Because the nature of acute schizophrenia is similar to depression, it is difficult to differentiate normal levels of depression in patients with schizophrenia from depressive levels in post-schizophrenic depression. "Prominent subjectively low mood, suggesting depression, and prominent blunting of affect, suggesting negative symptoms, are the two features which are most helpful in differentiating [schizophrenia and depression]." A number of researchers believe that depression is actually a symptom of schizophrenia that has been hidden by the psychosis. However, symptoms usually arise after the first psychotic episodes if they will arise at all. Officially, diagnosing post-schizophrenia depression in a patient requires for the patient to be experiencing a depressive episode of either short or long term following the overcoming of schizophrenia. The patient must still demonstrate some schizophrenic symptoms but those symptoms must no longer be the focus of the illness. Typically, the depressive symptoms are not severe enough to be classified as a severe depressive episode. Formally, diagnosis entails the patient having had schizophrenia within the past year, a number of schizophrenic symptoms, and depression being present for two weeks or more. Mild schizophrenic signs may be withdrawing socially, agitation or hostility, and irregular sleep such as in the case of insomnia and hypersomnia.

Post-schizophrenic depression is a "depressive episode arising in the aftermath of a schizophrenic illness where some low-level schizophrenic symptoms may still be present." Someone that suffers from post-schizophrenic depression experiences both symptoms of depression and can also continue showing mild symptoms of schizophrenia. Unfortunately, depression is a common symptom found in patients with schizophrenia and can fly under the radar for years before others become aware of its presence in a patient. However, very little research has been done on the subject, meaning there are few answers to how it should be systematically diagnosed, treated, or what course the illness will take. Some scientists would entirely deny the existence of post-schizophrenic depression, insisting it is a phase in schizophrenia as a whole. As of late, post-schizophrenic depression has become officially recognized as a syndrome and is considered a sub-type of schizophrenia.

Schizophrenia is a mental disorder that is expressed in abnormal mental functions and disturbed behavior.

The signs and symptoms of childhood schizophrenia are nearly the same as adult-onset schizophrenia. Some of the earliest signs that a young child may develop schizophrenia are lags in language and motor development. Some children engage in activities such as flapping the arms or rocking, and may appear anxious, confused, or disruptive on a regular basis. Children may experience symptoms such as hallucinations, but these are often difficult to differentiate from just normal imagination or child play. It is often difficult for children to describe their hallucinations or delusions, making very early-onset schizophrenia especially difficult to diagnose in the earliest stages. The cognitive abilities of children with schizophrenia may also often be lacking, with 20% of patients showing borderline or full intellectual disability.

Very early-onset schizophrenia refers to onset before the age of thirteen. The prodromal phase, which precedes psychotic symptoms, is characterized by deterioration in school performance, social withdrawal, disorganized or unusual behavior, a decreased ability to perform daily activities, a deterioration in self-care skills, bizarre hygiene and eating behaviors, changes in affect, a lack of impulse control, hostility and aggression, and lethargy.

Auditory hallucinations are the most common "positive symptom" in children. Positive symptoms have come to mean psychopathological disorders that are actively expressed, such as delusions, hallucinations, thought disorder etc.). A child's auditory hallucinations may include voices that are conversing with each other or voices that are speaking directly to the children themselves. Many children with auditory hallucinations believe that if they do not listen to the voices, the voices will harm them or someone else. Tactile and visual hallucinations seem relatively rare. The children often attribute the hallucinatory voices to a variety of beings, including family members or other people, evil forces ("the Devil", "a witch", "a spirit"), animals, characters from horror movies (Bloody Mary, Freddy Krueger) and less clearly recognizable sources ("bad things," "the whispers"). Command auditory hallucinations (also known as imperative hallucinations) were common and experienced by more than ½ of the group in a research at the Bellevue Hospital Center's Children's Psychiatric Inpatient Unit. And voices repeat and repeat: "Kill somebody!", "Kill her, kill her!". Delusions are reported in more than half of children with schizophrenia, but they are usually less complex than those of adults. Delusions often connected with hallucinatory experiences.. In a research delusions were characterized as persecutory for the most part, but some children reported delusions of control. Many said they were being tortured by the beings causing their visual and auditory hallucinations, some thought that if they disobeying their voices would cause them harm.

Some degree of thought disorder was observed in a test group of children in Bellevue Hospital. They displayed illogicality, tangentialiry (a serious disturbance in the associative thought process), and loosening of associations.

Negative ("deficit") symptoms in schizophrenia reflect mental deficit states such as apathy and aboulia, avolition, flattened affect, asthenia etc.

Research suggests that the severity of the delusions of grandeur is directly related to a higher self-esteem in individuals and inversely related to any individual’s severity of depression and negative self-evaluations. Lucas "et al." found that there is no significant gender difference in the establishment of grandiose delusion. However, there is a claim that ‘the particular component of Grandiose delusion’ may be variable across both genders. Also, it had been noted that the presence of GDs in people with at least grammar or high school education was greater than lesser educated persons. Similarly, the presence of grandiose delusions in individuals who are the eldest is greater than in individuals who are the youngest of their siblings.

According to the DSM-IV-TR diagnostic criteria for delusional disorders, grandiose-type symptoms include grossly exaggerated beliefs of:

- self-worth

- power

- knowledge

- identity

- exceptional relationship to a divinity or famous person.

For example, a patient who has fictitious beliefs about his or her power or authority may believe himself or herself to be a ruling monarch who deserves to be treated like royalty.

There are substantial differences in the degree of grandiosity linked with grandiose delusions in different patients. Some patients believe they are God, the Queen of England, a president's son, a famous rock star, and so on. Others are not as expansive and think they are skilled sports-persons or great inventors.

Childhood schizophrenia was not directly added to the DSM until 1968, when it was added to the DSM-II, which set forth diagnostic criteria similar to that of adult schizophrenia. "Schizophrenia, childhood type" was a DSM-II diagnosis with diagnostic code 295.8. It's equivalent to "schizophrenic reaction, childhood type" (code 000-x28) in DSM-I (1952). "Schizophrenia, childhood type" was successfully removed from the DSM-III (1980), and in the Appendix C they wrote: "there is currently no way of predicting which children will develop Schizophrenia as adults". Instead of childhood schizophrenia they proposed to use of "infantile autism" (299.0x) and "childhood onset pervasive developmental disorder" (299.9x).

In the DSM-III-R (1987), DSM-IV (1994), DSM-IV-TR (2000), DSM-5 (2013) there are no "childhood schizophrenia". The rationale for this approach was that since the clinical picture of adult schizophrenia and childhood schizophrenia is identical, childhood schizophrenia should not be a separate disorder.

In schizophrenia's "development and course" in the DSM-5 they wrote:

The diagnosis of schizophrenia was often given to children who by today’s standards would be diagnosed as having of autism or pervasive developmental disorder. This may be because the onset of schizophrenia is gradual, with symptoms relating developmental difficulties or abnormalities appearing before psychotic symptoms.

The main symptoms of paraphrenia are paranoid delusions and hallucinations. The delusions often involve the individual being the subject of persecution, although they can also be erotic, hypochondriacal, or grandiose in nature. The majority of hallucinations associated with paraphrenia are auditory, with 75% of patients reporting such an experience; however, visual, tactile, and olfactory hallucinations have also been reported. The paranoia and hallucinations can combine in the form of “threatening or accusatory voices coming from neighbouring houses [and] are frequently reported by the patients as disturbing and undeserved". Patients also present with a lack of symptoms commonly found in other mental disorders similar to paraphrenia. There is no significant deterioration of intellect, personality, or habits and patients often remain clean and mostly self-sufficient. Patients also remain oriented well in time and space.

Paraphrenia is different from schizophrenia because, while both disorders result in delusions and hallucinations, individuals with schizophrenia exhibit changes and deterioration of personality whereas individuals with paraphrenia maintain a well-preserved personality and affective response.

Pan-Neurosis is the existence of multiple neurotic symptoms such as:

- obsessions

- compulsions

- phobias

- hysteria

- depression

- hypochondriasis

- depersonalization

The diagnosis of pseudoneurotic schizophrenia can only be made with clinical observation by a mental health professional and by the patient's explanation of his or her experiences. A patient must identify with at least two of these symptoms in order to be distinguished as a pseudoneurotic schizophrenic. The intensity of a symptom may vary with the individual patient's severity of the disorder. The symptoms are organized into disorders of thinking and association, disorders of emotional regulation, disorders of sensorimotor and autonomic functioning, pan-anxiety, pan-neurosis, and pansexuality. The two symptoms can fall under any of these categories.

In the DSM-IV-TR, paranoia is diagnosed in the form of:

- paranoid personality disorder ()

- paranoid schizophrenia (a subtype of schizophrenia) ()

- the persecutory type of delusional disorder, which is also called "querulous paranoia" when the focus is to remedy some injustice by legal action. ()

According to clinical psychologist P. J. McKenna, "As a noun, paranoia denotes a disorder which has been argued in and out of existence, and whose clinical features, course, boundaries, and virtually every other aspect of which is controversial. Employed as an adjective, paranoid has become attached to a diverse set of presentations, from paranoid schizophrenia, through paranoid depression, to paranoid personality—not to mention a motley collection of paranoid 'psychoses', 'reactions', and 'states'—and this is to restrict discussion to functional disorders. Even when abbreviated down to the prefix para-, the term crops up causing trouble as the contentious but stubbornly persistent concept of paraphrenia".

At least 50% of the diagnosed cases of schizophrenia experience delusions of reference and delusions of persecution. Paranoia perceptions and behavior may be part of many mental illnesses, such as depression and dementia, but they are more prevalent in three mental disorders: paranoid schizophrenia, delusional disorder (persecutory type), and paranoid personality disorder.

A popular symptom of paranoia is the attribution bias. These individuals typically have a biased perception of reality, often exhibiting more hostile beliefs. A paranoid person may view someone else's accidental behavior as though it is with intent or threatening.

An investigation of a non-clinical paranoid population found that feeling powerless and depressed, isolating oneself, and relinquishing activities are characteristics that could be associated with those exhibiting more frequent paranoia.

Some scientists have created different subtypes for the various symptoms of paranoia including erotic, persecutory, litigious, and exalted.

Due to the suspicious and troublesome personality traits of paranoia, it is unlikely that someone with paranoia will thrive in interpersonal relationships. Most commonly paranoid individuals tend to be of a single status. According to some research there is a hierarchy for paranoia. The least common types of paranoia at the very top of the hierarchy would be those involving more serious threats. Social anxiety is at the bottom of this hierarchy as the most frequently exhibited level of paranoia.

Many patients who present with paraphrenia have significant auditory or visual loss, are socially isolated with a lack of social contact, do not have a permanent home, are unmarried and without children, and have maladaptive personality traits. While these factors do not cause paraphrenia, they do make individuals more likely to develop the disorder later in life.

Sluggish schizophrenia or slow progressive schizophrenia (, "vyalotekushchaya shizofreniya") was a diagnostic category used in Soviet Union to describe what they claimed was a form of schizophrenia characterized by a slowly progressive course; it was diagnosed even in a patient who showed no symptoms of schizophrenia or other psychotic disorders, on the assumption that these symptoms would appear later. It was developed in the 1960s by Soviet psychiatrist Andrei Snezhnevsky and his colleagues, and was used exclusively in the USSR and several Eastern Bloc countries, until the fall of Communism starting in 1989. The diagnosis has long been discredited because of its scientific inadequacy and its use as a means of confining dissenters. It has never been used or recognized outside of Soviet Union, or by international organizations such as the World Health Organization. It is considered a prime example of the political abuse of psychiatry in the Soviet Union.

Sluggish schizophrenia was the most infamous of diagnoses used by Soviet psychiatrists, due to its usage against political dissidents. After being discharged from a hospital, persons diagnosed with sluggish schizophrenia were deprived of their civic rights, credibility and employability. The usage of this diagnosis has been internationally condemned.

In the Russian version of the 10th revision of the International Statistical Classification of Diseases and Related Health Problems (ICD-10), which has long been used throughout present-day Russia, sluggish schizophrenia is no longer listed as a form of schizophrenia, but it is still included as a schizotypal disorder in section F21 of chapter V.

According to Sergei Jargin, the same Russian term "vyalotekushchaya" for sluggish schizophrenia continues to be used and is now translated in English summaries of articles not as "sluggish" but as "slow progressive".

The article "Betwixt Life and Death: Case Studies of the Cotard Delusion" (1996) describes a contemporary case of Cotard delusion, which occurred in a Scotsman whose brain was damaged in a motorcycle accident:

The article "Recurrent Postictal Depression with Cotard Delusion" (2005) describes the case of a fourteen-year-old epileptic boy whose distorted perception of reality resulted from Cotard syndrome. His mental health history was of a boy expressing themes of death, chronic sadness, decreased physical activity in playtime, social withdrawal, and disturbed biological functions. About twice a year, the boy suffered episodes that lasted between three weeks and three months. In the course of each episode, he said that everyone and everything was dead (including trees), described himself as a dead body, and warned that the world would be destroyed within hours. Throughout the episode, the boy showed no response to pleasurable stimuli and had no interest in social activities.

Oneirophenia and schizophrenia are often confused although there are distinct differences between the conditions. Oneirophrenia has some of the characteristics of simple schizophrenia, such as a confusional state and clouding of consciousness, but without presenting the dissociative symptoms which are typical of that disorder. Oneiophrenia often begins with the inability to focus on things while schizophrenia frequently starts with a traumatic event. Persons affected by oneirophrenia have a feeling of dream-like derealization which, in its extreme form, may progress to delusions and hallucinations. Therefore, it is considered a schizophrenia-like acute form of psychosis which remits in about 60% of cases within a period of two years. It is estimated that 50% or more of schizophrenic patients present oneirophrenia at least once.

The delusion of negation is the central symptom in Cotard's syndrome. The patient afflicted with this mental illness usually denies his or her existence, the existence of a certain body part, or the existence of a portion of their body. Cotard's syndrome exists in three stages: (i) Germination stage—the symptoms of psychotic depression and of hypochondria appear; (ii) Blooming stage—the full development of the syndrome and the delusions of negation; and (iii) Chronic stage—continued, severe delusions along with chronic psychiatric depression.

The Cotard syndrome withdraws the afflicted person from other people due to the neglect of their personal hygiene and physical health. The delusion of negation of self prevents the patient from making sense of external reality, which then produces a distorted view of the external world. Such a delusion of negation is usually found in the psychotic patient who also presents with schizophrenia. Although a diagnosis of Cotard's syndrome does not require the patient's having had hallucinations, the strong delusions of negation are comparable to those found in schizophrenic patients.

Dementia praecox (a "premature dementia" or "precocious madness") is a disused psychiatric diagnosis that originally designated a chronic, deteriorating psychotic disorder characterized by rapid cognitive disintegration, usually beginning in the late teens or early adulthood. Over the years, the term "dementia praecox" was gradually replaced by "schizophrenia", which remains in current diagnostic use.

The term "dementia praecox" was first used in 1891 by Arnold Pick (1851–1924), a professor of psychiatry at Charles University in Prague. His brief clinical report described the case of a person with a psychotic disorder resembling hebephrenia. German psychiatrist Emil Kraepelin (1856–1926) popularised it in his first detailed textbook descriptions of a condition that eventually became a different disease concept and relabeled as schizophrenia. Kraepelin reduced the complex psychiatric taxonomies of the nineteenth century by dividing them into two classes: manic-depressive psychosis and dementia praecox. This division, commonly referred to as the Kraepelinian dichotomy, had a fundamental impact on twentieth-century psychiatry, though it has also been questioned.

The primary disturbance in dementia praecox was seen to be a disruption in cognitive or mental functioning in attention, memory, and goal-directed behaviour. Kraepelin contrasted this with manic-depressive psychosis, now termed bipolar disorder, and also with other forms of mood disorder, including major depressive disorder. He eventually concluded that it was not possible to distinguish his categories on the basis of cross-sectional symptoms.

Kraepelin viewed dementia praecox as a progressively deteriorating disease from which no one recovered. However, by 1913, and more explicitly by 1920, Kraepelin admitted that while there may be a residual cognitive defect in most cases, the prognosis was not as uniformly dire as he had stated in the 1890s. Still, he regarded it as a specific disease concept that implied incurable, inexplicable madness.

Oneirophrenia can result from long periods of sleep deprivation or extreme sensory deprivation. The hallucinations in oneirophrenia are increased or derive under decreased sensory input. Psychoanalysts, such as Claudio Naranjo, in the sixties have described the value of ibogaine-induced oneirophrenia for inducing and manipulating free fantasy and dream-like associations in patients under treatment.

It can also be caused by drugs such as ibogaine, which has previously been used to induce the dream like state in some forms of treatment.

In the American Psychiatric Association's DSM-5, schizotypal personality disorder is defined as a "pervasive pattern of social and interpersonal deficits marked by acute discomfort with, and reduced capacity for, close relationships as well as by cognitive or perceptual distortions and eccentricities of behavior, beginning by early adulthood and present in a variety of contexts."

At least five of the following symptoms must be present: ideas of reference, strange beliefs or magical thinking, abnormal perceptual experiences, strange thinking and speech, paranoia, inappropriate or constricted affect, strange behavior or appearance, lack of close friends, and excessive social anxiety that does not abate and stems from paranoia rather than negative judgments about self. These symptoms must not occur only during the course of a disorder with similar symptoms (such as schizophrenia or autism spectrum disorder).

Schizotypal personality disorder (STPD) or schizotypal disorder is a mental disorder characterized by severe social anxiety, thought disorder, paranoid ideation, derealization, transient psychosis, and often unconventional beliefs. People with this disorder feel extreme discomfort with maintaining close relationships with people, mainly because they think that their peers harbor negative thoughts towards them, so they avoid forming them. Peculiar speech mannerisms and odd modes of dress are also symptoms of this disorder. Those with STPD may react oddly in conversations, not respond or talk to themselves.

They frequently interpret situations as being strange or having unusual meaning for them; paranormal and superstitious beliefs are common. Such people frequently seek medical attention for anxiety or depression instead of their personality disorder. Schizotypal personality disorder occurs in approximately 3% of the general population and is more common in males.

The term ""schizotype"" was first coined by Sandor Rado in 1956 as an abbreviation of ""schizophrenic phenotype"". STPD is classified as a cluster A personality disorder ("odd or eccentric disorders") and is considered to be the most impairing disorder in this cluster.

A self-disorder, also called ipseity disturbance, is a psychological phenomenon of disruption or diminishing of a person's sense of minimal (or basic) self. The sense of minimal self refers to the very basic sense of having experiences that are one's own; it has no properties, unlike the more extended sense of self, the narrative self, which is characterized by the person's reflections on themselves as a person, things they like, their identity, and other aspects that are the result of reflection on one's self. Disturbances in the sense of minimal self, as measured by the Examination of Anomalous Self-Experience (EASE), aggregate in the schizophrenia spectrum disorders, to include schizotypal personality disorder, and distinguish them from other conditions such as psychotic bipolar disorder and borderline personality disorder.

The Soviet model of schizophrenia is based on the hypothesis that a fundamental characteristic (by which schizophrenia spectrum disorders are distinguished clinically) is its longitudinal course. The hypothesis implies three main types of schizophrenia:

- Continuous: unremitting, proceeding rapidly ("malignant") or slowly ("sluggish"), with a poor prognosis

- Periodic (or recurrent): characterized by an acute attack, followed by full remission with little or no progression

- Mixed (; in German, "schub" means "phase" or "attack"): mixture of continuous and periodic types which occurs periodically and is characterized by only partial remission.

The classification of schizophrenia types attributed to Snezhnevsky is still used in Russia, and considers sluggish schizophrenia an example of the continuous type. The prevalence of Snezhnevsky's theories has particularly led to a broadening of the boundaries of disease such that even the mildest behavioral change is interpreted as indication of mental disorder.

The presence of self-disorders may have predictive power for whether those with an at risk mental state will develop psychosis; the risk of suicidal ideation and suicide by people with schizophrenia, though depression would also be an important factor; predicting initial social dysfunction in people with either schizophrenic or bipolar psychosis; and whether a person will move to a schizophrenia spectrum diagnosis later.

The presence of self-disorders may cause reduced person insight into their illness through the alteration of the basic structures of consciousness.

Self-disorders are difficult for the people experiencing them to articulate spontaneously; and are not well-known, by either the general public or professionals in the field. Because of this, people will often make vague, cliched complaints that mimic the symptoms of other mental disorders, symptoms such as "fatigue" or "concentration difficulties". Were a knowledgeable clinician to probe deeper, however, the underlying self-disorders may be assessed and help clarify the nature of the person's illness. In their review, Parnas, et al. (2014) say, "The psychiatrist’s acquaintance with the phenomenon of 'non-specific specificity' is, in our view, extremely important in the context of early diagnostic assessment, especially of people presenting with a vague, unelaborated picture of maladjustment, underperformance, chronic malaise and dysphoria, negative symptoms, or hypochondriac preoccupations." People with schizophrenia often describe their self-disorders as causing more suffering for them than psychosis.

Self-disorders underlie most of the first-rank symptoms, those often termed passivity phenomena. There is a current proposal to list self-disorder as one of the symptoms of schizophrenia in the upcoming ICD-11.

A mood disorder (affective) spectrum or bipolar spectrum or depressive spectrum. These approaches have expanded out in different directions. On the one hand, work on major depressive disorder has identified a spectrum of subcategories and subthreshold symptoms which are prevalent, recurrent and associated with treatment needs. People are found to move between the subtypes and the main diagnostic type over time, suggesting a spectrum. This spectrum can include already recognised categories of minor depressive disorder, 'melancholic depression' and various kinds of atypical depression.

Going in another direction, numerous links and overlaps have been found between major depressive disorder and bipolar syndromes, including mixed states (simultaneous depression and mania or hypomania). Hypomanic ('below manic') and more rarely manic signs and symptoms have been found in a significant number of cases of major depressive disorder, suggesting not a categorical distinction but a dimension of frequency which is higher in bipolar II and higher again in bipolar I. In addition, numerous subtypes of bipolar have been proposed beyond the types already in the DSM (which includes a milder form called cyclothymia). These extra subgroups have been defined in terms of more detailed gradations of mood severity, or the rapidity of cycling, or the extent or nature of psychotic symptoms. Furthermore, due to shared characteristics between some types of Bipolar disorder and Borderline Personality Disorder, some researchers have suggested they may both lie on a spectrum of affective disorders, although others see more links to post-trauma syndromes.

A spectrum disorder is a mental disorder that includes a range of linked conditions, sometimes also extending to include singular symptoms and traits. The different elements of a spectrum either have a similar appearance or are thought to be caused by the same underlying mechanism. In either case, a spectrum approach is taken because there appears to be "not a unitary disorder but rather a syndrome composed of subgroups". The spectrum may represent a range of severity, comprising relatively "severe" mental disorders through to relatively "mild and nonclinical deficits".

In some cases, a spectrum approach joins together conditions that were previously considered separately. A notable example of this trend is the autism spectrum, where conditions on this spectrum may now all be referred to as autism spectrum disorders. In other cases, what was treated as a single disorder comes to be seen (or seen once again) as comprising a range of types, a notable example being the bipolar spectrum. A spectrum approach may also expand the type or the severity of issues which are included, which may lessen the gap with other diagnoses or with what is considered "normal". Proponents of this approach argue that it is in line with evidence of gradations in the type or severity of symptoms in the general population, and helps reduce the stigma associated with a diagnosis. Critics, however, argue that it can take attention and resources away from the most serious conditions associated with the most disability, or on the other hand could unduly medicalize problems which are simply challenges people face in life.