Not Just Genes

Even researchers indifferent to traditional morality recognize that adolescent sexual activity entangles young people in serious problems. Pregnancy and Sexually Transmitted Diseases (STDs) are two problems that receive attention in a study of adolescents completed by researchers at the University of North Carolina at Chapel Hill. To understand why some young people are more at risk, the authors of the study analyze genetic data to assess the degree to which teens’ inherited DNA influences their sexual behavior. Statistical results do indeed suggest that a specific genetic profile increases the likelihood of teen sexual intercourse. But the data also identify a particular family configuration—the intact two-parent family—that shields teens from the perils of early sexual involvement.

To clarify the genes’ role in influencing teen sexual behavior, the authors of the study scrutinize data collected between 1994 and 2002 from 2,552 adolescents from fraternal and identical twins. The North Carolina researchers detect the influence of teens’ genetic inheritance in the fact that “identical twins correlate in the timing of first sexual intercourse to a substantially greater extent than do fraternal twins.” These researchers identify further genetic influence on sexual behavior in the fact that teens are especially likely to engage in early sexual intercourse if they have a particular genotype (a 48-hp repeat polymorphism in the dopamine receptor D4 gene).

Further parsing of the data, however, shows that genes are only part of the story. Family structure also strongly influences teen sexual behavior. Using a sophisticated statistical model, the researchers calculate that for their overall data sample, teens living in a single-parent family are about a quarter more likely to engage in sexual intercourse than are peers from intact two-parent families (23 percent; p=.001). Similarly, teens growing up in stepfamilies are more than a third more likely to engage in sexual intercourse than are peers from intact families (36 percent; p=.0001).

Looking at minority groups, the researchers note, “The effects of family structure in the African American sample are very similar to those in the all-ethnicities sample.” However, among Hispanics, the researchers see something different: “For Hispanics, growing up in a single-parent family appears to make one particularly vulnerable, being associated with an increase of 70 percent (p<.0001) in the risk of first sex.”

This study makes all the more puzzling why public health officials—despite their professed interest in reducing the incidence of pregnancy and STDs among teens—remain largely silent about the social costs of illegitimacy and family disintegration.

(Guang Guo and Yuying Tong, “Age at First Sexual Intercourse, Genes, and Social Context: Evidence from Twins and the Dopamine D4 Receptor Gene,” Demography 43 [2006]: 747–69.)