The Biologic Basis of Transgender Identity: 2D:4D Finger Length Ratios Implicate A Role for Prenatal Androgen Activity

      ABSTRACT

      Objective: The biologic basis for gender identity is unknown. Research has shown that the ratio of the length of the second and fourth digits (2D:4D) in mammals is influenced by biologic sex in utero, but data on 2D:4D ratios in transgender individuals are scarce and contradictory. We investigated a possible association between 2D:4D ratio and gender identity in our transgender clinic population in Albany, New York.
      Methods: We prospectively recruited 118 transgender subjects undergoing hormonal therapy (50 female to male [FTM] and 68 male to female [MTF]) for finger length measurement. The control group consisted of 37 cisgender volunteers (18 females, 19 males). The length of the second and fourth digits were measured using digital calipers. The 2D:4D ratios were calculated and analyzed with unpaired t tests.
      Results: FTM subjects had a smaller dominant hand 2D:4D ratio (0.983 ± 0.027) compared to cisgender female controls (0.998 ± 0.021, P = .029), but a ratio similar to control males (0.972 ± 0.036, P =.19). There was no difference in the 2D:4D ratio of MTF subjects (0.978 ± 0.029) compared to cisgender male controls (0.972 ± 0.036, P = .434).
      Conclusion: Our findings are consistent with a biologic basis for transgender identity and the possibilities that FTM gender identity is affected by prenatal androgen activity but that MTF transgender identity has a different basis.
      Abbreviations: 2D:4D = 2nd digit to 4th digit; FTM = female to male; MTF = male to female
      To read this article in full you will need to make a payment
      AACE Member Login
      AACE Members, full access to the journal is a member benefit. Use your society credentials to access all journal content and features.
      Already a print subscriber? Claim online access
      Already an online subscriber? Sign in
      Institutional Access: Sign in to ScienceDirect

      REFERENCES

        • Leinung M.C.
        • Urizar M.F.
        • Patel N.
        • Sood S.C.
        Endocrine treatment of transsexual persons: extensive personal experience.
        Endocr Pract. 2013; 19: 644-650
        • Saraswat A.
        • Weinand J.D.
        • Safer J.D.
        Evidence supporting the biologic nature of gender identity.
        Endocr Pract. 2015; 21: 199-204
        • Erickson-Schroth L.
        Update on the biology of transgender identity.
        J Gay Lesbian Ment Health. 2013; 17: 150-174
        • Breedlove M.S.
        Minireview: Organizational hypothesis: instances of the fingerpost.
        Endocrinology. 2010; 151: 4116-4122
        • Manning J.T.
        • Scutt D.
        • Wilson J.
        • Lewis-Jones D.I.
        The ratio of 2nd to 4th digit length: a predictor of sperm numbers and concentrations of testosterone, luteinizing hormone and oestrogen.
        Hum Reprod. 1998; 13: 3000-3004
        • Honekopp J.
        • Watson S.
        Meta-analysis of digit ratio 2D:4D shows greater sex difference in the right hand.
        Am J Hum Biol. 2010; 22: 619-630
        • Manning J.
        • Kilduff L.
        • Cook C.
        • Crewther B.
        • Fink B.
        Digit ratio (2D:4D): a biomarker for prenatal sex steroids and adult sex steroids in challenge situations.
        Front Endocrinol (Lausanne). 2014; 5: 9
        • van Honk J.
        • Montoya E.R.
        • Bos P.A.
        • van Vugt M.
        • Terburg D.
        New evidence on testosterone and cooperation.
        Nature. 2012; 485 (discussion E5–E6): E4-E5
        • Olshan J.
        • Eimicke T.
        • Belfort E.
        Gender incongruity in children with and without disorders of sexual differentiation.
        Endocrinol Metab Clin North Am. 2016; 45: 464-482
        • Wallien M.
        • Zucker K.
        • Steensma T.
        • Cohen-Kettenis P.
        2D:4D finger-length ratios in children and adults with gender identity disorder.
        Horm Behav. 2008; 54: 450-454
        • Brown W.M.
        • Hines M.
        • Fane B.A.
        • Breedlove S.M.
        Masculinized finger length patterns in human males and females with congenital adrenal hyperplasia.
        Horm Behav. 2002; 42: 380-386
        • Zheng Z.
        • Cohn M.J.
        Developmental basis of sexually dimorphic digit ratios.
        PNAS. 2011; 108: 16289-16294
        • Kraemer B.
        • Noll T.
        • Delsignore A.
        • Milos G.
        • Schnyder U.
        • Hepp U.
        Finger length ratio (2D:4D) in adults with gender identity disorder.
        Arch Sex Behavior. 2009; 38: 359-363
        • Schneider 1, H.J.
        • Pickel J.
        • Stalla G.K.
        Typical female 2nd-4th finger length (2D:4D) ratios in male-to-female transsexuals-possible implications for prenatal androgen exposure.
        Psychoneuroendocrinology. 2006; 31: 265-269
        • Ribeiro E.
        • Neave N.
        • Morais R.N.
        • Manning J.T.
        Direct versus indirect measurement of digit ratio (2d:4d): a critical review of the literature and new data.
        Evol Psychol. 2016; 14: 1-8
        • Vujović S.
        • Popović S.
        • Mrvošević Marojević L.
        • et al.
        Finger length ratios in Serbian transsexuals.
        ScientificWorldJournal. 2014; 2014: 763563
        • Hisasue S.
        • Sasaki S.
        • Tsukamoto T.
        • Horie S.
        The relationship between second-to-fourth digit ratio and female gender identity.
        J Sex Med. 2012; 9: 2903-2910
        • Grimbos T.
        • Dawood K.
        • Burriss R.P.
        • Zucker K.J.
        • Puts D.A.
        Sexual orientation and the second to fourth finger length ratio: A meta-analysis in men and women.
        Behav Neurosci. 2010; 124: 278-297