Comparative Study of Digit Ratios (2D:4D and Other) and Novel Measures of Relative Finger Length: Testing Magnitude and Consistency of Sex Differences across Samples

Digit ratios, such as the frequently studied second-to-fourth digit ratio (2D:4D), are already sexually differentiated in utero, postnatally fairly stable, and thus commonly held as possible markers for prenatal androgen levels. Based on six independent samples, totaling 381 male and 420 female subjects from different age bands, this study investigated the magnitude and cross-sample consistency of sex differences in all six possible digit ratios (2D:3D, 2D:4D, 2D:5D, 3D:4D, 3D:5D, 4D:5D) and in four novel relative finger-length measures (the ratio of a given finger to the sum of all four fingers) that were recently suggested by Loehlin, Medland, and Martin as possibly superior to digit ratios for discriminating between the sexes. However, based on the magnitude of sex effects and observed measurement repeatabilities, these novel measures were not superior to digit ratios. Across samples, among digit ratios the traditionally used 2D:4D never yielded the largest sex effect, 2D:5D was the most and 2D:3D the least sexually differentiated. Sex differences were generally larger for digit ratios calculated from nonadjacent than from adjacent fingers. Compared with females, males' relative finger length was clearly lower for 2D, somewhat lower for 3D, somewhat higher for 4D, and clearly higher for 5D. This gradient suggests mediolaterally arranged developmental fields that act sexually differentiated on finger-length expression. Implications of these findings for further digit ratio research are discussed.