Investigating the reliability and sex differences of digit lengths, ratios, and hand measures in infants

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.

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

“…Notably, research does not support reliable sex differences of 2D:4D in younger cohorts, as noted in another study of our group [ 49 ], and it appears that sex differences become larger and more stable during adolescence and adulthood [ 8 , 20 , 50 – 52 ]. Prepubertal girls and boys show nearly the same concentrations of fluctuating sex hormones [ 83 ].…”

“…Based on the assumption that 2D:4D is (at least partially) determined by prenatal androgens one can assume that the temporal stability should be high, especially before the onset of puberty. However, a review of relevant studies revealed limited evidence for the temporal stability of 2D:4D, and, moreover, the sex difference in prepubertal cohorts [49]. Longitudinal studies have shown an increase of 2D:4D with age, although with moderate to high correlations between the time points [20,[50][51][52].…”

Effects of caste, birth season, and family income on digit ratios

“…However, the above‐discussed findings suggest that, compared to palmar digit ratios, and especially the “classic” palmar 2D:4D ratio, the dorsal digit ratios constituting digit 5 are better markers of prenatal testosterone level. A recent study on infants also supports the likelihood of digit ratios constituting digit 5 as better markers of sexual dimorphism (Ernsten et al, 2021).…”

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“…A comparative analysis of the size of the hands also demonstrates that the length of the hand is longer in men than in women, while the same ratios were found for the metacarpal bones and phalanges of the fingers (Hsiao et al, 2015). In addition, one of the differences between the hands of men and women is the ratio of the lengths of the index and ring fingers (2d:4d ratio) -in men, the length of the ring finger prevails over the length of the index finger, while in women the opposite ratio is observed (Sanfilippo et al, 2013;de Sanctis et al, 2017;Ernsten et al, 2021). To study the variability of the hand associated with gender, the following are used: subjective criteria based on morphological features and objective criteria based on size analysis, including various measurements and indices (Kanchan and Krishan, 2011;Kondo et al, 2017).…”