Interest in sex differences in the brain has increased over the past 25 years. (a) The percentage of journal articles that mention sex or gender differences and brain (Web of Science) has approximately doubled. (b) The number of news articles about sex differences in the brain (Proquest) has increased approximately fivefold. Arrows indicate years during which a particular study or event contributed substantially to an increase in news stories. For the methods and raw data, see the electronic supplementary material, table S1. (Online version in colour.)
Some of the most-often cited sex differences in humans are characterized by extensive overlap (Δ). In each panel, d represents Cohen's d and Δ represents the percentage overlap. The graphs show the frequency distributions, or the number of individuals of each sex (y-axis) with any given measure or score (x-axis). (a) Distributions for human height  are shown for comparison. The effect size is large, but men and women overlap in height by 32%. (b) Total brain volume is larger in men than in women . (c) The volume of the hippocampus, corrected for total brain volume, has been reported to be larger in women than men [21,57]. (d) Intrahemispheric and (e) interhemispheric connectivity, measured via diffusion tensor imaging, varies slightly according to sex . The effect size plotted in (d) represents the average of 18 comparisons for which the values were higher in men. (f) Serotonin synthesis has been reported to be higher in women  and, (g) in a later study, higher in men . (h) Pain thresholds are generally reported to be higher in men (data shown from ; see  for review). (i) The dosage of morphine required for analgesia may vary according to sex but the degree of overlap is high (data shown from ; see  for review). (j) The drug zolpidem, a popular sleep aid, is cleared by women more slowly than by men . (k) The morning after taking zolpidem, women are more impaired than men during a driving task . For the values used to make the plots, see the electronic supplementary material, table S2. Distributions were assumed normal in each case. All graphs were made using the interactive tool at www.sexdifference.org. Readers are encouraged to use this tool to assess overlap for sex differences that they find in the literature or in their own research.
Analysis of a hypothetical dataset for ‘Dimorphinil’, a fictional drug. Normally distributed data were artificially generated for 40 ‘individuals’ of each sex. (a) The values both above and below the population mean (grey dotted line) consist of 18 members of one sex and 22 of the other. In other words, the numbers of above- and below-average individuals are about the same for each sex. (b) A Student's t-test shows a significant sex difference (p < 0.01). (c) The effect is medium-sized (d = 0.60) but there is a high degree of overlap (Δ = 76%). For the methods and raw data, see the electronic supplementary material, table S3.