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

Sanjay Kumar,Maharaj Singh,Martin Voracek

doi:10.3389/fendo.2019.00537

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.

Highlights

Studies have reported that sex differences in digit ratio (2D:4D) relate to prenatal sex-hormone levels [1, 2]
The results support the likelihood that the differential placing of flexion creases contributes to the occurrence of higher values of palmar 2D:4D ratio, as compared to bone 2D:4D ratio
The differential placing of flexion creases likely is a factor in the occurrence of sex differences in palmar 2D:4D ratios

Summary

INTRODUCTION

Studies have reported that sex differences in digit ratio (2D:4D) relate to prenatal sex-hormone levels [1, 2]. Because soft tissues (fingertip fat and the placing of flexion creases) are additional components of palmar digit length, these may well have a role in the occurrence of differential sex effects in bone vs palmar digit ratios. Studies have reported a stronger sexual dimorphism in digit index (i.e., the ratio of palmar digit 3 length by hand length, which yields lower values in men; 31), which may be a measure of the placing of flexion creases. 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]. Because fingertip fat and bone digit length are the common components in dorsal and palmar digit lengths, palmar-to-dorsal digit ratio may serve as a measure of the placing of basal flexion creases. Because pressing of fingers does not occur in the measurement procedure of palmar digit length, a comparison of observed effects in the two palmar-to-dorsal digit ratios, as derived from two different procedures of measuring digit length dorsally (involving pressing of fingers in one but not in other), may serve to clarify possible measurement-method effects due to pressing of fingers against a surface

MATERIALS AND METHODS

RESULTS

DISCUSSION

General Discussion

ETHICS STATEMENT