2D:4D, Sexual Orientation, East Versus West: Maybe It’s True the Twain Shall Never Meet (on Average)

Abstract

Evidence has been inconsistent regarding any links between the 2D:4D ratio and sexual orientation. The present study was undertaken to (a) further test the hypothesis that the 2D:4D ratio is associated with sexual orientation and (b) explore the possible confounding influence of ethnicity in these relationships. Samples of college students in Malaysia and the U.S. reported their 2D:4D ratios on their right hand (r2D:4D), sexual orientation, and ethnicity. Modest tendencies were found for homosexual preferences to be positively associated with r2D:4D among males and inversely associated among females, although there were ethnic differences in this regard. The ethnic differences in the r2D:4D ratios seemed to parallel Asian/Western differences in sexual orientation, with diminished opposite-gender preferences among Asians, especially Asian females. Analysis of how r2D:4D and sexual orientation were correlated revealed few significant relationships except among females. As theoretically predicted, correlations between r2D:4D and homosexual attractions were negative for the three Western ethnic samples (two of which were statistically significant). Unexpectedly, however, female homosexual attractions were significantly positively correlated with r2D:4D in all three Asian samples. Overall, findings suggest that at least among females, prenatal androgen exposure may be less responsible for variations in sexual orientation in Asian than in Western populations.Oh, East is East, and West is West, and never the twain shall meet.Rudyard Kipling, 1892IntroductionAccording to what has been termed evolutionary neuroandrogenic (ENA) theory (Ellis, 2011), sexual orientation is an evolved mammalian sex difference determined primarily by brain exposure to androgens during fetal development. Accordingly, the greater a mammal's exposure to androgens, the more likely he/she will be to exhibit highly masculine traits. One of these masculine traits is a tendency to prefer females as sex partners. Contrarily, the lower the exposure to androgens, the more feminine an individual will be, including exhibiting a preference for male sex partners. Of course, in general, male brains receive much higher androgen exposure than do female brains, thus accounting for substantial average gender differences in sexual orientation. Theoretically, therefore, in all mammalian species, the majority of both males and females should exhibit predominantly heterosexual preferences.Testing ENA theory in humans has been hampered by the fact that fetal brain exposure to androgens is difficult to measure and cannot be experimentally manipulated. Furthermore, one would have to compare prenatal exposure levels with sexual orientation at sexual maturity some twenty years following the exposure.Researchers have managed to estimate androgen levels in amniotic fluid (Baron-Cohen, Lutchmaya, & Knickmeyer, 2004; van de Beek, Thijssen, Cohen-Kettenis, van Goozen, & Buitelaar, 2004). While this should indeed approximate prenatal brain exposure, no study has yet managed to correlate this exposure to sexual orientation partly due to the large sample size that would be required given the relative rarity of non-heterosexuality.Over the past two decades, an obtuse method for assessing prenatal androgen exposure appears to have been identified, called the 2D:4D ratio (McIntyre, 2006; Nelson & Shultz, 2010). The assessment method involves simply measuring the relative length of the second (pointing) and fourth (ring) fingers. Studies suggest that the lower the ratio, the greater was an individual's prenatal exposure to testosterone (with estradiol, a postcursor of testosterone, also possibly being involved) (Manning, Scutt, Wilson, & Lewis-Jones, 1998; McIntyre, Ellison, Lieberman, Demerath, & Towne, 2005; van Anders, Vernon, & Wilbur, 2006). The most incontrovertible evidence to date of prenatal androgenic influences on the relative lengths of the second and fourth digits has come from recently published experiments with mice (Zheng & Cohn, 2011). …