APA Dictionary of Psychology

the theory that many behaviors are performed to claim desired identities, so that by behaving in a certain way one is symbolically proving oneself to be a certain kind of person. Insecurity about being the sort of person one wants to be is often the reason for engaging in such self-completing acts. For example, a person who takes pride in being very fit and active may respond to the first signs of illness or exhaustion by increasing, rather than reducing, their activities.