No effects of prenatal hormone transfer on digit ratio in a large sample of same- and opposite-sex dizygotic twins

Hormone-transfer theory proposes that the hormonal milieu of one twin may influence the other. In opposite-sex twins it has been proposed that hormonal transfer may produce a masculinizing effect in females. The second-to-fourth-finger ratio (2D:4D) has been proposed as an index of the level of prenatal androgens and could, theoretically, be used to detect hormone transfer. Previous examinations in smaller samples provide conflicting evidence. The present study attempted to clarify these results in a large sample of same-sex and opposite-sex dizygotic twins (867 individuals). No significant differences were found in the means, variances or covariance of same- and opposite-sex twins. Female 2D:4D was significantly higher than male (in both hands), as expected, but no sex effects were found on the variance or covariance. Thus, no evidence of hormonal transfer was found in this large twin sample.