An alter ego is a second self, a part of a person's personality which is in marked contrast with his or her regular personality. This concept is often used as a literary device, perhaps most famously in Dr. Jekyll and Mr. Hyde, but it has also been explored in philosophy, government, and even economics. The idea of a second distinct personality is intriguing to some people, and the alter ego concept is sometimes used to explain behavior which is viewed as irregular or abnormal.

This Latin term, which literally means “other self,” does not come up very often in clinical psychology or psychiatry. While some mental health conditions do involve the manifestation of alternate personalities, clinical terms like “schizophrenia” are usually preferred, as psychologists like to draw a clear line between psychology and disciplines like fiction and philosophy.

People refer to the alter ego in a number of different ways. Some people, for example, talk about close and inseparable friends as “alter egos,” referencing the idea that these friends have become enmeshed in their personal identity. It is certainly true that some people seem to come in pairs with their best friends.

Characters played by actors are sometimes referred to as “alter egos,” especially when those characters are important and famous, and they have made the actor's name well known. Sean Connery's alter ego, for example, is James Bond, because although other actors have portrayed Bond, Connery made the character famous, and Connery's trademark style is often pointed to as the “true” Bond.

When someone behaves differently in different situations, he or she is sometimes said to have an alter ego. In fact, the adjustment of one's behavior to meet social norms and expectations could be considered a social adaptation, but people who feel equally at ease in a variety of contrasting situations are sometimes viewed with suspicion by people who are less adaptable. Therefore, a loving mother who is also a sharp businesswoman might be accused of having an alter ego.

Someone with an especially well-developed alter ego in fiction and film may live a “double life.” For instance, a character in a novel who works as a spy might have a cover job as an auto mechanic, with characters who are in the know about the secret identity referencing the protagonist's double life and the emotional stress it creates. Double lives also occur in the real world, although they are less common than fiction would suggest.