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

Abstract

The present research examined whether differential placing of the basal flexion creases contributes to the occurrence of sex differences in digit ratio (2D:4D) derived from palmar digit lengths. The ratio of palmar-to-dorsal digit length, a measure of the placing of the basal flexion crease in the finger, was derived for the digits 2 and 4 of the right hand in two independent samples (Study I: N = 100; Study II: N = 200), in accordance with discovery-replication sample approach. The results show that men have lower palmar-to-dorsal digit ratios than women, and this effect is significantly stronger for digit 2 than for digit 4. Thus, the present study supports the likelihood that differential placing of flexion creases in the digits contributes to the occurrence of sex differences in palmar 2D:4D. In addition (Study II), the measurement procedure assessing the placing of flexion creases was validated. In conclusion, this evidence highlights potential conceptual shortcomings and technical limitations in the measurement conventions and methods currently employed in the field of 2D:4D research.

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

“…Moreover, because, in the present study, we found that sexual dimorphism occurs in palmar 2D:4D ratio but not in dorsal 2D:4D ratio (a faithful reflector of bone 2D:4D ratio, Auger & Eustache, 2011), and because placing of flexion creases is the only substantive difference between dorsal and palmar digit lengths, perhaps palmar 2D:4D ratio is reflecting sexual dimorphism in the placing of flexion creases between digits 2 and 4. This is consistent with the findings of earlier studies (Kumar et al, 2019, 2021). Thus, perhaps the placing of flexion creases and the bone digit length, the two different factors, combine to determine the occurrence of sexual dimorphism in the palmar 2D:4D ratio.…”

“…Thus, perhaps the placing of flexion creases and the bone digit length, the two different factors, combine to determine the occurrence of sexual dimorphism in the palmar 2D:4D ratio. Because of being different tissues, the placing of flexion creases and bone length may have different determinants of the sexual dimorphic effect (discussed in detail by Kumar et al, 2019). Moreover, because in the present study (discussed in the above paragraphs), we have identified other specific effects (family income and birth season on 2D:4D ratio) also, the motley of confounders disfavor the palmar 2D:4D ratio as an exclusive and faithful marker of prenatal hormones level.…”

“…If required, then the nails of the participants were cut. The dorsal length was measured as the distance from the tip of the finger (which clearly extends out of nails in the hand‐on‐table procedure; see Kumar et al, 2019) to the base of the dorsal phalange, whereas the palmar digit length was measured as the distance from the tip of the finger to the basal crease of the palmar phalange. All possible digit ratios, in both hands, were derived from the dorsal as well as from the palmar digit lengths (i.e.…”

“…Earlier studies have reported that, compared to the “classic” palmar 2D:4D ratio, sexual dimorphism is stronger in dorsal digit ratios with digit 5 as a constituent (Kumar et al, 2017). The dorsal digit ratio is a better proxy of the phalange‐bone‐length digit ratio (Auger & Eustache, 2011), and the placing of flexion creases moderates the sexual dimorphic effect in palmar digit ratios (Kumar et al, 2019). Therefore, the study of dorsal digit ratios may provide a better assessment of any likely difference in bone digit ratios.…”

The Relationship Between the Risk of Prostate Cancer and Second to Fourth Digit Ratio

“…There were 45 patients in each group. Finger lengths were measured by the distance from the proximal crease to the tip two times by a single observer using a digital caliper as in the study of Kumar et al (16), and the averages were recorded. To calculate the 2D:4D ratio, the length of the 2 nd finger was divided by the length of the 4 th finger.…”