The relationship between second-to-fourth digit ratio and female gender identity.
BACKGROUND
Gender identity and the second-to-fourth finger length ratio (2D : 4D) are discriminative between the sexes. However, the relationship between 2D : 4D and gender identity disorder (GID) is still controversial.


OBJECTIVE
The aim of this study is to investigate the relationship between 2D : 4D and score on the Gender Identity Scale (GIS) in female-to-male (FtM) GID subjects.


METHODS
Thirty-seven GID-FtM with testosterone replacement therapy from our clinic were included in this study. As controls, 20 male and 20 female volunteers participated from our institution (medical doctors and nurses). We photocopied left and right hands of the participants and measured the second and fourth finger lengths. Gender identity was measured with the GIS.


METHODS
2D : 4D digit ratio and GIS in male, female, and GID-FtM subjects.


RESULTS
The 2D : 4D (mean ± standard deviation) in male, female, and GID-FtM were 0.945 ± 0.029, 0.999 ± 0.035, and 0.955 ± 0.029 in right hand and 0.941 ± 0.024, 0.979 ± 0.040, and 0.954 ± 0.036 in left hand, respectively. The 2D : 4D was significantly lower in male controls in both hands and GID-FtM in the right hand than in female controls (P < 0.05, analysis of variance). Multiple linear regression analysis revealed that "consistent gender identity" score in the higher domain in GIS and "persistent gender identity" score in the lower domain are statistically significant variables correlating with 2D : 4D in the right hands among biological females.


CONCLUSIONS
The finger length ratio 2D : 4D in GID-FtM was significantly lower than in female controls in the right hand in this study. 2D : 4D showed a positive correlation with GIS score. Because 2D : 4D influences are assumed to be established in early life and to reflect testosterone exposure, our results suggest a relationship between GID-FtM and perinatal testosterone.
Department of Urology, School of Medicine, Teikyo University, Itabashi-ku, Tokyo, Japan. hisasue@med.teikyo-u.ac.jp
Shin-Ichi
S
Hisasue
Shoko
S
Sasaki
Taiji
T
Tsukamoto
Shigeo
S
Horie
D000328:Adult; D000886:Anthropometry; D005260:Female; D005385:Fingers / Q000033:anatomy & histology*; D005783:Gender Identity; D006801:Humans; D008297:Male; D012727:Sex Characteristics; D057829:Gender-Affirming Procedures; D011795:Surveys and Questionnaires; D013739:Testosterone / Q000008:administration & dosage; D055815:Young Adult
D013739:Testosterone