Like many mental illnesses, the diagnosis of schizophrenia is based upon the behavior of the person being assessed. There is a list of diagnostic criteria which must be met for a person to be so diagnosed. These depend on both the presence and duration of certain signs and symptoms.

The most commonly-used criteria for diagnosing schizophrenia are from the American Psychiatric Association's Diagnostic and Statistical Manual of Mental Disorders (DSM) and the World Health Organization’s International Statistical Classification of Diseases and Related Health Problems (ICD). The most recent versions are ICD-10 and DSM-IV-TR.

Below is an abbreviated version of the diagnostic criteria from the DSM-IV-TR, the full version is available here. (DSM cautionary statement)

To be diagnosed as having schizophrenia, a person must display:

A) Characteristic symptoms: Two or more of the following, each present for a significant portion of time during a one-month period (or less, if successfully treated)

delusions

hallucinations

disorganized speech (e.g., frequent derailment or incoherence). See thought disorder.

grossly disorganized or catatonic behavior

negative symptoms, i.e., affective flattening (lack or decline in emotional response), alogia (lack or decline in speech), or avolition (lack or decline in motivation).

Note: Only one Criterion A symptom is required if delusions are bizarre or hallucinations consist of hearing voices.

B) Social/occupational dysfunction: For a significant portion of the time since the onset of the disturbance, one or more major areas of functioning such as work, interpersonal relations, or self-care, are markedly below the level achieved prior to the onset.

C) Duration: Continuous signs of the disturbance persist for at least six months. This six-month period must include at least one month of symptoms (or less, if successfully treated) that meet Criterion A.

Historically, schizophrenia in the West was classified into simple, catatonic, hebephrenic, and paranoid. The DSM now contains five sub-classifications of schizophrenia. These are

catatonic type (where marked absences or peculiarities of movement are present),

disorganized type (where thought disorder and flat or inappropriate affect are present together),

paranoid type (where delusions and hallucinations are present but thought disorder, disorganized behavior, and affective flattening is absent),

residual type (where positive symptoms are present at a low intensity only) and

undifferentiated type (psychotic symptoms are present but the criteria for paranoid, disorganized, or catatonic types has not been met).

Symptoms may also be described as 'positive symptoms' (those additional to normal experience and behavior) and negative symptoms (the lack or decline in normal experience or behavior). 'Positive symptoms' describe psychosis and typically include delusions, hallucinations and thought disorder. 'Negative symptoms' describe inappropriate or nonpresent emotion, poverty of speech, and lack of motivation. In three-factor models of schizophrenia, a third symptom grouping, the so called 'disorganization syndrome' is also given. This considers thought disorder and related disorganized behavior to be in a separate symptom cluster from delusions and hallucinations.

Some symptoms, such as social isolation, may be caused or appear to be caused by a reaction of the individual to avoid psychosis or other more severe symptoms that are inconvenient or unbearable. The person may place limits on his environment or on his own behavior intended to avoid or limit whatever he experiences as causes for these symptoms. These limits or the resulting behavior may appear strange or inappropriate to other people.

It is worth noting that many of the positive or psychotic symptoms may occur in a variety of disorders and not only in schizophrenia. The psychiatrist Kurt Schneider tried to list the particular forms of psychotic symptoms which he thought were particularly useful in distinguishing between schizophrenia and other disorders which could produce psychosis. These are called first rank symptoms or Schneiderian first rank symptoms and include delusions of being controlled by an external force, the belief that thoughts are being inserted or withdrawn from your conscious mind, the belief that your thoughts are being broadcast to other people and hearing hallucinated voices which comment on your thoughts or actions, or may have a conversation with other hallucinated voices. As with other diagnostic methods, the reliability of 'first rank symptoms' has been questioned4, although they remain in use as diagnostic criteria in many countries.