Turkheimer (1998:782) suggested that “heritability and psychobiological association cannot be the basis for establishing whether behavior is genetic or biological, because to do so leads only to the banal tautology that all behavior is ultimately based in the genotype and brain.” Lest one interpret this as dismissive of past behavioral genetics research, Turkheimer (2000) published his “three laws of behavior genetics”: (1) All human behavior is heritable; (2) shared environmental influences are typically smaller than genetic influences on behavior; and (3) a great deal of behavioral variability is not based in either genetics or families. In addition, a recent review by Rose (1995:648-649) identified several emerging ideas and enduring issues in behavioral genetics, including a new design based on dividing pairs of MZ twins into those who shared a placenta in utero versus those who did not, and treatment of the question of whether (and how) family environment is relevant to development. Rose concluded: “Few areas of psychology are changing as rapidly as behavior genetics. Few are as filled with excitement and promise. Few are as surrounded by controversy.” Wahlsten (1999:599), in his review, described the blending of behavioral genetics and neurogenetics into the field of “neorobehavioral genetics, focusing on single-gene effects,” the type of consilience described in the opening sentences of this chapter. Finally, Rutter and Silberg’s (2002:465) review of behavioral genetics focused on gene-environment correlations and interactions. They emphasize that “the genes that influence sensitivity to the environment may be quite different from those that bring about main effects.”
In summary, therefore, while researchers need to be aware of the specific assumptions underlying behavioral genetics models (see Rutter, this volume), just as they must be aware of the assumptions underlying other domains of empirical work, recent methodological progress provides a powerful set of methods to investigate the relevance of genetic factors also in demography. The broad range of empirical and theoretical possibilities of how these methods can be used and integrated with demography is illustrated in two recent books on genetic influences on fertility and sexuality and the biodemography of fertility (Rodgers et al., 2000; Rodgers and Kohler, 2003).
Our goal in this section is to (begin to) explain a paradox. On the one hand, our previous analyses have shown that genetic factors contribute to variations in fertility outcomes, including the number of children, timing of the first child, and the age at first attempt to become pregnant (e.g., Rodgers et al., 2001a, 2001b). On the other hand, genetic influence related to natural selection changes very slowly; an exception is genetic influence related