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Comparative Study
, 5 (5), 643-6

Differences in the 2nd to 4th Digit Length Ratio in Humans Reflect Shifts Along the Common Allometric Line

Comparative Study

Differences in the 2nd to 4th Digit Length Ratio in Humans Reflect Shifts Along the Common Allometric Line

Lukás Kratochvíl et al. Biol Lett.


Ratios often lead to biased conclusions concerning the actual relationships between examined traits and comparisons of the relative size of traits among groups. Therefore, the use of ratios has been abandoned in most comparative studies. However, ratios such as body mass index and waist-to-hip ratio are widely used in evolutionary biology and medicine. One such, the ratio of the 2nd to the 4th finger (2D : 4D), has been the subject of much recent interest in both humans and animals. Most studies agree that 2D : 4D is sexually dimorphic. In men, the 2nd digit tends to be shorter than the 4th, while in women the 2nd digit tends to be of the same size or slightly longer than the 4th. Nevertheless, here we demonstrate that the sexes do not greatly differ in the scaling between the 2nd and 4th digit. Sexual differences in 2D : 4D are mainly caused by the shift along the common allometric line with non-zero intercept, which means 2D : 4D necessarily decreases with increasing finger length, and the fact that men have longer fingers than women. We conclude that previously published results on the 2D : 4D ratio are biased by its covariation with finger length. We strongly recommend regression-based approaches for comparisons of hand shape among different groups.


Figure 1.
Figure 1.
Scatterplot of the length of the fingers on right hands in 297 biology students. The common line estimated by the ordinary least-square linear regression is depicted as the solid line (length of the 2nd finger = 0.823 (length of the 4th finger) + 11.223; p < 0.00001; r2 = 0.81). Its intercept is significantly larger than zero (b = 11.223 ± 1.654; p < 0.00001), which means the 2D : 4D ratio necessarily decreases with increasing finger length. The sexes do not significantly differ in the scaling relationship between the length of the 2nd and 4th finger (see table 1 for statistics). Women, circles and dotted line; men, squares and broken line.

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