Meta-analysis reveals a lack of sexual dimorphism in human amygdala volume.
Level 1 - systematic review of randomized trials
Systematic review and meta-analysis of observational studies (level graded by design analogy, not clinical CEBM)
PubMed 27956206 · doi:10.1016/j.neuroimage.2016.12.021
What was done
The authors conducted a systematic review and random-effects meta-analysis of MRI studies comparing amygdala volume in age-matched healthy male and female cohorts. They identified 46 studies comprising 58 matched samples. Effect sizes were converted to Hedges g for uncorrected amygdala volume as well as amygdala volume normalized to total brain volume (TBV) or intracranial volume (ICV). Meta-regression was used to evaluate study year and average participant age.
What was found
Uncorrected amygdala volume was roughly 10% larger in males: right amygdala g = 0.581 (k = 28, n = 2022), left amygdala g = 0.666 (k = 28, n = 2006), and bilateral volume g = 0.876 (k = 16, n = 1585), all p < 0.001. However, males also had larger ICV (g = 1.186, 11.9% larger, k = 11, p < 0.001) and TBV (g = 1.278, 11.5% larger, k = 15, p < 0.001). Among studies reporting normalized volumes, sex differences were not statistically significant: right amygdala g = 0.171 (p = 0.206, k = 13, n = 1560; <0.1% larger in males), left amygdala g = 0.233 (p = 0.092, k = 12, n = 1512; 2.5% larger in males), and bilateral volume g = 0.257 (p = 0.131, k = 5, n = 1629).
Why it matters
This review shows that apparent sex differences in raw amygdala volume are driven by overall brain size differences rather than selective sexual dimorphism. It challenges a widespread assumption in affective and psychiatric neuroscience.
Limits
The abstract only covers gross macrostructural volumetric MRI in healthy populations, without examining amygdala subnuclei, structural connectivity, or functional activation. Only a smaller subset of studies reported brain-size-corrected volumes (k = 5 to 13), limiting sample size for the normalized analyses.
Cited by
- context On average, men tend to have a larger amygdala than women.