Author of the popular HBR article How to Stop Worrying About What Other People Think Of You, high-performance psychologist Michael Gervais argues that our fear of other people’s opinions — what he calls FOPO — may be the single greatest restrictor of human potential. In his new book, The First Rule of Mastery (Harvard Business Review Press), Gervais, who works with world-renowned athletes, artists, and musicians, as well as Fortune 100 leaders and teams, shares the mental skills and practices we need to overcome FOPO. This excerpt, which is lightly edited, explores our tendency to outsource our self worth — or look to others for external validation.