson and Stokes, 1976; Pullum and Wolf, 1991) have shown that there is usually a positive correlation between the number of children of parents and their offspring, while there is also the possibility of a negative relation due to cohort size effects (e.g., see Easterlin, 1980). Studies of intergenerational correlation without indicators of genetic relatedness can obviously not identify the contribution of genetic and social factors to this intergenerational transmission of fertility. Nevertheless, because (additive) genetic influences on fertility tend to cause a positive intergenerational correlation (though not necessarily if genetic factors operate through epistatis), findings that intergenerational correlations are also not weakened in posttransitional societies and more recent cohorts is supportive of our behavioral genetics analyses.
Murphy and Wang (2001), for instance, estimated the correlations between number of siblings and children for different contemporary developed countries, including Italy, Great Britain, Australia, Norway, and Germany, and found that the positive intergenerational relationship in fertility is not only substantial and present in all countries investigated but also that this relationship has been increasing in younger cohorts and persists even after controlling for socioeconomic characteristics. Similarly, in a study using Danish register data, Murphy and Knudsen (2002) did not find that the intergenerational fertility transmission weakened in younger cohorts, despite the fact that the socioeconomic and ideational changes experienced by these cohorts during the second demographic transmission would tend to attenuate parental influences and intergenerational transmission.
In summary, our empirical work falls into the category of research encouraged by Rutter and Silberg (2002) of gene-environment interplay. The specification of models in which behavioral genetics design/analysis is complemented by environmental measures is a natural way to formalize the goals of developing consilience between biodemographic and demographic approaches to studying fertility. Further, our empirical finding that the genetic variance implied by analysis of the twin design is strongly conditioned on educational level is an example of how the ultimate result of such efforts toward consilience can be greater than the sum of the parts.