Pupil Size and Intelligence: A Large-Scale Replication Study

A recent study by Tsukahara et al. (2016) found correlations between pupil size and measures of intelligence, with r values around .30. We attempted to replicate this association in a large dataset of US military personnel (n = 4,462). General intelligence, g, was extracted from 19 diverse tests. We first confirmed that right and left eye pupil size measures are strongly correlated (r = .97), suggesting high measurement reliability for this phenotype. However, unlike Tsukuhara et al., we could establish only small to nonexistent associations between cognitive ability and pupil size (r’s -.01 to .06, r with g specifically = .05). Regression analyses, controlling for multiple covariates, revealed that the association in this large representative sample was entirely attributable to confounding with race/ethnicity. Mean pupil size (mm) was 3.56, 3.35, and 3.23 for whites, Hispanics, and blacks, respectively. Relative to whites, this corresponds to effect sizes of 0.22 and 0.34 d. It is unclear why our results differ from those reported by Tsukahara et al. (2016), but the ethnic size rank order suggests an evolutionary explanation in terms of geo-bio-climatic selection for pupil size.