Extroversion is in our DNA—literally, according to some psychologists. The trait has been found to be less prevalent in Asia and Africa than in Europe and America, whose populations descend largely from the migrants of the world. It makes sense, say these researchers, that world travelers were more extroverted than those who stayed home—and that they passed on their traits to their children and their children’s children. “As personality traits are genetically transmitted,” writes the psychologist Kenneth Olson, “each succeeding wave of emigrants to a new continent would give rise over time to
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