Validity of mental illness diagnosis can be difficult to establish. Rosenhan experiment is good - (a summary)
http://psychrights.org/articles/rosenham.htm. Also, author called Szasz wrote on it - ("the myth of mental illness" - "the manufacture of madness" - "essays on the psychiatric dehumanization of man") unfortunately cannot find free full-text online for them. Older critiques of DSM and ICD often give arguments for mental illness as a construct (just search variations of 'DSM/ICD diagnostic validity/reliability').