Scores on the Wechsler Intelligence Scale for Children were analyzed in a sample of 7-year-old twins from the National Collaborative Perinatal Project. A substantial proportion of the twins were raised in families living near or below the poverty level. Biometric analyses were conducted using models allowing for components attributable to the additive effects of genotype, shared environment, and nonshared environment to interact with socioeconomic status (SES) measured as a continuous variable. Results demonstrate that the proportions of IQ variance attributable to genes and environment vary nonlinearly with SES. The models suggest that in impoverished families, 60% of the variance in IQ is accounted for by the shared environment, and the contribution of genes is close to zero; in affluent families, the result is almost exactly the reverse.

When genetic similarity is controlled, siblings often appear no more alike than individuals selected at random from the population. Since R. Plomin and D. Daniels' seminal 1987 review, it has become widely accepted that the source of this dissimilarity is a variance component called nonshared environment. The authors review the conceptual foundations of nonshared environment, with emphasis on distinctions between components of environmental variance and causal properties of environmental events and between the effective and objective aspects of the environment. A statistical model of shared and nonshared environmental variables is developed. A quantitative review shows that measured nonshared environmental variables do not account for a substantial portion of the nonshared variability posited by biometric studies of behavior. Other explanations of the preponderance of nonshared environmental variability are suggested.

In a longitudinal study of a national sample, more externalizing behavior problems were found among 222 children from never-married and 142 children from divorced families than among 840 children from married families. However, delinquent behavior reported when future mothers were single, childless adolescents prospectively predicted their future marital status and behavior problems among their offspring 14 years later. Maternal history of delinquent behavior accounted for much, but not all, of the relationship between marital status and children's externalizing behavior. Divorce and nonmarital childbirth do not occur at random, and these findings demonstrate that marital status is predicted by individual characteristics as well as by demographic factors. These findings highlight the importance of cautiously interpreting the much-discussed correlation between marital status and children's behavior problems. Much research has been conducted on the psychological adjustment of children who are born outside of marriage or whose parents divorce. Research has been stimulated by the dramatic increases in the rates of both divorce and nonmarital childbirth in the United States since the 1960s. Rates have stabilized and are slowly showing a downward trend, but about 40% of all children born to married parents still are predicted to experience their parents' divorce (U.S. Census Bureau, 1992), and about 25% of all children in the United States

The present study examines the relations between adolescent motherhood and children's behavior, substance use, and internalizing problems in a sample of 1,368 children of 712 female twins from Australia. Adolescent motherhood remained significantly associated with all mental health problems, even when using a quasiexperimental design capable of controlling for genetic and environmental confounds. However, the relation between adolescent motherhood and offspring behavior problems and substance use was partially confounded by family background variables that influence both generations. The results are consistent with a causal relation between adolescent motherhood and offspring mental health problems, and they highlight the usefulness of behavior genetic designs when examining putative environmental risks for the development of psychopathology. The generalizability of these results to the United States, which has a higher adolescent birth rate, is discussed.Correspondence concerning this article should be addressed to K. Paige Harden, Box 400400, Department of Psychology, University of Virginia, Charlottesville, VA 29904-4400. kph3k@virginia.edu. NIH Public AccessAuthor Manuscript J Abnorm Psychol. Author manuscript; available in PMC 2010 July 14. NIH-PA Author ManuscriptNIH-PA Author Manuscript NIH-PA Author ManuscriptKeywords adolescent motherhood; children-of-twins; teenage pregnancy; internalizing; externalizingThe most obvious correlate of adolescent childbearing is privation: Teenage mothers are twice as likely to be impoverished when adults (Hoffman, Foster, & Furstenberg, 1993) and more likely to receive welfare (Moore et al., 1993), although the extent to which these adverse economic outcomes are a product of adolescents' disadvantaged background, versus the pregnancy itself, is debated (Geronimus & Korenman, 1992;Lee & Gramotnev, 2006). In addition to the negative socioeconomic correlates of adolescent childbearing, the children of adolescent mothers are at elevated risk for diverse forms of psychopathology, including depressive and anxiety disorders (Hofferth, 1987;Moore, Morrison, & Greene, 1997), antisocial behavior, and other externalizing disorders (Jenkins, Shapka, & Sorenson, 2006;Levine, Pollack, & Comfort, 2001;Nagin, Pogarsky, & Farrington, 1997;Spieker, Larson, Lewis, Keller, & Gilchrist, 1999;Wakschlag et al., 2000). The disparity in adjustment between children of adolescent and adult mothers seems only to widen over children's life spans, with the most dramatic disparities evident in adolescence and adulthood (Brooks-Gunn & Furstenberg, 1986;Furstenberg, Brooks-Gunn, & Morgan, 1987).Consequently, several authors have concluded that a mother's age at her first birth influences her child's cognitive and psychosocial development and eventual adult adjustment. This has not remained a purely academic hypothesis: American organizations with disparate political ideologies, such as The Heritage Foundation (The Heritage Foundation, 2004) and Planned Parenthood (Planned Parenthood, 2006), pu...

We examine interactive effects of childhood sexual abuse (CSA) on heritable variation in age at first consensual sexual intercourse in a young cohort of 3,350 female and 2,724 male Australian twins. Consistent with hypotheses, genetic influences explained little if any variation in age at first consensual sexual intercourse for female twins reporting CSA (CSA(+)), with shared environment explaining 73%. For female twins reporting no history of CSA (CSA(-)), 39% of variation in age at first consensual sexual intercourse was explained by genetic effects, with shared environment accounting for 30%. For male twins, significant interactive effects of CSA on genetic and environmental variation in age at first consensual sexual intercourse were not observed. Overall genetic influences explained 51% of variation in age at first consensual sexual intercourse for male twins, with shared environment accounting for 8%. For both female and male twins, results from models that included conduct disorder as a covariate were near identical to results from models without conduct disorder.