“Everyone from pop-psych authors to business-school professors to astrologers has come up with her own system for sizing up people. If it were possible, wouldn’t one method have prevailed by now? In fact, two recent paradigm-breaking studies suggest that personality traits can shift slowly yet drastically over time, and quite quickly after therapeutic interventions. This is a legitimate question for theoretical mathematicians, but in the science of personality (unlike mathematics) perception trumps precision most of the time. This fluid state of affairs is often captured best by writers, who tend to have an agenda when delineating characters.”