Sexual Dimorphism in Digit Ratios Derived from Dorsal Digit Length among Adults and Children

Abstract

Sexual dimorphism in ventrally measured digit ratios (2D:4D and other) has been related to prenatal sex-hormone levels. In the present series of three studies, we measured all digit lengths (excluding the thumb) on the dorsal, rather than the ventral, side of left and right hands and investigated the sexual dimorphism in digit ratios in three independent samples, two of them comprising adults (Study I, N = 104; Study II, N = 154), and one further, comprising kindergarten children (Study III, N = 64). Results show that men have lower digit-ratio values compared to women in digit ratios that include digit 5 as one of the constituents of the ratio (i.e., the 4D:5D, 3D:5D, and 2D:5D ratios). Boys have lower values compared to girls for the 4D:5D and 3D:5D ratios, and there is a similar trend of sexual dimorphism in the 2D:5D ratio. Thus, based on the evidence from dorsally measured digit ratios, the present findings from three samples are consistent with the idea that early sex-hormonal effects might be stronger for digit ratios involving digit 5, as compared to the classic, and frequently studied, ventrally measured 2D:4D ratio.

Sexual Dimorphism of Digit‐Length Ratio in a Viviparous Lizard: Influence of Age, but not Preservation State or Sex of Interuterine Twin

“…Our qualitative assessment for amniotes suggests that if there is such a trend, it may affect (or be most detectable on) the limb of mammals and reptiles with the longest digits, rather that the foreor hindlimbs specifically (in birds, only the feet are measured). Furthermore, in our study as in many others (e.g., for mice: Brown et al, 2002a;humans: Manning et al, 1998;McFadden and Shubel, 2002;H€ onekopp and Watson, 2010;for birds: Saino et al, 2007), effects are more often detected for right limbs than for left (but see Kumar et al, 2017 below for a different result in humans when measurements are made with a novel procedure). In mice, the embryonic onset of this directional asymmetry (Zheng and Cohn, 2011) implies differences in sensitivity of the right and left paws to prenatal sex steroids (Manning et al, 2014).…”

“…Studies on humans show that four methods of measuring 2D:4D on the ventral surface (physical, photocopies, printed scans and computer‐assisted image analysis) yield different mean values and different interobserver consistency, with computer‐assisted image analysis (similar to our study) being the most reliable (Allaway et al, ). Furthermore, in a recent study in which measurements were taken using a novel, dorsal approach with the hand in flexion, no effect of sex on 2D:4D was detected, whereas left‐hand finger ratios involving digit 5 showed consistent female‐larger values in both adults and children (Kumar et al, ). Equivalent comparisons among methods of measurement for lizards are lacking, and even whether measurements were collected ventrally or dorsally is not always explicit.…”

Differential Placing of Flexion Creases Contributes to Sex Differences in the Second-to-Fourth Digit Ratio (2D:4D)

“…The dorsal digit (dD) length measures the distance between the fingertip and the base of the proximal dorsal phalanx. The investigator asked participants to put their right hand on the edge of a smooth table, with fingers making an angle of 90° to the palm, following the procedure of Kumar et al [(32); see Figure 2]. The palmar digit (pD) length measures the distance between the fingertip and the basal crease of the proximal palmar phalanx (Figure 3).…”

“…Thus, although no prior study has studied the differential occurrence of sex effects in the placing of flexion creases in different digits, the account of Kulaksiz and Gozil (31) lends some support to this idea. Moreover, a recent study has supported the likelihood that the placing of flexion creases is a contributing factor in the occurrence of sex differences in digit ratios (32).…”

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

“…By computing digit ratios with every possible digit (excluding the thumb), i.e. the second (2D), third (3D), fourth (4D), and fifth digit (5D), significant differences between male and female ratios in different age groups could be observed in various studies 22 – 26 . However, a study regarding directly measured digit ratios in children aged 2–18 years revealed relevant fluctuations and argues that digit ratios other than 2D:4D do not serve as reliable indicators of prenatal androgen exposure 12 .…”