On the Real Magnitude of Psychological Sex Differences

Abstract

A comprehensive evolutionary theory of sex differences will benefit from an accurate assessment of their magnitude across different psychological domains. This article shows that mainstream research has severely underestimated the magnitude of psychological sex differences; the reason lies in the common practice of measuring multidimensional differences one dimension at a time, without integrating them into a proper multivariate effect size (ES). Employing the Mahalanobis distance D (the multivariate generalization of Cohen's d) results in more accurate, and predictably larger, estimates of overall sex differences in multidimensional constructs. Two real-world examples are presented: (1) In a published dataset on Big Five personality traits, sex differences on individual scales averaged d = .27, a typical ES conventionally regarded as “small.” However, the overall difference was D = .84 (disattenuated D = .98), implying considerable statistical separation between male and female distributions. (2) In a recent meta-analytic summary of sex differences in aggression, the individual ESs averaged d = .34. However, the overall difference was estimated at D = .75 – .80 (disattenuated D = .89–1.01). In many psychological domains, sex differences may be substantially larger than previously acknowledged.