Abstract
Hands and digits tend to be sexually dimorphic and may reflect prenatal androgen exposure. In the past years, the literature introduced several hand and digit measures, but there is a lack of studies in prepubertal cohorts. The available literature reports more heterogeneous findings in prepubertal compared to postpubertal cohorts. The comparability of the available studies is further limited by the study design and different measurement techniques. The present study compared the reliability and sex differences of available hand and digit measures, namely digit lengths of 2D, 3D, 4D, 5D, digit ratios 2D:4D, 2D:5D, 3D:4D, 3D:5D, 4D:5D, relative digit lengths rel2, rel3, rel4, rel5, directional asymmetry of right and left 2D:4D (Dr-l), hand width, length, and index of 399 male and 364 female 6-month-old German infants within one study using only indirect and computer-assisted measurements. The inter-examiner reliability was excellent while the test-retest reliability of hand scans was only moderate to high. Boys exhibited longer digits as well as wider and longer hands than girls, but smaller digit ratios, with ratios comprising the fifth digit revealing the largest effect sizes. Other hand and digit ratios revealed sex differences to some extent. The findings promote the assumption of sexual dimorphic hand and digit measures. However, by comparing the results of the available literature, there remains an uncertainty regarding the underlying hypothesis. Specifically in prepubertal cohorts, i.e. before the influence of fluctuating hormones, significant effects should be expected. It seems like other factors than the influence of prenatal androgens contribute to the sexual dimorphism in hand and digit lengths.
Highlights
Hands and digits tend to be sexually dimorphic and may reflect prenatal androgen exposure
Research suggested that hands and digits may serve as an indicator for sexual differentiation, as it is hypothesized that they are associated to the HOX g enes[6], which promote the development of the urogenital tract and external genitalia as well as the limb development[7,8], and reflect differences in prenatal androgen exposure[9]
The most discussed indicator for prenatal androgen exposure is the second to fourth digit ratio (2D:4D) that has been associated with an amount of behavioral outcomes that are known to differ between males and females[18]
Summary
Hands and digits tend to be sexually dimorphic and may reflect prenatal androgen exposure. In prepubertal cohorts, i.e. before the influence of fluctuating hormones, significant effects should be expected It seems like other factors than the influence of prenatal androgens contribute to the sexual dimorphism in hand and digit lengths. To the best of our knowledge, no study has considered the relative digit lengths in prepubertal cohorts Another aspect to consider when looking at sexual dimorphism of the hand is that authors postulate that prenatal androgens have a different influence on the right versus left side of the body and digit ratios may differ in the right versus left hand[11,31,32]. Relatively little research has considered this variable in terms of sexual dimorphism and Dr-l in newborns remained uncorrelated towards testosterone levels in amniotic fluid and showed no significant sex difference[34]
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