The advantage of this approach is that it provides an estimate of the overall relevance of genetically mediated biological effects on variations in fertility behavior. This information about the overall relevance of genetically mediated variations is interesting in itself. Moreover, this information will guide future research with respect to the scope of investigating specific mechanisms and pathways of biological influences: If the net overall variation attributed to genetic factors is high, the search for specific pathways (or possibly even specific gene factors) is likely to be more promising compared to a situation in which the overall influence is found to be low. In addition, studies of overall genetic variations in fertility outcomes can suggest specific socioeconomic contexts of cohorts that seem to facilitate genetically-mediated variation in fertility behavior, and these genetic-socioeconomic interactions will provide considerable scope for integrating sociological and biological theories about reproductive behavior.
Although most behavioral genetics work, including that applied to fertility, still focuses on partitioning variances into genetic and environmental components, broader applications are emerging (see Rutter, this volume). On the one hand, more sophisticated modeling and theorizing are supported by an increased availability of data, including large-scale twin and family data that are rich in socioeconomic life course information pertaining to education, marriage/union or labor market history, and genetic relatedness. Such data can be used to estimate multivariate behavioral genetics models that disentangle the pathways of genetic influences, such as the processes affecting marriage/union formation, age at first birth, educational attainment, and so forth. For an example of such an application, see the study by Rodgers et al. (2003), which uses age at first pregnancy attempt as an indicator of (volitional) fertility motivation and lag to pregnancy as a measure of (nonvolitional) fecundity. The study implements a methodology that allows competition between these two domains—the psychological and the biological—in accounting for the genetic variance underlying fertility outcomes.
In addition to suggesting the joint investigation of genetic dispositions and life course processes/decisions, the framework outlined above implies that the relevance of genetic factors for variations in fertility outcomes within cohorts is likely to be strongly conditioned by the socioeconomic context of the cohorts. A possibly surprising—but robust—finding in some of our earlier analyses (Kohler et al., 1999, 2002b), for instance, is a systematic relationship between fertility transitions and patterns in both heritabilities and shared environmental variance in data on female Danish twins: Increased opportunities for education and labor market participation and the emergence of relaxed and flexible reproductive norms in recent decades seem to have strengthened the genetic component in fertility outcomes. Finding these varying influences is consistent with, even predicted by, our