The relationship between baseline pupil size and intelligence:

The relationship between baseline pupil size and intelligence
Jason S. Tsukahara , Tyler L. Harrison, Randall W. Engle (2016)

"at the bivariate level we found that WMC explained 6% of the variance in baseline pupil size and with each 1 SD increase in WMC there was a 0.30 mm increase in baseline pupil diameter, b = 0.30, r = 0.24, t(335) = 4.44, p < 0.05. Fluid intelligence explained 12% of the variance in baseline pupil size and each 1 SD increase in Gf was associated with a 0.45 mm increase in baseline pupil diameter, b = 0.45, r = 0.35, t(335) = 6.90, p < 0.05."