Finger Length Ratios of Identical Twins with Discordant Sexual Orientations

Abstract

A proposed marker of prenatal androgen exposure is the ratio of the index finger to ring finger (2D:4D). Within each sex, this ratio may be lower for those who were exposed to higher levels of androgens and become attracted to women, as compared to those who were exposed to lower levels of androgens and become attracted to men. We examined these patterns in identical twins with discordant sexual orientations. Because these twins are genetically identical, differences in prenatal androgen exposure, as reflected in their different finger length ratios, might contribute to their discordance. For 18 female twin pairs, non-straight (bisexual or lesbian) twins had significantly lower, or more masculinized, 2D:4D ratios than their straight co-twins, but only in the left hand. For 14 male pairs, non-straight twins had, contrary to our prediction, more masculinized finger length ratios than straight co-twins, but this difference was not significant. A reanalysis of present and previous data (Hall & Love, 2003; Hiraishi, Sasaki, Shikishima, & Ando, 2012) suggested that these patterns were robust. Furthermore, males had more masculinized 2D:4D ratios than females. This sex difference did not vary by sexual orientation.

Sexual Orientation, Sexual Arousal, and Finger Length Ratios in Women

“…The evidence for a link between women's sexual orientation and 2D:4D has not been consistent across hands, with some studies finding a relationship only in the right hand and not in the left (Swift-Gallant et al, 2020). However, the aforementioned meta-analysis found an association between women's 2D:4D and sexual orientation in both hands (Grimbos et al, 2010), and we found an association between 2D:4D and sexual orientation in the left hand in a previous project (Watts et al, 2018a). Thus, in the present research, we performed all analyses on both hands.…”

“…Sexual orientation in women has repeatedly been linked to 2D:4D, with homosexual women having lower (more maletypical) 2D:4D than heterosexual women on average (Kraemer et al, 2006;Putz et al, 2004;Rahman, 2005;Wallien et al, 2008;Watts et al, 2018a), and we predicted the same pattern in present research (Hypothesis 2). A meta-analysis of 34 independent samples totaling 5,828 participants confirmed that women with a non-heterosexual orientation had lower 2D:4D in the left and right hands, compared with heterosexual women, Hedge's g's = 0.23 and 0.29, 0.04 < 95% CIs < 0.51 (Grimbos et al, 2010).…”

“…These studies computed correlations between female sexual orientation and either genital arousal or pupil dilation to male or female sexual stimuli, or computed a correlation between female sexual orientation and 2D:4D measures. The reported correlations were 0.20, 0.27 and 0.30, respectively (Rieger et al, 2016;Watts et al, 2018a). A power analysis conducted in G*Power determined that a sample size of 195 would be necessary for the smallest estimated effect (r = 0.20) to achieve significant results with a power of 0.80.…”

“…The mean (SD) age of the sample was 24.22 (6.98), and most were Caucasian (78%), followed by Chinese (5%), Black (4%), and other ethnicities. Eighteen of the 199 participants in the present study were identical twins whose data have been previously published (Watts et al, 2018a(Watts et al, , 2018c. Since identical twins are comparable to non-twins in their sexual arousal patterns, and in the interest of maximizing statistical power in the present sample, we decided to include these individuals in the analyses.…”

Masculinized Second-to-Fourth Digit Ratio (2D:4D Ratio) Is Associated With Lower Cortisol Response in Infant Female Rhesus Monkeys (Macaca mulatta)

“…Studies show that variation in PAE contributes to stable individual differences in brain function and behavior (Hines et al, 2016 ; Spencer et al, 2017 ; Del Giudice et al, 2018 ). Second-to-forth digit ratio (2D:4D ratio) is associated with sex differences in behavior, for example, one well-replicated line of research shows the same-sex attraction in women is associated with a more masculinized 2D:4D ratio (Williams et al, 2000 ; Kraemer et al, 2006 ; Watts et al, 2018 ), although the inverse relationship is not always found in men (Williams et al, 2000 ; Voracek et al, 2005 ). Thus, PAE plays a complex and important modulating role in the development of typical sex differences through its organizational effect on the brain.…”