The Second to Fourth Digit Ratio (2D:4D) in a Japanese Twin Sample: Heritability, Prenatal Hormone Transfer, and Association with Sexual Orientation

The second to fourth digit ratio has been argued to reflect prenatal hormonal influences and is reportedly associated with
various psychological and behavioral traits, such as sexual orientation, cognitive abilities, and personality. We examined
genetic and environmental influences on the second to fourth digit ratio (2D:4D) using a Japanese twin sample (N = 300). The genetic analysis showed substantial additive genetic influences for both right and left hand 2D:4D. The rest
of the variance was explained mainly by environmental influences not shared within twin pairs. These findings were, in general,
in accordance with preceding studies with primarily Caucasian twin samples. The bivariate genetic analysis revealed that the
additive genetic influences were largely shared between the right and left hand, while the non-shared environmental influences
were largely unique to each hand. Results from a comparison of opposite-sex and same-sex twins were not significant, although
they were in the predicted direction according to the prenatal hormone transfer hypothesis. Female monozygotic twin pairs
discordant in sexual orientation showed significant within-pair differences in left hand 2D:4D, where non-heterosexual twins
had lower (more masculinized) 2D:4D. In addition, we found that non-heterosexual male MZ twins had larger (more feminized)
2D:4D than their heterosexual co-twins. These results suggest the existence of non-shared environmental influences that affect
both 2D:4D and sexual orientation.