The S-index: Summarizing patterns of sex differences at the distribution extremes

Sex differences researchers are becoming increasingly interested in how differences in averages and variability jointly influence male and female representation at the tails of the distribution. This paper introduces the S -index, a novel index that provides a simple and robust summary of the shape of sex differences at the distribution extremes. The use of S is illustrated with a selection of real-world datasets of personality and cognitive ability, and a R function is provided to calculate S and draw intuitive proportion plots of sex differences across the distribution. The S-index is not limited to the study of sex differences; it can be applied to other domains as long as the groups to be compared are about equally represented in the population and the variables of interest are approximately bell-shaped.