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SummaryThis paper applies models of the onset of adolescent sexual intercourse using national data from Denmark and the USA. The model gave excellent fits to data on Danish Whites and a good fit to American Whites, but the model-fits for American Blacks and Hispanics were not as good. The weakness of the latter model fits may reflect either real processes that the model does not capture or problems in the reliability of adolescent sexuality data.

This study presents a multivariate behavioural genetic analysis of the relationship between education, intelligence and age of first birth. Analyses investigated the mediational role of education in explaining the relationship between intelligence and age of first birth at both the phenotypic and behavioural genetic level. The data come from the National Longitudinal Survey of Youth (NLSY), a nationally representative survey that included genetically informative full- and half-sibling pairs (n=1423 pairs). Respondents were aged 14 to 22 when contacted in 1979. Heritability estimates were 0·32, 0·50 and 0·06 for IQ, education and age of first birth, respectively. Shared environment estimates were 0·35, 0·23 and 0·20 respectively. Common genetic and shared environmental factors were substantial in explaining the relationship between intelligence and education, and also education and age of first birth. Education partially mediated the relationship between intelligence and age of first birth only in the phenotypic analyses. After considering the genetic and shared environmental factors that influence all three variables, evidence for mediation was less convincing. This pattern of results suggests that the apparent mediational role of education at the phenotypic level is in fact the result of underlying genetic and shared environmental influences that affect education, IQ and age of first birth in common.

This article investigates the fertility of Danish twins born during the periods 1870–1910 and 1953–64 in order to pursue two central questions for understanding human reproduction: Do genetic dispositions influence fertility and fertility‐related behavior? Does the relevance of the "nature versus nurture" debate shift over time or with demographic regimes? The authors find that genetic influences on fertility exist, but that their relative magnitude and pattern are contingent on gender and on the socioeconomic environment experienced by cohorts. Among females born in 1880–90 and after 1955, about 30–50 percent of the variance in fertility is due to genetic influences; these influences are substantially smaller for earlier and for interim birth cohorts. Male fertility is generally subject to smaller genetic and larger shared‐environment effects than female fertility. Because genetic effects are most prevalent in situations with deliberately controlled fertility and relatively egalitarian socioeconomic opportunities, the authors propose that the genetic dispositions affect primarily fertility behavior and motivations for having children. Analyses of fertility motivations, measured by age of first attempt to have a child, support this interpretation.