Studies that verify the presence of DID using multiple resources add credibility to the diagnosis. Research on individuals with DID that have little to no media exposure to information on the illness lends further credibility to the reliability of the existence of this mental health condition.
Why is a just a rather odd wh -word. Its distribution is very limited -- it can only have the word reason as its antecedent, and since it's never the subject it's always deletable. Consequently it behaves strangely, as you and others point out.
The question is: why did the English adapt the name pineapple from Spanish (which originally meant pinecone in English) while most European countries eventually adapted the name ananas, which came from the Tupi word nanas (also meaning pineapple).