[tgv]

> How did they determine cognitive ability here?

A few tests, among which specific memory tests. However, the memory tests didn't correlate, they claim only "fluid intelligence" correlates (even though they've also claimed the two are correlated). Fluid intelligence was measured "with the Raven’s Advanced Progressive Matrices, letter sets, and number series."

The problem, for me, is that there is nothing to relate intelligence (fluid or otherwise) and pupil size. Without something plausible linking the two, this is just meaningless.

And of course they rely on NHST statistics and a whole raft of modeling techniques, which is enough to call the whole article in doubt.

I dare say they used very visual tasks, which may be harder for people with small pupils under less than optimal lighting conditions.

[zrm]

> The problem, for me, is that there is nothing to relate intelligence (fluid or otherwise) and pupil size. Without something plausible linking the two, this is just meaningless.

Catecholamines:

https://en.wikipedia.org/wiki/Catecholamine

They're both neurotransmitters and vasoconstrictors, but the latter effect is reversed in the pupils, so they dilate your pupils. If you're low on catecholamines then your working memory is impaired and your pupils will be smaller.

This is why amphetamine dilates your pupils and is prescribed to treat ADHD.