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

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

Sanjay Kumar et al. Front Endocrinol (Lausanne).

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.

Keywords: digit ratio (2D:4D); dorsal finger length; fingertip; flexion creases; prenatal testosterone; sex differences.

Figures

Figure 1
Figure 1
Flow chart of the scheme of selection of participants.
Figure 2
Figure 2
Image showing the measurement method for dorsal digit length through the hand-on-table (HT) procedure. Distance between “a” and “b” represents the dorsal digit length.
Figure 3
Figure 3
Image showing the measurement method of palmar digit length. Distance between “c” and “d” represents the palmar digit length.
Figure 4
Figure 4
Image showing the measurement method of dorsal digit length through the hand-in-air (HA) procedure.
Figure 5
Figure 5
Line graph showing sex differences in palmar-to-dorsal digit ratios of digit 2 (Δ) and digit 4 (O), measured through the HT (__) and the HA (−−−) procedures. (i) Men have lower palmar-to-dorsal digit ratios than women, and this effect is stronger for digit 2 than for digit 4. (ii) The pattern of sex differences observed in digits 2 and 4 is similar for both (HT or HA) procedures. (iii) Digit ratios derived from the HT procedure are higher than those derived from the HA procedure, and, among both men and women, this effect is stronger for digit 2 than for digit 4.

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