[verylittlemeat]

Couldn't it be that a high level of education/intelligence is also a sign of high socioeconomic status which would imply better access to healthcare and more psych diagnoses?

This might be wrong but it seems like an obvious superficial criticism.

[cJ0th]

This would mean that you wouldn't find a difference in poor and highly educated Canadians. I don't have data on this but I doubt it.

[verylittlemeat]

You might be right but it looks like this specific article is USA specific (at least the paper it's referencing is).