I'm not up to speed with the current thinking on this, but some have suggested a biological basis for introversion-extraversion. It's been suggested that introverts have a higher resting level of sympathetic nervous system arousal. This is why they need down time and quiet - they are already close to their optimum without any extra stimulation. By contrast, the extravert has low resting arousal, so he's always looking for stimulation to push him up to the optimum.
One bit of evidence for this is that extraverts tend to have higher pain thresholds than introverts. The extravert's whole nervous system just isn't as responsive.
Taking the argument to the extreme, it's been suggested that psychopaths may have ultra-low levels of arousal. When you and I were little, we learned moral rules because when we did something wrong, we got an angry parent on our case - too much stimulation. But for a psychopath, that same lsituation isn't stressfu and aversivel, it's mildly pleasant. They welcome the extra stimulation. As a consequence, they just don't go through the same conditioning process that other people do.
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