Abstract
Previously reported associations between second-to-fourth digit length ratio (2D:4D), a proxy for prenatal androgen load, and transgender identity have been inconsistent. The objectives of the present study were to provide additional original data and an updated meta-analysis concerning this association. In a study of 464 participants, we compared the 2D:4D of transgender individuals with age- and sex-matched controls. Patients were recruited at a specialized psychiatrist’s medical office, whereas controls were hired via flyers, advertisements, and as convenience sample. A random-effects meta-analysis of the literature (17 samples, n = 3674) also quantifies the overall magnitude of the difference in 2D:4D between transgender individuals and controls. In our study providing new original data, we found a significantly higher (i.e. feminized) left-hand 2D:4D in the male-to-female transgender (MtF) identity [mean age: 32.3 (18; 61)] than in the male control group [mean age: 34.5 (18; 65)] with a Cohen’s d = 0.271. Concordantly, the meta-analytic results suggest a significant difference in 2D:4D among MtF individuals compared to male controls [g = 0.153; 95% CI (0.063; 0.243)], which was even more pronounced when individuals had been diagnosed by a clinician instead of self-identified as transgender [g = 0.193; 95% CI (0.086; 0.300)]. In both studies, no significant results were revealed for female-to-male transgender individuals [mean age: 26.1 (18; 53)] versus female controls [mean age: 27.2 (18; 55)]. This original investigation and the updated meta-analysis clarify the association between transgender identity and 2D:4D indicating the influence of prenatal androgen on the development of gender identity in subjects born as males.
Highlights
Our analyses revealed no significant association between the transgender individuals’ sexual orientation and mean of right-hand and left-hand 2D:4D (M2D):4D, right-hand 2D:4D (R2D):4D, left-hand 2D:4D (L2D):4D, and 2D:4Dr-l (Supplementary Table S3)
We found significantly feminized 2D:4D in male-to-female transgender (MtF) individuals compared to male controls in the left hand, but not in the right hand and no significant results in female-to-male transgender (FtM) individuals versus female controls
In accordance with our hypotheses, we found associations of small effect size between feminized 2D:4D and MtF gender identity that were more pronounced in MtF individuals diagnosed by a specialized clinician
Summary
From 2016 to 2019, we recruited MtF and FtM patients and male and female control groups. The control subjects were hired via flyers and advertisements (Facebook advertisements, posts on black boards) or as convenience sample (neighborhood in Erlangen and Bergisch-Gladbach, Germany) They were recruited by a medical student (I.D.) in the Department of Psychiatry and Psychotherapy of the University Hospital Erlangen, Germany. Independent from each other, three study team members (T.M., C.M., B.L.) translated the 54 items of the Gender Identity/Gender Dysphoria Questionnaire for Adolescents and Adults (GIDYQ-AA; 27 for the male and 27 for the female questionnaire)[36] and the 24 items of the Utrecht Gender Dysphoria Scale (UGDS)[37] into German. The data were analyzed using IBM SPSS statistics Version 24 for Windows (SPSS Inc., Chicago, IL, USA)
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