No study has investigated the association between intelligence quotient (IQ) and disability (i.e., difficulties in activities of daily living [ADL] or instrumental activities of daily living [IADL]) in the general population.

The goal of this nationally representative study was to analyse the potential IQ–disability association in England and identify influential factors in this association.

Cross-sectional data were analyzed from the 2007 Adult Psychiatric Morbidity Survey (n=6872). IQ was assessed by using the National Adult Reading Test, which consists of a list of 50 words and is scored by counting the number of errors in reading the words aloud. Disability was defined as difficulties in at least 1 of the 7 domains of ADL and IADL. Regression and mediation analyses were conducted to analyze the association between IQ and disability and identify potential factors involved in this relationship, estimating odds ratios (ORs) and 95% confidence intervals (CIs).

Among the 6872 participants, the mean (SD) age was 46.9 (18.9) years; 51.8% were women. The prevalence of disability increased from 27.7% with IQ 120–129 to 51.0% with IQ 70–79. After adjusting for sex, age and ethnicity, as compared with IQ 120–129, with IQ 110–119, 100–109, 90–99, 80–89, and 70–79, the probability of disability was increased (OR 1.22 [95% CI 1.01–1.48], 1.42 [1.16–1.72], 1.86 [1.54–2.25], 2.41 [1.92–3.03], and 4.71 [3.56–6.17], respectively). In addition, we found a positive association between a 1-SD decrease in IQ and disability (OR 1.53, 95% CI 1.43–1.63). Finally, income (mediated percentage 26.9%), social class (18.0%) and education (11.6%) strongly affected the IQ–disability association, and these socioeconomic factors collectively explained 37.1% of the association.

Low IQ was positively associated with disability in England, and socioeconomic status explained more than one-third of this relationship.