Finger Length Ratios of Identical Twins with Discordant Sexual Orientations
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.
KeywordsSexual orientation Sexual differentiation Finger length ratio 2D:4D Twins
This research was funded by the American Institute of Bisexuality.
Compliance with Ethical Standards
Conflict of interest
All authors declare that they have no conflict of interest.
All procedures performed in studies involving human participants were in accordance with the ethical standards of the institutional committee and with the 1964 Helsinki Declaration and its later amendments or comparable ethical standards. The University of Essex’s Ethics Committee approved this study (“GR1303”).
Informed consent was obtained from all individual participants included in the study.
- Allaway, H. C., Bloski, T. G., Pierson, R. A., & Lujan, M. E. (2009). Digit ratios (2D:4D) determined by computer-assisted analysis are more reliable than those using physical measurements, photocopies, and printed scans. American Journal of Human Biology, 21, 365–370. https://doi.org/10.1002/ajhb.20892.CrossRefPubMedPubMedCentralGoogle Scholar
- Auyeung, B., Baron-Cohen, S., Ashwin, E., Knickmeyer, R., Taylor, K., Hackett, G., et al. (2009). Fetal testosterone predicts sexually differentiated childhood behavior in girls and in boys. Psychological Science, 20, 144–148. https://doi.org/10.1111/j.1467-9280.2009.02279.x.CrossRefPubMedPubMedCentralGoogle Scholar
- Bailey, J. M., Dunne, M. P., & Martin, N. G. (2000). Genetic and environmental influences on sexual orientation and its correlates in an Australian twin sample. Journal of Personality and Social Psychology, 78, 524–536. https://doi.org/10.1037/0022-35184.108.40.2064.CrossRefPubMedPubMedCentralGoogle Scholar
- Hiraishi, K., Sasaki, S., Shikishima, C., & Ando, J. (2012). The second to fourth digit ratio (2D:4D) in a Japanese twin sample: Heritability, prenatal hormone transfer, and association with sexual orientation. Archives of Sexual Behavior, 41, 711–724. https://doi.org/10.1007/s10508-011-9889-z.CrossRefPubMedGoogle Scholar
- Kinsey, A. C., Pomeroy, W. B., & Martin, C. E. (1948). Sexual behavior in the human male. Philadelphia: Saunders.Google Scholar
- McFadden, D., Loehlin, J. C., Breedlove, S. M., Lippa, R. A., Manning, J. T., & Rahman, Q. (2005). A reanalysis of five studies on sexual orientation and the relative length of the 2nd and 4th fingers (the 2D: 4D ratio). Archives of Sexual Behavior, 34, 341–356. https://doi.org/10.1007/s10508-005-3123-9.CrossRefPubMedGoogle Scholar
- Meyer-Bahlburg, H. F. L., Dolezal, C., Baker, S. W., & New, M. I. (2008). Sexual orientation in women with classical or non-classical congenital adrenal hyperplasia as a function of degree of prenatal androgen excess. Archives of Sexual Behavior, 37, 85–99. https://doi.org/10.1007/s10508-007-9265-1.CrossRefPubMedPubMedCentralGoogle Scholar
- Pasterski, V., Acerini, C. L., Dunger, D. B., Ong, K. K., Hughes, I. A., Thankamony, A., et al. (2015). Postnatal penile growth concurrent with mini-puberty predicts later sex-typed play behavior: Evidence for neurobehavioral effects of the postnatal androgen surge in typically developing boys. Hormones and Behavior, 69, 98–105. https://doi.org/10.1016/j.yhbeh.2015.01.002.CrossRefPubMedGoogle Scholar
- Putz, D. A., Gaulin, S. J. C., Sporter, R. J., & McBurney, D. H. (2004). Sex hormones and finger length: What does 2D:4D indicate? Evolution and Human Behavior, 25, 182–199. https://doi.org/10.1016/j.evolhumbehav.2004.03.005.CrossRefGoogle Scholar