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.

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Change history

  • 11 December 2018

    Readers should note that the confidence intervals for the effect of sexual orientation on 2D:4D in the left hand of female twins are incorrectly reported in Table 1 of this article. In particular, the upper limit of the confidence intervals is missing a minus sign.

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Funding

This research was funded by the American Institute of Bisexuality.

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Correspondence to Tuesday M. Watts.

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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”).

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Informed consent was obtained from all individual participants included in the study.

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Watts, T.M., Holmes, L., Raines, J. et al. Finger Length Ratios of Identical Twins with Discordant Sexual Orientations. Arch Sex Behav 47, 2435–2444 (2018). https://doi.org/10.1007/s10508-018-1262-z

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Keywords

  • Sexual orientation
  • Sexual differentiation
  • Finger length ratio
  • 2D:4D
  • Twins