> 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.
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.
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.