While the movie states that Kevin suffers from DSD, the book makes it clear that the experts are not sure exactly what Kevin has. They speculate that it is some never before seen combination of DSD and Schizophrenia.
Consider this: While most people suffering from DSD are treated with the goal of curing them of this disorder, Kevin spent over 10 years having his disorder nurtured and feed by his crazy aunt and uncle. Who knows what DSD might evolve into when treated this way?
