Sex differences
The 3D model on this site is an average of 40 people and is neither female nor male. Real brains differ, and some differences between women and men are large on average. Every one of them overlaps: the curves below show how much. The numbers come from one large study, Ritchie et al. 2018: brain scans of 2,750 women and 2,466 men aged 44–77 (mean 61.7) in the UK Biobank.
How to read the numbers: d is the difference between the averages in standard deviations, women minus men (negative: smaller in women). Overlap is the share two equally spread bell curves that far apart would have in common, computed here from d. The study computed d after adjusting for age and ethnicity, while the curves show raw volumes, so a curve pair can look a little closer than its d (grey matter most).
Overall size
Men’s brains are larger on average. The study found that regional differences were not fully explained by overall size or by height. These are the largest differences it reports, and the curves still overlap by about half.
Total brain volume
d = -1.41 · overlap about 48% · variance ratio 0.82 (below 1: men vary more).
Grey matter volume
d = -1.28 · overlap about 52% · variance ratio 0.81 (below 1: men vary more).
White matter volume
d = -1.49 · overlap about 46% · variance ratio 0.82 (below 1: men vary more).
Deep structures
Every deep structure measured is larger in men on average. Part of that follows from overall size: after adjusting for total brain volume, the differences in the hippocampus, caudate and thalamus were no longer statistically significant. Those in the amygdala, putamen, pallidum and nucleus accumbens remained (marked ✓).
| Structure | Women, cm³ | Men, cm³ | d | Overlap | After size adjustment |
|---|---|---|---|---|---|
| Left hippocampus | 3.73 ± 0.42 | 3.94 ± 0.46 | -0.55 | 78% | no significant difference |
| Right hippocampus | 3.82 ± 0.42 | 4.04 ± 0.48 | -0.54 | 79% | no significant difference |
| Left amygdala | 1.21 ± 0.22 | 1.35 ± 0.25 | -0.59 | 77% | ✓ still differs |
| Right amygdala | 1.18 ± 0.24 | 1.31 ± 0.27 | -0.51 | 80% | ✓ still differs |
| Left thalamus | 7.54 ± 0.64 | 8.11 ± 0.72 | -0.98 | 62% | no significant difference |
| Right thalamus | 7.34 ± 0.62 | 7.92 ± 0.69 | -1.03 | 61% | no significant difference |
| Left caudate | 3.28 ± 0.38 | 3.54 ± 0.41 | -0.66 | 74% | no significant difference |
| Right caudate | 3.45 ± 0.4 | 3.72 ± 0.44 | -0.65 | 75% | no significant difference |
| Left putamen | 4.61 ± 0.5 | 5.07 ± 0.56 | -1.01 | 61% | ✓ still differs |
| Right putamen | 4.64 ± 0.49 | 5.13 ± 0.55 | -1.08 | 59% | ✓ still differs |
| Left pallidum | 1.69 ± 0.21 | 1.85 ± 0.22 | -0.77 | 70% | ✓ still differs |
| Right pallidum | 1.74 ± 0.2 | 1.89 ± 0.22 | -0.78 | 70% | ✓ still differs |
| Left nucleus accumbens | 0.49 ± 0.11 | 0.53 ± 0.12 | -0.39 | 85% | ✓ still differs |
| Right nucleus accumbens | 0.4 ± 0.1 | 0.42 ± 0.11 | -0.31 | 88% | ✓ still differs |
Mean ± SD, raw volumes. d and the adjustment result as reported by Ritchie et al. 2018, Table 1.
The cortex: larger in men, thicker in women
Across the 68 cortical regions of the Desikan-Killiany atlas, men had larger volume in every region (d from -0.24 in the right temporal pole to -1.03 in the right insula; mean -0.67) and larger surface area (mean d -0.83, up to -1.2 in the left superior frontal region).
Thickness ran the other way. Women had thicker cortex in 47 regions (mean d 0.22, from 0.07 to 0.45 in the left inferior parietal region); 20 regions showed no difference, and men were thicker only in the right insula (d = 0.14).
“Unlike volume and surface area, females had thicker cortex across almost the entire brain.”
See the regions on the model in the atlas, and their template areas on the Data page.
What this does and does not show
- Averages and spread in one population: UK adults aged 44–77. Younger people and other populations may differ.
- Structure only: sizes and thicknesses measured on scans. This page says nothing about abilities or behaviour.
- The curves are drawn as normal distributions from the reported mean and SD, over ±3.5 SD; the study reports no ranges.
- For every measure here, many women have higher values than many men, and the reverse.
“Males had higher raw volumes, raw surface areas, and white matter fractional anisotropy; females had higher raw cortical thickness and higher white matter tract complexity. There was considerable distributional overlap between the sexes.”
Reference
Ritchie SJ, Cox SR, Shen X, Lombardo MV, Reus LM, Alloza C, Harris MA, Alderson HL, Hunter S, Neilson E, Liewald DCM, Auyeung B, Whalley HC, Lawrie SM, Gale CR, Bastin ME, McIntosh AM, Deary IJ. Sex Differences in the Adult Human Brain: Evidence from 5216 UK Biobank Participants. Cerebral Cortex 2018;28(8):2959-2975. doi:10.1093/cercor/bhy109 Open access, CC BY 4.0.