Schizophrenia is a mental disorder characterized by abnormal social behavior and failure to understand what is real. Common symptoms include false beliefs, unclear or confused thinking, hearing voices that others do not hear, reduced social engagement and emotional expression, and a lack of motivation. People with schizophrenia often have additional mental health problems such as anxiety, depressive, or substance-use disorders. Symptoms typically come on gradually, begin in young adulthood, and last a long time. The causes of schizophrenia include environmental and genetic factors. Possible environmental factors include being raised in a city, cannabis use during adolescence, certain infections, parental age and poor nutrition during pregnancy. Genetic factors include a variety of common and rare genetic variants. Diagnosis is based on observed behavior, the person's reported experiences and reports of others familiar with the person. During diagnosis a person's culture must also be taken into account. As of 2013 there is no objective test. Schizophrenia does not imply a "split personality" or "dissociative identity disorder" – conditions with which it is often confused in public perception.