The human hippocampus is not sexually-dimorphic: Meta-analysis of structural MRI volumes.
Level 1 - systematic review of randomized trials
Systematic review and meta-analysis of observational MRI datasets (graded 1 by design analogy, not clinical CEBM)
PubMed 26334947 · doi:10.1016/j.neuroimage.2015.08.050
What was done
The authors conducted a systematic review and random-effects meta-analysis of structural MRI studies in healthy participants of all ages to determine whether hippocampal volume (HCV) differs between males and females. They analyzed 68 matched samples (4,418 total participants) reporting uncorrected HCVs and 36 samples (2,183 participants) reporting HCVs adjusted for total brain volume (TBV) or intracranial volume (ICV). Meta-regression was performed to assess the effect of age.
What was found
Uncorrected HCV was significantly larger in males across all measures: left hippocampus (Hedges' g = 0.545), right hippocampus (Hedges' g = 0.526), and bilateral hippocampus (Hedges' g = 0.557). Overall head size was also substantially larger in males (TBV: g = 1.085; ICV: g = 1.272). When HCV was corrected for TBV or ICV, the sex difference was no longer significant, with effect sizes ranging from Hedges' g = +0.011 to -0.206. Meta-regression showed no moderating effect of age.
Why it matters
These findings challenge the widespread belief that women have disproportionately larger hippocampi than men, demonstrating that observed absolute volume differences are fully accounted for by overall differences in total brain and intracranial volume.
Limits
The analysis relies on aggregate data from published literature rather than individual participant data. Only about half of the included sample cohorts reported measures corrected for TBV or ICV, and specific MRI acquisition and segmentation protocols varied across the underlying studies.
Cited by
- contradicts Women tend to have a larger hippocampus up until a certain age, while men tend to have a larger cerebellum.