therefore account for major gene-environment interactions that occur across cohorts. An alternative possibility is to test gene-environment interactions by incorporating additional socioeconomic conditions and information about the parental household and spouses of the twins. Moreover, these twin models sometimes allow for explicit tests of alternative genetic models, such as dominance effects. An additional development of particular value to demographers is the identification of kinship structure in large national datasets like the National Longitudinal Survey of Youth (NLSY). Using information from the survey, Rodgers et al. (1994) developed an algorithm that specified kinship structure in the children of the NLSY youth data and in another study Rodgers et al. (1999) reported a similar linking algorithm that specified kinship links for the NLSY youth respondents. These links open up the potential to do behavioral genetics analyses on national probability samples (a common criticism of the twins study is its low external validity) using the rich longitudinal and multivariate structure of such data sources.

Behavioral genetics methods have long been criticized by those outside the field (e.g., Lewontin et al., 1984, have defined a popular set of criticisms of the basic behavioral genetics design, including in particular the confounded nature of the genotype of monozygotic twins genotype and similar treatment by their families and peers).2 Interestingly, the behavioral genetic community itself is filled with internal critics who have carefully scrutinized and criticized these methods, and (arguably) the strongest and most cogent criticisms arise from behavioral geneticists themselves. For example,


In fact, reviewers of the empirical results presented later in this chapter raised the issue of whether the results could have been caused by the greater similarity in appearance/attractiveness of MZ twins compared to dizygotic (DZ) twins. For example, could MZ twin correlations in fertility that are higher than DZ twin correlations in fertility be caused by the greater similarity in appearance/attractiveness for MZ twins? This concern is a form of the well-studied equal environments assumption of behavioral genetics modeling. This assumption has been studied in several ways. “Mislabeling studies” have studied persons (especially twins) misdiagnosed for zygosity; typically, their correlational patterns are similar to their biological zygosity, rather than their presumed (incorrect) zygosity (Scarr and Carter-Saltzman, 1979). Other studies have directly addressed similarity of appearance (e.g., Loehlin and Nichols, 1976). Plomin et al. (1990:319) reviewed these studies and concluded that the data “strongly support the reasonableness of the equal environments assumption.” More specifically related to the current research are a number of past studies of fertility based on family designs that come to similar conclusions as our studies of twins (see Rodgers et al., 2001a, for a review). Those family studies include cousins, half siblings, and full siblings (in addition to a few twins). Though siblings are more genetically related than half siblings (for example), they are not likely to be much more similar in appearance or attractiveness. Though the concern over the appearance confound is well founded, evidence has not yet emerged that either behavioral traits in general or fertility in particular are especially influenced by violations of the equal environments assumption. Nevertheless, behavioral genetics texts recommend continued caution.

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