The relationship between digit ratio (2D:4D) and muscular fitness: A systematic review and meta-analysis.

ABSTRACT

BACKGROUND: Digit ratio (2D4D), a marker of prenatal testosterone exposure, is a weak negative correlate of sports/athletic/fitness performance. While numerous studies have examined the relationship between 2D4D and physical fitness, there has never been a comprehensive study that has synthesized studies examining relationships between 2D4D and muscular fitness. OBJECTIVES: To systematically review and meta-analyze the relationship between 2D4D and muscular fitness measured as handgrip strength (HGS). METHODS: We systematically searched five electronic databases, reference lists, topical systematic reviews/meta-analyses, and personal libraries in November 2020. Peer-reviewed, cross-sectional studies that reported Pearson's correlation coefficients between objectively measured 2D4D and HGS were included. We used random-effects meta-analysis to estimate the pooled correlation and the 95% confidence interval (95%CI), and moderator analyses to estimate the influence of sex and age. RESULTS: Data from 22 studies, representing 5271 individuals from 11 countries ranging in (mean) age from 10.4 to 58.0 years, were included. Overall, there was a weak negative correlation between 2D4D and HGS (r = -0.15, 95%CI = -0.20 to -0.09), indicating that individuals with low 2D4Ds had high HGS. We found substantial heterogeneity between studies (Q = 123.4, p < .0001; I2  = 74%), but neither sex (Q = 0.003, p = .96) nor age (Q = 0.46, p = .50) significantly moderated the pooled correlation. CONCLUSIONS: We found a weak negative relationship between 2D4D and HGS, which showed substantial heterogeneity between studies, but was neither moderated by sex nor age. Our finding probably reflects both the long-term (organizational) and short-term (activational) benefits of testosterone.