Extroversion-introversion was one of the best-studied systems of traits in all psychological theory, with great masses of evidence from many different cultures supporting the objective reality of the concept. Not as a simple duality of course; one did not label a person plainly this or that, but rather placed them on a scale, rating them for such qualities as sociability, impulsiveness, changeability, talkativeness, outgoingness, activity, liveliness, excitability, optimism, and so on. These measurements had been done so many times that it was statistically certain that the various traits did
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