Dissociative identity disorder
There are two main ideas about what causes DID, but there is still disagreement about which is correct.  The trauma theory says that DID is caused by parents or families that abuse or neglect their children.  Some of these children deny the abuse is happening or pretend it is happening to someone else, and this eventually becomes a different identity or alter.  Most people diagnosed with DID say they remember being abused by their parents or other caregivers when they were children.  When asked about their childhood, patients with DID are more likely to say they were abused or neglected than people with any other diagnosis.  However, the idea that DID develops in childhood is disputed because childhood memories are not very reliable and it is not clear if the abuse actually happened.  Though DID has been diagnosed in children, there are reasons to believe the diagnosis is because of the influence of parents and doctors rather than abuse.  There is not enough good scientific evidence about DID in childhood to be sure DID is actually caused by abuse or something else.