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Comparative Study
, 272 (1576), 1995-2001

Second to Fourth Digit Ratio and Face Shape

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Comparative Study

Second to Fourth Digit Ratio and Face Shape

Bernhard Fink et al. Proc Biol Sci.

Abstract

The average human male face differs from the average female face in size and shape of the jaws, cheek-bones, lips, eyes and nose. It is possible that this dimorphism is determined by sex steroids such as testosterone (T) and oestrogen (E), and several studies on the perception of such characteristics have been based on this assumption, but those studies focussed mainly on the relationship of male faces with circulating hormone levels; the corresponding biology of the female face remains mainly speculative. This paper is concerned with the relative importance of prenatal T and E levels (assessed via the 2D : 4D finger length ratio, a proxy for the ratio of T/E) and sex in the determination of facial form as characterized by 64 landmark points on facial photographs of 106 Austrians of college age. We found that (i) prenatal sex steroid ratios (in terms of 2D : 4D) and actual chromosomal sex dimorphism operate differently on faces, (ii) 2D : 4D affects male and female face shape by similar patterns, but (iii) is three times more intense in men than in women. There was no evidence that these effects were confounded by allometry or facial asymmetry. Our results suggest that studies on the perception of facial characteristics need to consider differential effects of prenatal hormone exposure and actual chromosomal gender in order to understand how characteristics have come to be rated 'masculine' or 'feminine' and the consequences of these perceptions in terms of mate preferences.

Figures

Figure 1
Figure 1
(a) An example face with 64 predefined landmarks. The grey-filled circles indicate classical landmarks that can be identified unambiguously, the white-filled circles are semi-landmarks that lie on a curve (see §2), and the forehead boss points (solid black) are used for visualization only and are not included in the statistical analyses. (b) All 106 landmark configurations superimposed by the Procrustes fit. These coordinates are the basis for further statistical analysis.
Figure 2
Figure 2
Visualization of the shape regression on 2D : 4D ratio (averaged among both hands) within males. The middle face with an undeformed square grid is the average landmark configuration and corresponds to the average digit ratio for males. The right grids show deformations from the mean face to faces that are predicted for higher 2D : 4D ratios (0.068=2 s.d. and 0.136=4 s.d., respectively, higher than the average). The left faces correspond to low 2D : 4D ratios (−2 s.d. and −4 s.d.). The ±4 s.d. values are outside the data range.
Figure 3
Figure 3
Shape regression within males on the 2D : 4D ratio of the left hand (upper and lower left figures), the right hand (middle figures), and the mean 2D : 4D ratio (right figures). The three upper figures are visualizations of predicted faces for 2D : 4D ratio 4 s.d. higher than the average. Accordingly, the lower figures are predicted faces for 2D : 4D ratio 4 s.d. lower than the average.
Figure 4
Figure 4
Shape regression within females on the 2D : 4D ratio of the left hand (left figures), the right hand (middle figures), and the mean 2D : 4D ratio (right figures). The three upper figures are visualizations of predicted faces for 2D : 4D ratio 6 s.d. higher than the average. Accordingly, the lower figures are predicted faces for 2D : 4D ratio 6 s.d. lower than the average.
Figure 5
Figure 5
Sexual dimorphism in facial shape. The two inner deformation grids are thin-plate spline visualizations of the shape differences between the average male face (m) and the average female face (f). To enhance the details these differences were exaggerated by two in the outer grids.

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