Digit ratio (2D:4D) and transgender identity: new original data and a meta-analysis

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.

An anthropometric study of sexual orientation and gender identity in Thailand

“…First, contrary to the prediction that more masculine groups would show lower 2D:4D, toms had higher left 2D:4D than lesbian women. That said, lack of support for our prediction is not necessarily out of step with other literature given recent meta-analytic findings suggesting no differences in 2D:4D between heterosexual women and trans men 16 . As such, processes contributing to digit ratio might not be related to the development of masculine identity among individuals assigned female at birth.…”

“…As such, this pattern runs contrary to a recent study that reported digit ratio was more female-typical among gay men who expressed feminine gender role behavior 57 . Further, meta-analyses have suggested digit ratio is more female-typical (vs. heterosexual men) among trans women 16 , but not gay men 12 (but see findings with an adolescent sample 58 ). Given the discrepant findings, it will be important for future research to continue to examine digit ratio in relation to both sexual orientation and gender identity/expression.…”

“…Some studies suggest digit ratios were more consistent with gender identity/expression than with birth-assigned sex 13 – 15 . A recent study and meta-analysis indicated no difference in 2D:4D between trans men and cisgender women, whereas trans women had a more female-typical digit ratio compared with cisgender men 16 .…”

A Follow-Up Study of Boys With Gender Identity Disorder

“…All factor loadings were ≥0.30 (median, 0.86; range, 0.34-0.96). The GIDYQ-AA has strong evidence for discriminant validity and a high threshold for specificity (i.e., low false positive rate for non-GID individuals) [see (95,96,(99)(100)(101)(102)].…”

Beyond the Gender Binarism: The Neural Correlates of Trans Men Investigated in a Functional Connectivity–Resting State fMRI Study

“…Both biological (Sadr et al, 2020;Siegmann et al, 2020) and psychosocial (Schnarrs et al, 2019) factors are involved in the etiology of GD. Structural magnetic resonance imaging tecniques highlighted neuroanatomical similarities between transgender and cisgender individuals sharing the same birthassigned sex (Khorashad et al, 2021), i.e.…”