Sex differences in directional asymmetry of digit length and its effects on sex differences in digit ratio (2D:4D).

Patterns of directional asymmetry in the length of index and ring finger (2D and 4D) may be sexually differentiated. Martin, Puts, and Breedlove (2008) found leftward bias, i.e., both a longer 2D and 4D in the left than in the right hand, more frequently in men, whereas rightward bias occurred more frequently in women. Further, rightward vs leftward bias seemed a substantial source of variation in digit ratio (2D:4D), a putative marker for prenatal androgen levels. Given the generally small effects, unreplicable results, and inconsistent findings of 2D:4D research, these findings potentially are important as the type of directional digit asymmetry could moderate or obfuscate real associations between 2D:4D and target traits. Based on six samples totaling about 3,000 individuals, the present study replicated the first finding of Martin, Puts, and Breedlove (2008): leftward bias was more frequent among men and rightward bias more frequent among women. This sex effect was small. However, the present study did not replicate the second finding of Martin, et al. (2008): relative to participants' sex the contributions of leftward vs rightward asymmetry to individual variation in 2D:4D were smaller by orders of magnitude and not significant. Implications of these findings for 2D:4D research are discussed.