If we were to keep it simple I'd always imagine the breakdown to work like this.
Ferengi Obsessive compulsive, hyper anxious.
Klingons ADHD, Borderline, and Bipolar.
Betazoids Hyper empathetic, hyper social.
Cardassians, paranoid, narcissistic, anxious(ocd), and Psychopathic
Vulcans, autism.
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Romulans, narcissism, psychopathy.
The vulcan romunlan split however is the truly interesting question.
I've come up with a few theory of the pre logic vulcans.
1) Vulcans were just like the current Romulans, however over time historical revisionism portrayed natural Vulcanians as mentally unstable.
2) Vulcans and Romulans were split along physiological lines dividing two group by some form of spectrum(autistism?)
3) Romulans after leaving vulcan interbred with another species bringing stability to their people.
4) Romulans underwent chemical treaments to control their emotions in contrast to the vulcans that underwent some form of therapy(logic.)