Country-level gender inequality is associated with structural differences in the brains of women and men

**Background:** Gender inequality varies across countries and is associated with worse mental health and lower educational attainment in women. Prior research has linked brain structure to social and environmental factors, such as early stimulation, adverse childhood experiences, and poverty. The authors hypothesized that women living in societies with high gender inequality experience greater adversity, which could negatively impact brain development, leading to structural differences between sexes that are more pronounced in unequal countries.

**Methods:** The study included 139 samples from 29 countries, totaling 7,876 MRI scans from healthy adults aged 18–40 years (4,078 women, 3,798 men). Nearly 35.26% of participants lived in low- and middle-income countries. The median mean age across samples was 24.19 years (range 18.83–31.69). Brain MRI data were analyzed using FreeSurfer to measure cortical thickness, surface area, and hippocampal volume. Country-level gender inequality was quantified by combining the Gender Gap Index and the Gender Inequality Index into a composite Z-score. A random-effects meta-analysis with meta-regression was used to examine associations between gender inequality and sex differences in brain structure, controlling for age and, in surface area and hippocampal analyses, total intracranial volume. Regional analyses corrected for multiple comparisons using false discovery rate (FDR).

**Key Results:** Hemisphere-wide analyses revealed a significant association between gender inequality and sex differences in right hemisphere cortical thickness (beta 0.012, 95% CI 0.0033–0.020, P = 0.006, R² = 4.34%). In countries with greater gender equality, there were practically no sex differences in right hemisphere cortical thickness; in countries with greater inequality, men had thicker cortices than women. The association remained significant after controlling for per capita GDP (beta 0.015, 95% CI 0.004–0.026, P = 0.0065). No significant association was found for the left hemisphere (beta 0.008, 95% CI −0.001–0.0155, P = 0.07). In women-only analyses, right hemisphere cortical thickness tended to decrease with higher inequality, but this was not significant (beta −0.022, 95% CI −0.047–0.0036, P = 0.093). No association was found in men (beta −0.0075, 95% CI −0.036–0.021, P = 0.6). Regional analyses identified three brain regions where sex differences in cortical thickness significantly correlated with gender inequality after FDR correction: right caudal anterior cingulate gyrus (beta 0.036, 95% CI 0.014–0.058, P_FDR = 0.040), right medial orbitofrontal gyrus (beta 0.028, 95% CI 0.014–0.043, P_FDR = 0.012), and left lateral occipital cortex (beta 0.017, 95% CI 0.006–0.028, P_FDR = 0.0495). In all three regions, women had thicker or similar cortices in more equal countries, but thinner cortices in more unequal countries. The association for the right caudal anterior cingulate remained significant after controlling for GDP (beta 0.055, 95% CI 0.027–0.084, P_FDR = 0.0098), but not for the orbitofrontal (P_FDR = 0.064) or occipital (P_FDR = 0.13) regions. No significant associations were found for hemispheric or regional surface area, hippocampal volumes, or total intracranial volume. Results were robust to sensitivity analyses excluding large samples (China and the US), using individual inequality indices, restricting to 3T scanners, excluding small studies, and using only visually quality-checked data.

**Clinical Implications:** This study provides evidence that country-level gender inequality is associated with structural brain differences between women and men, specifically in cortical thickness of the right hemisphere and regions involved in emotional control and stress processing. The findings suggest that macrosocial factors like gender inequality may contribute to sex differences in brain structure, potentially through mechanisms involving chronic stress, reduced access to enriched environments, or disparities in education and healthcare. These results highlight the importance of considering the social environment in neuroscience research and may inform policies aimed at reducing gender inequality to improve mental health outcomes. The study is limited by its cross-sectional design, reliance on non-random samples, and inability to determine causality or the specific mechanisms underlying the associations. Future longitudinal studies are needed to examine the timing and mediators of these effects.