Digit Ratio (2D:4D), Transgendered Belief, and Transsexual Drug Therapy in the BBC Internet Study

Abstract

Transgendered belief—the conviction that one is the opposite gender to one’s natal gender—may be influenced by prenatal sex steroids. We consider this possibility by examining the relationship between digit ratio (2D:4D—a suggested correlate of fetal testosterone and estrogen), natal gender, felt gender, and transsexual drug therapy in a large online survey (the BBC Internet Study). There were 209,317 participants who reported their gender, their felt gender, and whether they were taking/had taken transsexual drug therapy (male-to-female (MtF) or female-to-male (FtM)). Participants included natal males who felt male (M→M, n = 104,939) and those who felt female (M→F, n = 4760) and natal females who felt female (F→F, n = 84,904) and those who felt male (F→M, n = 4705). Transsexual drug therapy (MtF and FtM) was reported by 830 and 223 participants, respectively. Digit length was determined by direct self-measurement. Mean 2D:4D of M→F and MtF individuals was higher (more “feminized”) than for M→M and natal males, respectively. These effects were found in the total sample, the most numerous ethnic group (Whites) and the two largest national White samples (the UK and the USA). The mean 2D:4D of F→M and FtM participants did not differ from that of F→F and natal females, respectively. We conclude that M→F and MtF individuals may have experienced lower prenatal testosterone and higher estrogen than M→M and natal males, respectively. There was no evidence for an effect of prenatal sex steroids on transgendered belief or transsexualism in F→M and FtM individuals.