Abstract
The length ratio between the second and the fourth digit (2D : 4D) is a retrospective, non-invasive biomarker for prenatal androgen exposure. It was found to be negatively correlated with handgrip strength (HGS) in men, but the evidence for women is mixed. Such studies in women call for increased detection sensitivity. The present study was designed to reduce potential confounding factors, especially age and ethnicity variation. We measured the digit ratios and HGS of 125 healthy women between 19 and 31 years of age from a remote region in Austria. 2D : 4D of both hands was significantly and negatively correlated with HGS (n = 125, right hand: r = –0.255, p = 0.002, left hand: r = –0.206, p = 0.011). Size, direction and significance of correlation coefficients remained stable when statistically controlling for age, body weight, body height, body mass index or hours of exercise per week. This yields theory-consistent evidence that HGS and 2D : 4D are clearly associated in women—when sufficiently reducing genetic variation (confounding 2D : 4D), the ontogenetic environment and age ranges (confounding HGS) in the study population. This finding implies similar organizing effects of prenatal androgens as in men, pointing to a more parsimonious developmental mechanism and a new look into its proximate and ultimate causes.
Highlights
Since pre- and perinatal hormones organize the architecture of the mammalian body and brain [1,2], early exposure to higher levels of testosterone (T) may cause fewer female-like and more male-like characteristics, such as increased muscle mass [3]
This study supports the biological hypothesis that higher prenatal T exposure, as approximated by 2D : 4D, and 2D : 4D asymmetry (Dr − l), increases handgrip strength (HGS) in women
The data analysis showed a significant negative correlation between 2D : 4D and HGS: women with relatively shorter second digits had on average a higher HGS
Summary
Since pre- and perinatal hormones organize the architecture of the mammalian body and brain [1,2], early exposure to higher levels of testosterone (T) may cause fewer female-like and more male-like characteristics, such as increased muscle mass [3]. Reports of correlations between 2D : 4D and HGS that included both sexes yielded significant negative associations for men but not for women (for adults [28] and for children [40], but see [41] for a null result). These studies are based on large samples and it is likely that there is a negative relationship between 2D : 4D and HGS in males (both children and adults). Negative correlations have been reported between 2D : 4D ratio and physical fitness/athletic prowess for European female participants [21,22] This calls for re-examining 2D : 4D and HGS in a more homogeneous European population. Hartigan’s dip test statistic for unimodality was calculated using the ‘diptest’ package [77] in R v. 3.3.2 [78]
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