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.
