In general terms, abnormal behavior is behavior that is maladaptive and harmful. There are many types of abnormal behavior, and some major categories include: Anxiety Disorder, Dissociative Disorders, multiple personality disorder, mood disorders, depression, schizophrenia.

There are several theories which try to explain why some people have abnormal behavior. One of them is called the behaviorist approach, where they think that abnormal behavior is a disease caused by internal problems.

The second school of thought is an interaction theory where abnormal behavior is caused by a combination of biological factors (brain processes), psychological factors and social factors.

Part 1: identifying common psychological disorders, and explain why these behaviors are abnormal.

The first main category of abnormal behavior is called anxiety disorder, Here a person experiences symptoms such as trembling, inability to relax, dizziness; a racing heart, perspiration, and irrational and destructive thoughts. There are a few main types of anxiety disorder such as panic disorder (Reoccurring sudden onset of intense apprehension or terror), Obsessive compulsive disorder (Individual has anxiety provoking thoughts that will not go away (obsession) and/or urges to perform repetitive, ritualistic behaviors to prevent or produce some future situation), post-traumatic stress disorder (Anxiety symptoms that may immediately follow the trauma or by months of years).

A second category is personality disorder. A personality disorder is identified
by a pervasive pattern of experience and behavior that is abnormal with respect to any two of the following: thinking, mood, personal relations, and the control of impulses.

There are many types of personality disorders:
Paranoid: This personality disorder is characterized by a distrust of others and a constant suspicion that people around you have sinister motives.
Antisocial: antisocial personality disorder is characterized by a lack of conscience. People with this disorder are prone to criminal behavior, believing that their victims are weak and deserving of being taken advantage of.
Borderline: Borderline personality disorder is characterized by mood instability and poor self-image

Depression is also a type of abnormal behavior. Major depression occurs when Individual is deeply unhappy, demoralized, self-derogatory, and bored, showing changes in appetite, and sleep patterns, decreased energy, feelings of worthlessness, concentration problems, and guilt feelings that might prompt thoughts of suicide.

There are now three theories which explain depression - a genetic component, a biological component and an environmental component.

Schizophrenic Disorders are characterized by distorted thoughts and perceptions, odd communication, inappropriate emotion, abnormal motor behavior, and social withdrawal. Individual's mind is split from reality, and personality loses its unity.

Part 2: Differences between anxiety, depression and schizophrenia

Anxiety is characterized by abnormal thoughts, actions and reactions to situations. Anxiety could be irrational such as a person who has a phobia, or it could be rational (everyone has some type of anxiety in their lives.)

Depression is characterized by a person's depressed mood, erratic sleep patterns, low energy level, thoughts of suicide.

Schizophrenia is characterized by distorted thoughts, thinking people are talking to them in their mind, social withdrawal.

So there are many overlapping features in each of these three disorders. Someone with agoraphobia is scared of going outside, and that is similar to a person who suffers from schizophrenia and who goes through social withdrawal. A person may have erratic sleep patterns because they are depressed or because they are going through post- traumatic stress disorder.

In fact, depression and anxiety disorders frequently co-occur. This condition, known as "comorbidity" indicates that an individual meets diagnostic criteria for both types of disorders. In addition to comorbid diagnoses, many individuals with anxiety have symptoms of depression, even though they do not meet diagnostic criteria for the disorder.

So what differentiates these? Environmental factors, genetic factors and biological factors, and treatment options.

Often, someone with an anxiety disorder experienced a traumatic experience such as a death or a loved on, a fear that spiraled out of control..,. and these are all environmental factors.

A major factor in depression is biological factors, where there is an imbalance of neurotransmitters in the brain.

Schizophrenia seems to be highly genetic in nature, as well, people who suffer from this might also have a chemical imbalance.
Treatment options also differ. For anxiety disorders, behavioral therapy works well with people have ODC. For depression, administering a anti-depressant as well as therapy might work. For schizophrenics there are well developed anti-psychotic drugs. Please note that these drugs do not cure the disease, unlike depression where the drugs can restore the neurotransmitter balance in the brain and lead towards recovery.

Part 3: difference between personality disorder and mental illness

Mental illness is a broad name for illnesses that may include affective or emotional instability, behavioral dysregulation, and/or cognitive dysfunction or impairment. These include what we discussed above - depression, anxiety disorder and schizophrenia.

Personality disorders on the other hand are disorders which are related to a person's ability to function in a normal setting. A person who is depressed has trouble going about their normal day, but this does not effect their personality. Somone with a personality disorder behaves and thinks differently then most people in society. A depressed persons may change their routine but they still convey social norms and act in a normal way, but a person with personality is likely to deviate from what is socially accepted (for example, a person with anti-social behavior only experience a limited range of emotions).