How can you tell when a behavior is psychologically abnormal and when it is merely "weird?"

There is no one standard of abnormal behavior; in a very real way abnormality is in the eye of the beholder. Behavior that would seem perfectly normal to members of one culture might seem completely bizarre to those of another. Standards that psychologists in Western society typically use to define abnormal behavior include: behavior that harms the individual or others, and behavior that prevents the individual from functioning effectively in society. It is, of course, hard to measure just when a behavior crosses the line into "abnormality." The truth is that we are all capable of performing all kinds of behaviors for all kinds of reasons.

What is the most serious psychological disorder? How does it develop?

While many disorders can lead to severe disruption of a person's life, schizophrenia is perhaps one of the most disruptive, and one of the most difficult to treat. Schizophrenia is characterized by severe distortion of reality, such as delusions; withdrawal; disruption of thought and language; and inappropriate emotional responses. No clear picture of how it develops is available. It seems as though people may be genetically predisposed to develop schizophrenia, but that environmental stressors play a role as well.