  A stronger but similar argument was made by Horwitz and Wakefield (2007). They claim that only a hypothesis about causes distinguishes a normal population from a disordered one. They begin with the DSM criteria for diagnosing major depression. A subject must suffer from five of the following nine symptoms in a two week period (including either or both of depressed mood or diminished interest or pleasure in almost all activities): depressed mood; diminished interest; weight gain or loss (without dieting) or change in appetite; insomnia or excessive sleep; observable psychomotor agitation or retardation; fatigue or loss of energy; feeling worthless or excessively guilty; diminished ability to think or concentrate, or indecisiveness; recurrent thoughts of death or suicide or a planned or attempted suicide.