No association between fluctuating asymmetry in highly stabilized traits and second to fourth digit ratio (2D:4D) in human fetuses

Recent studies have suggested that the ratio of the length of the second and fourth digit (2D:4D) may be associated with developmental instability (DI) as measured by the left-right asymmetry of the same digits. Because the 2D:4D ratio is amongst others, determined prenatally as a result of exposure to sex hormones, such an association could indicate that the same prenatal developmental processes determine levels of DI. In this study we criticize these earlier findings and show by simulations that they are confounded by the fact that (non-) linear combinations of the digit lengths are used as both dependent (average asymmetry in digits 2 and 4) and independent (ratio of the lengths of digits 2 and 4) variable. We therefore studied associations between 2D:4D ratios and asymmetry not only in digits but also in several other skeletal elements in deceased human fetuses. In contrast to the earlier studies, we did not find an association between 2D:4D ratios and asymmetry in digits 2 and 4. We argue that this may be due to the low levels of DI in this study, which limits the confounding effects of DI. Also, no associations were detected with the asymmetry of all other trait either. Thus, there appears to be very little evidence of any link between DI and 2D:4D in this population for limb measurements. We conclude that highly stabilized and functionally important traits such as human limbs may in general show limited increases in asymmetry with prenatal stress.