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

Abstract

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.

The Relationship Between Digit Ratio (2D:4D) and Sexual Orientation in Men from China

“…The ethnic differences between Chinese and Western populations may explain the conflicting results we found here. Moreover, our results were consistent with one studythatexploredthe relationshipbetween2D:4D andmale sexual orientation in Japan (Hiraishi et al, 2012). The ethnic difference between Chinese and Japanese participants is smaller than that between Chinese and Western populations, which supports the view that ethnic differences may be an important influence on the 2D:4D digit ratio.…”

“…This result supports the findings of some previous studies (Hall & Schaeff, 2008;Hiraishi et al, 2012;McFadden & Shubel, 2002), but conflicts with other work (Lippa, 2003;Manning et al, 2007). Manning et al (1998) suggested that the right hand 2D:4D is likely to reflect prenatal androgen exposure more strongly than the left hand.…”

Prenatal Influences on Human Sexual Orientation: Expectations versus Data

“…Hall and Love (2003), examining just seven pairs of monozygotic twins who were discordant for sexual orientation, found that the lesbian twins, on average, had more masculine digit ratios than the straight twins. This finding was replicated nine years later in a Japanese study of eight pairs of twins (Hiraishi, Sasaki, Shikishima, & Ando, 2012), which was published after the 2010 meta-analysis. In the midst of a so-called “replication crisis” in psychology (Maxwell, Lau, & Howard, 2015) as well as life sciences and physical sciences (Baker, 2016), it is noteworthy that so many different labs have found this difference between lesbians and straight women.…”

Analysis of different sources of measurement error in determining second-to-fourth digit ratio, a potential indicator of perinatal sex hormones exposure

“…Voracek, Manning, and Dressler (2007) recommended marking the points used for determining length on the participant's hand with a marker before scanning. The authors further suggested covering the hand with foil in order to achieve a higher contrast, while Hiraishi, Sasaki, Shikishima, and Ando (2012) recommended a white cloth to the same purpose.…”

“…The experimenter controlled participants followed this instruction and checked that their hand position was in accordance to the guidelines provided by Mayhew et al (2007). As suggested by Hiraishi et al (2012), white cloth was put on the hands by the experimenter in order to achieve more contrast and an easy determination of points on the scanned pictures. After scanning participants lifted their hands and then put them on the scanner again to repeat the procedure.…”