The effects of sex, nation, ethnicity, age and self-reported pubertal development on participant-measured right-left 2D:4D (Dr-l) in the BBC internet study.

Digit ratio (2D:4D) - a proxy for prenatal sex steroids - shows sex, nationality and ethnic differences and is linked to pubertal onset. It is unclear whether right-left 2D:4D (Dr-l) also correlates with prenatal sex steroids, as evidence of these differences has been less conclusive. The present study examined the effects of sex, nation, ethnicity, age and self-reported pubertal development (i.e. the rate of physical development and age at menarche [females] or first shave [males]) on Dr-l in a large online study (the BBC internet study). Digit lengths were self-measured in 201,865 adults (110,955 males) and the sample of nations included 41 countries. Participants reported the self-perceived rate of physical pubertal development on a five-point scale (from very slow to very fast) and provided information on the age at menarche or first shave. Adult (>17 years) males had lower Dr-l than females with weak effect size across 41 nations (males-females; d = -.065, p < .0001). There were sex and ethnicity effects on Dr-l across seven ethnic groups with males < females and lower Dr-l in Whites and Middle/Near Eastern participants compared to Asian, Black and Chinese respondents. Considering age effects, the authors focused on participants >12 years; there were stable sex differences and a weak positive effect of age on Dr-l. Dr-l showed a positive relationship with the rate of physical development and a negative relationship with age at menarche or first shave. Relationships were present in males and females with stronger effects in the latter. It is concluded that Dr-l shows a weak sex difference (males < females) independent of nation, ethnicity and age, and suggest that Dr-l is a proxy for prenatal sex steroids.