This is a difficult question. Even though a mental illness must cause distress or dysfunction, to some extent we judge the pathology of behavior by its relationship to cultural norms. Therefore our concept of mental illness is tied in part to our ideas of what is normal. This raises the question of whether all abnormal behavior is pathological and whether all normal behavior is mentally healthy. Clearly, people can engage in unusual behavior that is not pathological. We do not want to diagnose any kind of original or unconventional behavior as mentally ill. Nor is all normal behavior healthy. Drug abuse, violence, and anorexia can be very common in certain social groups, but all cause distress or dysfunction. Thus, although it can be very easy to recognize mental illness in the most extreme cases (such as acute psychosis or severe depression), there are many circumstances where the boundaries between mental health and mental illness are not so clear.