On genetic variation in menarche and age at first sexual intercourse:

Abstract 

The association of age of menarche, nonvirginity status, and age of first sexual intercourse was investigated in teenage, female twins (mean age, 17 years) in the National Longitudinal Study of Adolescent Health (Add Health). The sample sizes were all relatively small, so complex biometric models were not fit to twin covariance matrices; rather, rough estimators of genetic effects were used. Accordingly, all three characteristics were influenced by genetic variation, with higher heritabilities on nonvirginity status and age of menarche than on age of first sex. In MZ twins, the phenotypic (i.e., on individuals) correlation between menarcheal age and age of first sexual intercourse was .27. The association of menarcheal age in identical (MZ) Twin 1 and sexual onset age in MZ Twin 2, and vice versa, was .25. The genetic correlation between them, r_g, was roughly estimated to be .72. These findings weaken a conditional adaptation interpretation of this association as proposed by Belsky and Draper, suggesting instead that heritable individual differences may give rise to this association.

Keywords:  Add Health, Menarche, Sexual intercourse, Conditional adaptation