Jan. 21, 2008 -- NFL superstar Herschel Walker has long suffered from multiple personality disorder, the Heisman Trophy winner discloses in an upcoming autobiography.
Former teammates were surprised to learn Walker, 46, who played for several professional teams, suffered from the mental disorder. People with the condition develop multiple alternative identities, each identity sometimes unaware of the memories and actions of the others. The disorder often stems from a childhood trauma.
"No one ever would have thought he was sick," said former Dallas Cowboys quarterback Danny White, who played with Walker from 1985 to 1987.
"Herschel was always a little bit different," White told ABC NEWS.com. "He kept very much to himself and was a real loner. From our standpoint though, no one ever thought there was anything wrong with him."
The book, "Breaking Free," will be released in August or September, said Marcia Burch, a publicist at publisher Simon & Schuster. She confirmed that the book contains material about his disorder but declined to give further details or provide a copy.
"Everyone's questions will be answered when the book comes out," she said.
Walker won the Heisman Trophy in 1982 after leading the University of Georgia to a 33-1 record. He left the school in his junior year to play for the New Jersey Generals in the now-defunct United States Football League.
He went on to play, mostly as a running back, in the National Football League for the Dallas Cowboys, Minnesota Vikings and New York Giants, retiring in 1989.
Calls to Walker by ABC NEWS.com were not returned.
White, who described Walker as a friend, said he last saw him last year at a flag football game in Dallas and he "seemed his normal self."
In his playing days, White said Walker "rarely would hang out with other players after practice or after games. We knew almost nothing about his private life. Unlike others who were always doing things with each other he really kept to himself. … But he was always a great teammate and great athlete."
People often don't know that someone they work with is living with multiple personality disorder, now known as dissociative identity disorder, or DID, said Helen Friedman, a St. Louis psychologist who specializes in treating DID.
"That his teammates didn't know doesn't surprise me," she said. "It is usually not something very obvious to other people."
Friedman said the disorder is sparked by an early childhood trauma, such as abuse, and develops as a coping mechanism.
"When there is no other safe harbor, children create one for themselves. It is a very primitive defense. A child with constant trauma can't say, 'I believe this isn't happening.' But he can say, 'I believe this isn't happening to me."
Some DID patients can have hundreds of personalities, each intended to deal with a different set of circumstances. The actions of one personality may not be known to another personality. Language, behavior, even handwriting can alter depending on the personality presenting at any given time, Friedman said.
Treatment usually involves psychotherapy in which the personalities are coaxed to communicate with one another and merge into one.