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

Abstract

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

Handgrip strength and 2D : 4D in women: homogeneous samples challenge the (apparent) gender paradox

“…In a recent meta-analysis, Pasanen et al [ 42 ] found that study-sex-specific correlation coefficients ranged from –0.62 to 0.08 (85% negative); overall it was –0.15. They concluded that sex did not moderate the relationship between 2D : 4D and HGS.…”

“…Such comparisons would also be valuable for understanding the association between 2D : 4D and aerobic exercise (e.g. long-distance running) that generally yields a stronger relationship with 2D : 4D than anaerobic exercise (such as handgrip or sprinting performance) [ 21 , 38 , 42 ].…”

“…Nonetheless, if the magnitude of the relationship for men is quite small (e.g. r = about –0.30 in a number of studies [ 42 ]), then demonstrating the correlation in women would require that confounding factors such as ethnicity and exercise regime be carefully controlled.…”

The association of prenatal amniotic sex hormones and digit ratio (2D:4D) in children aged 5 to 70 months: A longitudinal study

“…However, the validity of 2D:4D as a marker for prenatal testosterone exposure has been questioned [ 20 , 25 , 26 ]. Nonetheless, according to a PubMed search using the keywords “2D:4D” and “digit ratio” there were more than 100 newly published studies from 2020 till today that have investigated associations between 2D:4D and various behaviors, traits, disorders, or diseases, like muscular strength and fitness [ 5 , 27 ], parental income [ 28 ], sexual preferences [ 29 ], concentration of steroids like cortisol and vitamin D [ 30 ], thyroid disease [ 31 ], and migraine in adults [ 32 ].…”

Digit ratio (2D:4D) and maternal testosterone-to-estradiol ratio measured in early pregnancy

“…Despite enduring popularity, results from studies that have tried to validate digit ratio measures have been equivocal 10 , 11 . As there is a vast and rapidly growing literature examining 2D:4D in relation to an extensive range of variables across multiple research fields (e.g., psychiatry 12 , social science 13 , 14 , cancer research 15 , criminology 16 , sports science 17 , 18 ), it is important to consider this lack of consistent evidence rather than rely on the assumption that 2D:4D is a valid and reliable proxy for the prenatal hormonal environment.…”