Is it possible to create mental worlds in your mind?

A lot of people still call this disorder multiple personality disorder but it's actually called Dissociative Identity Disorder (DID). It manifests to protect the person from an experience they'd rather not face/relive, etc. and a person can develop many, many personalities depending on their defensive needs. Some personalities are aware of others but some are not. Personalities can be of the same , age, race, etc. or they can be vastly different (for example, a 25 gear old woman could have an alternate personality of a 6 year old boy). It's a very fascinating disorder, yet very devastating as well. The website for the National Institute of Mental Health (www.nimh.net or .com) will give you lots of information on this disorder and lots of others.