Muscle physiology distinguishes between tonic activity (the baseline of electrical activity when the muscle is idling) and phasic activity (the burst of electrical activity when the muscle is challenged and contracts). Most of psychology is about tonic activity; introversion, high IQ, depression, and anger, for example, are all measured in the absence of any real-world challenge, and the hope of the psychometrician is to predict what a person will actually do when confronted with a phasic challenge. How well do tonic measures fare? Does a high IQ predict a truly canny response to a customer saying no? How well does tonic depression predict collapse when a person is fired? "Moderately well, but imperfectly" is the best general answer. Psychology as usual predicts many of the cases, but there are huge numbers of high-IQ people who fail, and another huge number of low-IQ people who succeed when life challenges them to do something actually intelligent in the world. The reason for all these errors is that tonic measures are only moderate predictors of phasic action. I call this imperfection in prediction the Harry Truman effect. Truman, after an undistinguished life, to almost everyone's surprise rose to the occasion when FDR died and ended up becoming one of the great presidents.
We need a psychology of rising to the occasion, because that is the missing piece in the jigsaw puzzle of predicting human behavior. In the evolutionary struggle for winning a mate or surviving a predator's attack, those of our ancestors who rose to the occasion passed on their genes; the losers did not. Their tonic characteristics — depression level, sleep patterns, waist size — probably did not count for much, except insofar as they fed the Harry Truman effect. This means that we all contain ancient strengths inside of us that we may not know about until we are truly challenged. Why were the adults who faced World War II the "greatest generation"? Not because they were made of different stuff than we are, but because they faced a time of trouble that evoked the ancient strengths within.
When you read about these strengths in Chapters 8 and 9 and take the strengths survey, you will find that some of your strengths are tonic and some are phasic. Kindness, curiosity, loyalty, and spirituality, for example, tend to be tonic; you can display these several dozen times a day. Perseverance, perspective, fairness, and valor, at the other extreme, tend to be phasic; you cannot demonstrate valor while standing in a check-out line or sitting in an airplane (unless terrorists hijack it). One phasic action in a lifetime may be enough to demonstrate valor. When you read about these strengths, you will also find some that are deeply characteristic of you, whereas others are not. I call the former your signature strengths, and one of my purposes is to distinguish these from strengths that are less a part of you. I do not believe that you should devote overly much effort to correcting your weaknesses. Rather, I believe that the highest success in living and the deepest emotional satisfaction comes from building and using your signature strengths. For this reason, the second part of this book focuses on how to identify these strengths.