BackgroundChildhood obesity has been shown to affect human capital—ie, an individual's ability to contribute economically to society—and social outcomes later in life. Yet, evidence of the causal nature of this link is scarce and the pathways are not well understood. We aimed to investigate the effect of childhood obesity and overweight on cognitive performance in adolescence and educational attainment in early childhood. MethodsThis study was a secondary analysis of two longitudinal UK cohort studies: the Avon Longitudinal Study of Parents and Children (ALSPAC), which included children born in 1991–92 in the former county of Avon, UK, and the National Child Development Study (NCDS), which included children born in 1958 in England, Scotland, and Wales. All children from singleton births, of European ancestry, and without developmental delays were included in our analyses. We obtained data on childhood body-mass index (BMI; Z score and BMI category at age 11 years), cognitive performance (maths scores at age 16 years), and educational attainment (tertiary qualification at age 23 years). We used ordinary least squares and logistic regression, value-added sex-stratified models, and mendelian randomisation models to explore the effect of childhood overweight and obesity on educational outcomes. The Managing Ethico-social, Technical and Administrative issues in Data Access ethics committee approved this project. FindingsIn ALSPAC (n=5373), female individuals who had been overweight at age 11 years scored 1·00 point (95% CI −1·58 to −0·36; p=0·028) less on their maths GCSE exam than their healthy-weight peers, and girls who had been obese at age 11 years scored 1·66 points (–3·15 to −0·18; p=0·0021) less. Female individuals who had been obese at age 11 years were less likely to graduate from university than their healthy-weight peers (odds ratio [OR] 0·75, 95% CI 0·59 to 0·97; p=0·030); the effect on graduation for those who had been overweight was less conclusive (OR 0·85, 0·71 to 1·01; p=0·060). Male individuals who had been overweight scored 1·21 points (95% CI −2·84 to −0·81; p=0·0011) less and those who had been obese 2·24 points (95% CI −3·46 to −1·02; p<0·0001) less on their GCSE maths exam than their non-obese peers, but there was no association between male childhood weight and university graduation (overweight: OR 1·07, 0·95 to 1·21; p=0·26; obesity: OR 0·89, 0·66 to 1·21; p=0·47). No associations were found in the NCDS cohort (n=5346). InterpretationOverweight and obesity in the ALSPAC cohort were linked with lowered educational outcomes, especially for female individuals. Our findings are robust to various causal methods and might help to inform interventions to address this issue. Cross-cohort comparisons suggest that there might be a generational effect of overweight and obesity on educational outcomes, since the younger cohort was susceptible to some negative consequences of childhood obesity but the older cohort was not; this needs to be explored in further research. FundingHealth Foundation's Social and Economic Value of Health Programme (award 809008).