People with schizophrenia perceive things without a stimulation, which are termed hallucinations. These people also suffer delusions, which are mistaken beliefs that are contrary to facts. Their speech is disorganized and can be incoherent.

In obsessive-compulsive disorders, people have obsessions, which are recurrent thoughts, and compulsions, which are recurrent, irrestible actions. People who experience episodes of depression with mania are considered to have bipolar disorder. Antisocial personality disorder describes people who lie, cheat, and steal, and have no sense of responsibility and no guilt about their behavior. Post-traumatic stress disorder occurs when a person who has gone through a significant trauma shows stress symptoms that impair the person's ability to function. 

A delusion of persecution is when a person believes everyone is trying to hurt (persecute) him or her; Lucy's belief about random planes bombing her and strangers plotting to hurt her are two examples of such perceived persecution. 

Positive symptoms are the presence of emotions or behaviors that are usually absent in a healthy patient. Negative symptoms are the absence of emotions or behaviors that are usually present in a healthy patient.

Flat affect is a negative symptom of schizophrenia, since it is a disruption in normal emotional functioning. Delusions, hallucinations, and catatonia are examples of positive symptoms, whereas impaired executive functioning is an example of a cognitive symptom.

While hallucinations, delusions, and incoherent speech are all noted symptoms of schizophrenia- new feelings or behaviors not present in healthy people-, flat affect (a lack of emotional expression and decrease in talkativeness) is a noted negative symptom, something that healthy people have/do that schizophrenia sufferers do not.

The causes of schizophrenia are varied but one of the leading suspected factors is an excess of the chemical dopamine.

A negative symptom is a symptom that represents something that the sufferer does not have but is regularly found in unaffected people. In the case of a schizophrenic, he or she may not display any emotion where an unaffected person would. This is known as a "flat effect.” 

There are four types of schizophrenia, one of them being paranoid schizophrenia. The identifying and differentiating symptom of paranoid schizophrenia is the delusion that someone or something is “out to get” the person with the disorder.

A schizophrenic is susceptible to certain kinds of delusions—namely those of grandeur, reference, and persecution. Delusions of grandeur lead the schizophrenic to believe they are greater than they are such as thinking they are a prophet or a god. Those of reference may lead them to believe secret messages are being sent to them and they must decode them. Those of persecution incite the paranoid belief that people are out to get them.

A positive symptom is something that is present in the affected person that is not normally found in unaffected persons. Essentially, it's not positive in the sense of "good" but of something being added. One out of several positive symptoms of schizophrenia is the presence of delusional thinking, where one thinks there are things that are present that actually are not.

A positive symptom is a gained behavior, while a negative symptom is a loss of normal function. The gained paranoia behaviors are therefore 'positive.'