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.

J.T Manning,A Kasielska-Trojan,B Fink,R Trivers,L Mason

doi:10.1017/s0021932022000049

Abstract

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.

Highlights

Digit ratio (2D:4D) – the relative lengths of the second (2D) and fourth (4D) fingers – is thought to be a negative correlate of prenatal testosterone (T) and a positive correlate of prenatal oestrogen (E) (Zheng & Cohn, 2011; Manning, 2011)
The side difference in 2D:4D suggests that the magnitude of right-left 2D:4D (Dr-l) may be a correlate of prenatal sex steroids. This possibility was first suggested in connection with hand preference (Manning et al, 2000) and considered in detail by Manning (2002; for overall sex differences p. 21-22, spermatogenesis p. 27-30, waist-to-hip ratio p. 32-37, oestrogen levels p. 37-38)
The dimorphism remained significant after the effect of age was removed

Summary

Introduction

Digit ratio (2D:4D) – the relative lengths of the second (2D) and fourth (4D) fingers – is thought to be a negative correlate of prenatal testosterone (T) and a positive correlate of prenatal oestrogen (E) (Zheng & Cohn, 2011; Manning, 2011). The first detailed consideration of these associations reported: i) 2D:4D was sexually dimorphic (males < females), ii) it showed stability across the age range of 2 to 25 years, and iii) its associations with target traits (in this case aspects of spermatogenesis) were strongest for right 2D:4D. The latter observation led to the suggestion that right. The side difference in 2D:4D suggests that the magnitude of right-left 2D:4D (Dr-l) may be a correlate of prenatal sex steroids. Low Dr-l (putatively high prenatal T and low prenatal E) has been linked to a diverse set of sex-dependent traits such as the implicit need for power (Schultheiss et al, 2019) and elite performance in rugby (Bennett et al, 2010)

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Conclusion