Multilevel multiverse meta-analysis indicates lower IQ as a risk factor for physical and mental illness.

Is lower intelligence in early life an overlooked risk factor for later physical and mental illness? Intelligence shapes decision-making, career paths, and other health-relevant factors. However, our understanding of its association with health remains limited because there is no quantitative synthesis of the literature. Here, we conducted a comprehensive systematic review and meta-analysis of associations between intelligence test scores and mental and physical health. We included studies reporting standardized intelligence test scores obtained in childhood, adolescence, or early adulthood (<21 years of age) and their association with later-life health outcomes. We excluded studies limited to clinical populations without healthy controls. Our three-level multiverse analyses of 49 studies (N > 2,900,000) showed a 15-point IQ disadvantage in early life was associated with a 22 percent higher risk of later mental and physical illness (logHR = 0.20, 95% CI [0.13, 0.26]). Lower IQ predicted disease risk across various conditions, including schizophrenia, depression, dementia, and diabetes. Notably, the association between IQ and future health diminished with improved healthcare quality and when education was statistically held constant. Nevertheless, a meaningful effect of intelligence remained after adjusting for these variables. Multiple methods for detecting dissemination bias indicated that risk of bias was low. While our summary effect estimates are precise, all included data were collected in highly developed nations. Further, samples were predominantly male, potentially limiting generalizability. We show that lower IQ scores in early life are linked to a higher risk of later physical and mental illness. Improving education and healthcare quality appears as potential measures to address the issue. This research received no specific funding.