Extroversion and Depression
In my life I've never experienced depression and I think this is because I'm very introverted. I think the two are absolutely linked.
The brains of introverted and extroverted people behave differently. An extrovert requires more stimulation than an introvert. An introvert playing a computer game can experience as much stimulation as an extrovert white water riding or parachuting. An introvert with three people in a quiet room can feel as stimulated as an extrovert at a party with forty people with loud music. In that situation an introvert would be completely overwhelmed and must essentially shut off. The person at a party who stands in a corner gazing at a pot plant isn't anti-social (why would an anti-social person go to a party?), but is over stimulated and seeking quietness to reset their neurochemical imbalance.
Psychological studies have shown that general happiness is linked to social activity, and social activity creates stimulation, the pleasant rush of pleasant brain chemicals that equate to happiness. Antidepressants artificially create those chemicals.
I postulate that it is a lack of sufficient stimulation that creates depression; thus it will affect extroverts more than introverts. Extroverted people lacking sufficient social contact will become depressed, and those people will benefit from (and enjoy) sports, games, parties and pleasant team events. Such activities would create natural antidepressants and present a much better way of dealing with the problem. Introverts can gain happiness with a much smaller quantity of stimulation and could gain pleasure from a book or solving a new and fantastical mathematics problem, and as such should be much less likely to experience depression.