Sexual dimorphism in the prenatal digit ratio (2D:4D)

Abstract

The second to fourth digit ratio (2D:4D) is smaller in human males than in females and hence this trait is sexually dimorphic. The digit ratio is thought to be established during early prenatal development under the influence of prenatal sex hormones. However, the general assumption of early establishment has hardly been studied. In our study, we analyzed the 2D:4D ratio in 327 deceased human fetuses. We measured digit lengths in 169 male and 158 female fetuses ranging from 14 to 42 weeks old. Our results showed a slight, but significant, sexual dimorphism in the expected direction, i.e., females had, on average, a ratio of 0.924 and males a ratio of 0.916. There was no significant relationship with the presence or absence of minor and major or single and multiple congenital abnormalities. There was a minimal, but significant difference between digit ratios based on digit lengths including and excluding the non-bony fingertip with the values being strongly correlated (r = .98). The prenatal 2D:4D ratio was lower than has thus far been reported for children and adults both for males and females. The extent of the sexual dimorphism in fetuses was similar to that found for children, but lower than for adults. The 2D:4D ratio, thus, seems to increase after birth in both men and women, with the second digit growing faster than the fourth digit (positive allometric growth of digit two) and perhaps more so in women than in men. Therefore, the sexual dimorphism is probably determined by prenatal as well as by postnatal developmental processes.

Fig. 1

Radiograph of right hand. Lines indicate length of the midpoint of the proximal end of the proximal phalanx to the midpoint of the distal end of the distal phalanx. The fourth digit is longer than the second digit and this is the case in all but one specimen

Fig. 2

The 2D:4D ratio in deceased male and female human fetuses. Males have, on average, a slightly lower ratio than females for both hands (.91 vs. .92). The ratio slightly, but significantly declines for the left hand, but not for the right hand