Martin Seligman spent his early career on learned helplessness: after repeated uncontrollable adversity, people stop trying even when they could succeed. It became a leading model for depression.
Then he inverted it. If helplessness can be learned, so can optimism. He identified explanatory style as the mechanism — pessimists attribute bad events to causes that are internal, stable and global, and those attributions can be disputed and changed.
Elected president of the American Psychological Association in 1998 by the widest margin in its history, he used the position to launch positive psychology as a field. An American psychologist turned optimism into a research discipline and then into a training programme.