Mental disorders are characterized by abnormalities in thoughts, perceptions, mood, and behavior that deviate from social norms. Psychopathology is the study of these deviations and symptoms of mental disorders. Descriptive psychopathology involves close observation of patients' behavior and exploration of their subjective experiences, without assumptions about causes. Symptoms can be delusions, hallucinations, disturbances in thoughts, mood, perception, and memory. Cultural variations exist in how symptoms are expressed and understood.