Fluctuating Asymmetry and Digit Ratio (2D:4D) in Reared-Apart Twins

Reared-apart twin designs offer a powerful means for investigating the relative contributions of genetic and environmental factors to individual differences in physical and behavioral traits. The current study analyses data from the Minnesota Study of Twins Reared Apart (MISTRA) for digit lengths, digit ratios (2D:4D), and fluctuating asymmetry (FA). The ratio of length between the second (index) and fourth (ring) fingers—2D:4D—is thought by some to be a proxy for variations in prenatal sex hormone levels, with low ratios indicating high androgen exposure. FA represents deviations from bilateral symmetry for paired traits. It is typically examined as a proxy for developmental instability (DI), with larger deviations from symmetry indicating greater DI. The current study investigated the contributions of genetic and environmental factors to individual differences in 2D:4D and FA using a sample of reared-apart twins. In addition, we examined 2D:4D in relation to the twin testosterone transfer (TTT) hypothesis. It has been reasoned that androgens can transfer between twins during gestation, and twins with male co-twins are exposed to higher levels of androgens than twins with female co-twins. Digit lengths and ratios all showed evidence of heritability, but no such effects were observed for FA. A group difference in directional asymmetry in 2D:4D (D[R−L]), consistent in direction with the TTT hypothesis, emerged in analyses without multiple-comparison correction, though the result warrants cautious interpretation.