Gender is the most salient human characteristic. Males and females are far more similar than they are different. Nevertheless, humans are drawn to differences and tend to exaggerate their relevance, giving rise to stereotypes.

Research on gender differences relies on meta-analysis. Researchers use the means and standard deviations for males and females reported in many studies to calculate an effect size. Effect size is measured in comparison to “no effect,” as in “gender has no effect on aggression.” Large-scale studies report gender differences in terms of variance, a statistical measure of how much of one variable (aggression) can be explained by another variable (gender).

When researchers look for gender differences, they examine the distribution of the scores for males and females. Most often, the distributions overlap. The area in which they do not overlap is the area of statistical difference. A small difference may be statistically significant, yet be of no practical importance in terms of explaining the difference or justifying interventions to eliminate it.

In 1974, Maccoby and Jacklin reported four areas of male-female differences: verbal ability, quantitative ability, spatial ability, and aggression. Since then, meta-analyses have challenged or explained those differences.