Ethically, my fear is how politicians and the general public would interpret the finding of ‘smaller brains in ADHD’ This has the (sickly) smell of Eugenics and Social Darwinism, with the potential to reduce reasonable adjustments to help these children learn in their preferred way. Also, when selecting for apprentice schemes and jobs, could a diagnosis of ADHD based on MRI scans limit success in employment, limit driving licences being given? What happened in Germany in the late 1920’s (A life not worth of life) is not that long ago.
All of this is beside the point that Scott is trying to make though but certainly interesting to think about. Lots of professions have intelligence floors then select for other traits over the floor level. Some professions just need as much raw brainpower as humans can provide and so select only for that. Other professions have low intelligence floors but high floors for other traits such that high intelligence people do them better all else being equal (they give IQ tests to every NFL draft prospect, for example) but the people in that profession aren’t selected specifically for intelligence.