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, andpersonality.We examined geneticandenvironmental influencesonthe secondto 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 variancewas explainedmainly by environmental influences not shared within twin pairs.Thesefindings were,ingeneral, inaccordancewithpreceding studies with primarilyCaucasiantwinsamples. Thebivariate 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 twinshadlarger (morefeminized)2D:4Dthantheir heterosexual co-Twins. These results suggest the existence of non-shared environmental influences that affect both 2D:4D and sexual orientation.