Obesity and IQ: What the Link Is and Which Way It Runs

People with obesity score a little lower on IQ tests on average, and the best long-term studies suggest the link mostly runs from childhood IQ to later weight rather than from weight to IQ. In the largest British cohort, each standard deviation (15 points) lower in IQ at age 11 raised the odds of obesity at 42 by about 26% in men and 38% in women, and most of that link disappeared once adult education was taken into account.

That is a modest association, and it says nothing about any individual. Plenty of people with high IQs have obesity, and plenty with lower scores do not. This page covers what the cohort studies and meta-analyses show, the evidence on causal direction, what happens to thinking after weight-loss surgery, and the confounds that make this topic easy to misread.

The obesity IQ association has been measured in several large samples, and the results are consistent in direction and small in size.

• British 1958 birth cohort: Tarani Chandola, Ian Deary, David Blane and David Batty followed the National Child Development Study, a cohort of 17,414 births in Great Britain. Childhood IQ was measured at 11, and body mass index (BMI) at 16, 23, 33 and 42. Per standard deviation lower IQ, the odds ratio for obesity at 42 was 1.38 in women and 1.26 in men. After adjustment for adult characteristics, especially education, those fell to 1.11 and 1.10 and were no longer statistically significant.

• Meta-analysis: A team led by Z. B. Yu reviewed 26 studies in 2010. Children with obesity scored 2.8 full-scale IQ points lower than controls on average. In adults, and in studies linking childhood IQ to adult BMI, the association became nonsignificant after adjustment for educational attainment.

• A dissenting reading: Satoshi Kanazawa reanalyzed the same British cohort with IQ measured at 7, 11 and 16 and BMI at 51. He reported that childhood intelligence had a direct effect on adult BMI and obesity even after controlling for education, earnings, parents' BMI and childhood social class. The difference from Chandola's result depends on which adult factors are treated as confounders and which as part of the pathway.

To translate the British numbers: a child scoring 85 rather than 100 at age 11 had roughly 26% to 38% higher odds of obesity in middle age, before adjustment. Odds ratios of that size describe a modest shift in risk across a large population, and most children at either score never develop obesity.

There are three possibilities. Lower IQ could lead to weight gain. Obesity could harm the brain and lower IQ. Or a third set of factors, such as family income and education, could drive both. Cross-sectional studies, which measure IQ and weight at the same time, cannot tell these apart.

The most informative design follows the same people from early childhood. Daniel Belsky, Avshalom Caspi, Terrie Moffitt and colleagues did this with the Dunedin study, which followed 1,037 children born in New Zealand in 1972 to 1973 through their fourth decade of life with 95% retention. As expected, cohort members who became obese had lower adult IQ scores. But they showed no extra decline in IQ over time. Their scores had been lower since childhood, and the gap was already visible at age 3, long before most of them developed obesity. That held even for those with metabolic syndrome or raised markers of inflammation.

Mendelian randomization offers another angle. It uses genetic variants linked to a trait as a kind of natural experiment, since genes are assigned at conception and are less tied to later lifestyle. Neil Davies, George Davey Smith and colleagues applied it to UK Biobank data in 2019. In single-exposure models, one standard deviation of higher genetically predicted intelligence was associated with a BMI about 0.91 points lower. When intelligence and education were modeled together, the authors found "some evidence of a direct effect of education, but not intelligence, on BMI." That matches the cohort studies: education looks like the main route between IQ and weight.

Kanazawa reviewed the longitudinal studies from Sweden, New Zealand and Britain in 2014 and argued that they point from lower intelligence to obesity rather than the reverse. On this narrow point his reading agrees with the Dunedin findings, even though his reanalysis of the British cohort differs from Chandola's on the role of education.

If obesity itself impaired thinking, large weight loss might improve it. Bariatric surgery studies have tested this, and they do find gains, though in specific skills and in small samples.

• Twelve weeks: John Gunstad and colleagues tested 109 surgery patients and 41 comparison patients with obesity who did not have surgery. Before surgery, between 4.6% and 23.9% of surgery patients scored in the impaired range, depending on the test. At 12 weeks, the surgery group's memory had improved while the comparison group's memory had declined.

• Two years: A follow-up by Michael Alosco, Mary Beth Spitznagel, Gunstad and colleagues with 63 surgery patients and 23 comparison patients found the memory improvements still present at 24 months. Lower BMI at 24 months showed only a trend toward better memory (p = 0.075).

These studies measure memory and attention on computerized batteries, not full-scale IQ, and patients were not randomized to surgery. Surgery also changes blood sugar control and blood pressure, so any cognitive gain may reflect better metabolic health more than weight itself. In a follow-up of 85 patients by Rachel Galioto and colleagues, falling insulin resistance predicted better working memory and attention at 12 months, while change in BMI did not predict cognitive scores once baseline performance was controlled. The evidence supports the idea that severe obesity with its medical complications can dampen some cognitive functions, which is a separate question from whether body weight lowers measured intelligence across the population.

Several factors sit underneath the obesity IQ correlation and explain much of it.

• Education: In the British cohort, adjusting for adult education did most of the work in removing the association. People with more years of schooling, on average, have jobs, incomes and health knowledge that affect weight.

• Family background: Childhood social class and parental income predict both children's test scores and later weight. The general relationship between IQ and socioeconomic status means many studies are partly measuring family circumstances.

• Health behaviors: Chandola's team found that adult diet partly mediated the link, and Kanazawa reported that higher-IQ children went on to eat more healthily and exercise more often as adults. Whether exercise improves test scores in turn is a different question, covered in our page on whether exercise can affect your IQ score.

• Medical complications: In Gunstad's surgery sample, patients without high blood pressure had better short-delay recall at 12 weeks than those with it, and insulin resistance tracked cognitive change in the later follow-up. Conditions that travel with severe obesity can therefore make weight itself look like the cause.

A useful comparison is height, another physical trait with a small positive correlation with IQ, discussed on our page about IQ and height. In both cases, a real population-level correlation is far too weak to judge any one person's intelligence from their body. Anyone concerned about weight or about changes in memory and concentration is best served by a primary care physician, who can refer to an endocrinologist, dietitian or neuropsychologist as needed.

The best long-term evidence says not much, if at all. In the Dunedin study, people who became obese had lower scores since early childhood and showed no extra decline as adults.

On average, slightly. Lower childhood IQ predicts higher odds of adult obesity, but most of that link runs through education and adult lifestyle.

A 2010 meta-analysis found that children with obesity scored about 2.8 IQ points lower than their peers. In adults the gap mostly disappears once education is controlled.

Studies find improvements in memory a few months and two years after bariatric surgery. They did not measure full-scale IQ, and better metabolic health may explain the gains.

No. The correlation is weak, and the overlap between groups is enormous, so body size tells you almost nothing about an individual's ability.

The obesity IQ link is real but small, and the strongest studies suggest it runs mainly from childhood ability and education toward adult weight rather than from weight toward lower intelligence. Severe obesity and its medical complications may affect memory and attention, and those functions can improve with better metabolic health. Neither finding supports judging anyone's intelligence by their size. If you want an accurate picture of your own reasoning skills, you can take the RIOT IQ test and see how your results compare with modern norms.

1. Chandola, T., Deary, I. J., Blane, D., & Batty, G. D. (2006). Childhood IQ in relation to obesity and weight gain in adult life: The National Child Development (1958) Study. International Journal of Obesity, 30(9), 1422-1432. [doi.org/10.1038/sj.ijo.0803279](. doi.org

2. Yu, Z. B., Han, S. P., Cao, X. G., & Guo, X. R. (2010). Intelligence in relation to obesity: A systematic review and meta-analysis. Obesity Reviews, 11(9), 656-670. [doi.org/10.1111/j.1467-789X.2009.00656.x](. doi.org

3. Kanazawa, S. (2013). Childhood intelligence and adult obesity. Obesity, 21(3), 434-440. [doi.org/10.1002/oby.20018](. doi.org

4. Belsky, D. W., Caspi, A., Goldman-Mellor, S., Meier, M. H., Ramrakha, S., Poulton, R., & Moffitt, T. E. (2013). Is obesity associated with a decline in intelligence quotient during the first half of the life course? American Journal of Epidemiology, 178(9), 1461-1468. [doi.org/10.1093/aje/kwt135](. doi.org

5. Davies, N. M., Hill, W. D., Anderson, E. L., Sanderson, E., Deary, I. J., & Davey Smith, G. (2019). Multivariable two-sample Mendelian randomization estimates of the effects of intelligence and education on health. eLife, 8, e43990. [doi.org/10.7554/eLife.43990](. doi.org

6. Kanazawa, S. (2014). Intelligence and obesity: Which way does the causal direction go? Current Opinion in Endocrinology, Diabetes and Obesity, 21(5), 339-344. [doi.org/10.1097/MED.0000000000000091](. doi.org

7. Gunstad, J., Strain, G., Devlin, M. J., Wing, R., Cohen, R. A., Paul, R. H., Crosby, R. D., & Mitchell, J. E. (2011). Improved memory function 12 weeks after bariatric surgery. Surgery for Obesity and Related Diseases, 7(4), 465-472. [doi.org/10.1016/j.soard.2010.09.015](. doi.org

8. Alosco, M. L., Spitznagel, M. B., Strain, G., Devlin, M., Cohen, R., Paul, R., Crosby, R. D., Mitchell, J. E., & Gunstad, J. (2014). Improved memory function two years after bariatric surgery. Obesity, 22(1), 32-38. [doi.org/10.1002/oby.20494](. doi.org

9. Galioto, R., Alosco, M. L., Spitznagel, M. B., Strain, G., Devlin, M., Cohen, R., Crosby, R. D., Mitchell, J. E., & Gunstad, J. (2015). Glucose regulation and cognitive function after bariatric surgery. Journal of Clinical and Experimental Neuropsychology, 37(4), 402-413. [doi.org/10.1080/13803395.2015.1023264](. doi.org

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