Background: Digit ratio (2D:4D), a marker of prenatal testosterone exposure, is a weak negative correlate of sports/athletic/fitness performance. While numerous studies have examined the relationship between 2D:4D and physical fitness, there has never been a comprehensive study that has synthesized studies examining relationships between 2D:4D and muscular fitness. Objectives: To systematically review and meta-analyze the relationship between 2D:4D 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 2D:4D 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 2D:4D and HGS (r = À0.15, 95% CI = -0.20 to À0.09), indicating that individuals with low 2D:4Ds had high HGS. We found substantial heterogeneity between studies (Q = 123.4, p < .0001; I 2 = 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 2D:4D and HGS, which showed substantial heterogeneity between studies, but was neither moderated by sex nor age. Our finding probably reflects both the longterm (organizational) and short-term (activational) benefits of testosterone. | INTRODUCTIONDigit ratio, also referred to as 2D:4D, is the ratio of the length of the second digit (2D, the index finger) to the length of the fourth digit (4D, the ring finger). Although 2D:4D is thought to be the result of exposure to prenatal testosterone and estrogen (Manning, 2011;Zheng & Cohn, 2011), there is ongoing scientific debate regarding the association between 2D:4D and prenatal sex steroids (see Swift-Gallant et al. (2020) for those in favor and McCormick and Carré (2020) for those opposed). 2D:4D appears to be set as early as the second trimester in