Psychiatric disorders: a disorder of psychological function sufficiently severe to require treatment by a psychiatrist or clinical psychologist. 2 main difficulties in diagnosing: patients suffering from same disorder often display different symptoms, patients suffering from different disorders often display the same symptoms. About 1% of individuals of all races & cultures. Case of lena: stuporous catatonic schizophrenic characterized by long period of immobility and waxy flexibility (not resisting movement but holding new positionuntil moved again). Symptoms overlap with many other disorders & change as the disorder progresses. Negative symptoms: a reduction or loss of normal function: affective flattening, alogia (reduction/absense of speech), avolition (reduction/absense of motivation), anhedonia (inability to experience pleasure) Recurrence of any 2 of these symptoms for 1 month is sufficient for the diagnosis of schizophrenia. Only 1 symptom necessary if bizarre delusion of auditory hallucination. Genetic component 10% of it developing in an immediate relative, even if adopted by a healthy family.