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.

Digit ratio (2D:4D) in newborns: Influences of prenatal testosterone and maternal environment

“…Our data shows that the 2D:4D dimorphism is already present at birth, confirming previous reports that it shows as early as at 14 weeks of gestation [16]. As expected, males had lower mean 2D:4D ratio than females, but like previous authors [2,14] we found that the difference is subtle and there is a large overlap of 2D:4D distributions between sexes (Fig.…”

“…Our data also support claims that 2D:4D increases from birth to adulthood [14][15][16][17], as mothers had significantly larger 2D:4D than their NBs (Table 2) for both sexes and hands. Postnatal developmental processes should thus be involved in the determination of 2D:4D sexual dimorphism later found in adults.…”

“…Similarly, Galis et al [16] working with 327 deceased human fetuses (14 to 42 weeks old) reported that the prenatal 2D:4D in both sexes was lower than has been reported for children and adults, suggesting that it may increase after birth. More recently, McIntyre and Alexander [17], in a sample of 74 young children (3-5 months old) also found relatively low 2D:4D values, and Knickmeyer et al [18] followed 2D:4D during the postnatal testosterone surge (0-2 years old) to conclude that 2D:4D ratios change, apparently modulated by postnatal testosterone, with such individual variability that led the authors to suggest that 2D:4D measured in early childhood may not correlate well with prenatal exposure to testosterone.…”

Click-Evoked Otoacoustic Emissions in Children and Adolescents with Gender Identity Disorder

“…Men generally have lower 2D:4D ratios compared to women. This sex difference has been shown to be already present in young children (Manning et al, 1998;McIntyre et al, 2006) and even prenatally (Galis, Ten Broek, Van Dongen, & Wijnaendts, 2010;Malas et al, 2006). Furthermore, 2D:4D ratios in children at 2 years of age were significantly predicted by the ratio of testosterone to estradiol levels in amniotic fluid (Lutchmaya et al, 2004;McIntyre, 2006) and digit ratios were associated with circulating sex hormone levels in adults (Manning et al, 1998).…”

Environmental levels of oestrogenic and antiandrogenic compounds feminize digit ratios in male rats and their unexposed male progeny

“…Box plot displaying the 10th, 25th, 50th, 75th and 90th percentile values, the extreme 5th and 95th percentiles (circles) and the mean value (thick white line) for right forepaw 2D : 4D in (a) the F 1 males exposed to the compounds and (b) in their unexposed male offspring (F 2 ) carried by unexposed dams. Mann-Whitney non-parametric comparisons (#p , 0.001, ***p , 0.005, **p , 0.01, *p , 0.05) versus controls (abbreviations for both the F 1 Relative AGD did not differ significantly between the seven groups tested, in the male F 1 or F 2 (F ¼ 1.18, p ¼ 0.32 and F ¼ 0.82, p ¼ 0.55, respectively) despite mean values being lower in all exposure groups than in controls (data not shown). Overall, relative AGDs were not associated with digit ratios in the F 1 or F 2 (data not shown) in all studied animals or in the controls only (for the 2D : 4D digit ratio, see the electronic supplementary material, table S5).…”