In psychiatry, derailment (also loosening of association, asyndesis, asyndetic thinking, knight's move thinking, or entgleisen) is a thought disorder characterized by discourse consisting of a sequence of unrelated or only remotely related ideas. The frame of reference often changes from one sentence to the next.

In a mild manifestation, this thought disorder is characterized by slippage of ideas further and further from the point of a discussion. Derailment can often be manifestly caused by intense emotions such as euphoria or hysteria. Some of the synonyms given above ("loosening of association", "asyndetic thinking") are used by some authors to refer just to a "loss of goal": discourse that sets off on a particular idea, wanders off and never returns to it. A related term is tangentiality—it refers to off-the-point, oblique or irrelevant answers given to questions. In some studies on creativity, "knight's move thinking", while it describes a similarly loose association of ideas, is not considered a mental disorder or the hallmark of one; it is sometimes used as a synonym for lateral thinking.

Risk factors (constructs which increase the likelihood or intensity of homesickness) and protective factors (constructs that decrease the likelihood or intensity of homesickness) vary by population. For example, a seafarers on board, the environmental stressors associated with a hospital, a military boot camp or a foreign country may exacerbate homesickness and complicate treatment. Generally speaking, however, risk and protective factors transcend age and environment.

The risk factors for homesickness fall into five categories: experience, personality, family, attitude and environment. More is known about some of these factors in adults—especially personality factors—because more homesickness research has been performed with older populations. However, a growing body of research is elucidating the etiology of homesickness in younger populations, including children at summer camp, hospitalized children and students.

- Experience factors: Younger age; little previous experience away from home (for which age can be a proxy); little or no previous experience in the novel environment; little or no previous experience venturing out without primary caregivers.

- Attitude factors: The belief that homesickness will be strong; negative first impressions and low expectations for the new environment; perceived absence of social support; high perceived demands (e.g., on academic, vocational or sports performance); great perceived distance from home

- Personality factors: Insecure attachment relationship with primary caregivers; low perceived control over the timing and nature of the separation from home; anxious or depressed feelings in the months prior to the separation; low self-directedness; high harm avoidance; rigidity; a wishful-thinking coping style.

- Family factors: Low decision control (e.g., caregivers forcing a young child to spend time away from home against her wishes); governments forcing a person to be in a novel environment (e.g., being drafted into military service away from home or being sentenced to prison); unsupportive caregiving; caregivers who express anxiety or ambivalence about the separation (e.g., "Have a great time away. I don't know what I'll do without you.")

- Environmental factors: High cultural contrast (e.g., different language, customs, food); threats to physical and emotional safety; dramatic alternations in daily schedule; lack of information about the new place; perceived discrimination

Finally, research has provided no support for a few factors that conventional wisdom had once held to be risk factors. These include: recent separation or divorce of primary caregivers; geographic distance from home; or recent geographic move. Most likely, it is not a change in family structure, distance or dwelling that predicts homesickness, but whether these changes have left unanswered (potentially preoccupying) questions in the person's mind.

Thought disorder (TD) or formal thought disorder (FTD) refers to disorganized thinking as evidenced by disorganized speech. Specific thought disorders include derailment, poverty of speech, tangentiality, illogicality, perseveration, and thought blocking.

Psychiatrists consider formal thought disorder as being one of two types of disordered thinking, with the other type being delusions. The latter involves "content" while the former involves "form". Although the term "thought disorder" can refer to either type, in common parlance it refers most often to a disorder of thought "form" also known as formal thought disorder.

Eugen Bleuler, who named schizophrenia, held that thought disorder was its defining characteristic. However, formal thought disorder is not unique to schizophrenia or psychosis. It is often a symptom of mania, and less often it can be present in other mental disorders such as depression. Clanging or echolalia may be present in Tourette syndrome. Patients with a clouded consciousness, like that found in delirium, also have a formal thought disorder.

However, there is a clinical difference between these two groups. Those with schizophrenia or psychosis are less likely to demonstrate awareness or concern about the disordered thinking. Clayton and Winokur have suggested that this results from a fundamental inability to use the same type of Aristotelian logic as others. On the other hand, patients with a clouded consciousness, referred to as "organic" patients, usually do demonstrate awareness and concern, and complain about being "confused" or "unable to think straight"; Clayton and Winokur suggest that this is because their thought disorder results, instead, from various cognitive deficits.

A mental breakdown (also known as a nervous breakdown) is an acute, time-limited mental disorder that manifests primarily as severe stress-induced depression, anxiety, or dissociation in a previously functional individual, to the extent that they are no longer able to function on a day-to-day basis until the disorder is resolved. A nervous breakdown is defined by its temporary nature, and often closely tied to psychological burnout, severe overwork, sleep deprivation, and similar stressors, which may combine to temporarily overwhelm an individual with otherwise sound mental functions.

The terms "nervous breakdown" and "mental breakdown" have not been formally defined through a medical diagnostic system such as the DSM-5 or ICD-10, and are nearly absent from current scientific literature regarding mental illness. Although "nervous breakdown" is not rigorously defined, surveys of laypersons suggest that the term refers to a specific acute time-limited reactive disorder, involving symptoms such as anxiety or depression, usually precipitated by external stressors. Many health experts today refer to a nervous breakdown as a "modern mental health crisis."

Specific cases are sometimes described as a "breakdown" only after the emotional and physical demands on a person's life are so great as to prevent him or her from performing activities of daily living or, less strictly, only when those demands prevent him/her from performing his/her familial or occupational duties.

Nervous breakdowns are often caused by serious ongoing mental health disorders.

According to research by RM Carney et al., any history of child depression influences the occurrence of adolescent cardiac risk factors, even if individuals no longer suffer from depression. They are much more likely to develop heart disease as adults.

The concept of thought disorder has been criticized as being based on circular or incoherent definitions. For example, thought disorder is inferred from disordered speech, based on the assumption that disordered speech arises because of disordered thought. Incoherence, or word salad, refers to speech that is unconnected and conveys no meaning to the listener.

Furthermore, although thought disorder is typically associated with psychosis, similar phenomena can appear in different disorders, potentially leading to misdiagnosis—for example, in the case of incomplete yet potentially fruitful thought processes.

It has been suggested that individuals with autism spectrum disorders (ASD) display language disturbances like those found in schizophrenia; a 2008 study found that children and adolescents with ASD showed significantly more illogical thinking and loose associations than control subjects. The illogical thinking was related to cognitive functioning and executive control; the loose associations were related to communication symptoms and to parent reports of stress and anxiety.

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.

According to research conducted by Laura P. Richardson et al., major depression occurred in 7% of the cohort during early adolescence (11, 13, and 15 years of age) and 27% during late adolescence (18 and 21 years of age). At 26 years of age, 12% of study members were obese. After adjusting for each individual's baseline body mass index (calculated as the weight in kilograms divided by the square of height in meters), depressed late adolescent girls were at a greater than 2-fold increased risk for obesity in adulthood compared with their non-depressed female peers (relative risk, 2.32; 95% confidence interval, 1.29-3.83). A dose-response relationship between the number of episodes of depression during adolescence, and risk for adult obesity was also observed in female subjects. The association was not observed for late adolescent boys or for early adolescent boys or girls.

Neurosis is a class of functional mental disorders involving chronic distress but neither delusions nor hallucinations. The term is no longer used by the professional psychiatric community in the United States, having been eliminated from the "Diagnostic and Statistical Manual of Mental Disorders" in 1980 with the publication of DSM III. It is still used in the .

Neurosis should not be mistaken for psychosis, which refers to a loss of touch with reality. Neither should it be mistaken for neuroticism, a fundamental personality trait proposed in the big Five personality traits theory.

In psychology and psychiatry, clanging refers to a mode of speech characterized by association of words based upon sound rather than concepts. For example, this may include compulsive rhyming or alliteration without apparent logical connection between words. This is associated with the irregular thinking apparent in psychotic mental illnesses (e.g. mania and schizophrenia). Gustav Aschaffenburg found that manic individuals generated these "clang-associations" roughly 10-50 times more than non-manic individuals. Aschaffenburg also found that the frequency of these associations increased for all individuals as they became more fatigued.

Clanging refers specifically to behavior that is situationally inappropriate. While a poet rhyming is not evidence of mental illness, disorganized speech that impedes the patient's ability to communicate is a disorder in itself, often seen in schizophrenia.

As with most mental disorders, BDD's causation is likely intricate, altogether biopsychosocial, through an interaction of multiple factors, including genetic, developmental, psychological, social, and cultural. BDD usually develops during early adolescence, although many patients note earlier trauma, abuse, neglect, teasing, or bullying. Though twin studies on BDD are few, one estimated its heritability at 43%. Yet BDD's causation may also involve introversion, negative body image, perfectionism, heightened aesthetic sensitivity, and childhood abuse and neglect. Media influence has also been identified as a factor causing poor body image.

Reported prevalence of STPD in community studies ranges from 0.6% in a Norwegian sample, to 4.6% in an American sample. A large American study found a lifetime prevalence of 3.9%, with somewhat higher rates among men (4.2%) than women (3.7%). It may be uncommon in clinical populations, with reported rates of 0% to 1.9%.

Together with other Cluster A personality disorders, it is also very common among homeless people.

A University of Colorado Colorado Springs study comparing personality disorders and Myers-Briggs Type Indicator types found that the disorder had a significant correlation with the Introverted (I), Intuitive (N), Thinking (T), and Perceiving (P) preferences.

Endogenous depression occurs as the results of an internal stressor—commonly cognitive or biological—and not an external factor. Potential risk factors include these cognitive or biological factors. Patients with endogenous depression often are more likely to have a positive family history of disorders and fewer psychosocial and environmental factors that cause their symptoms. A family history of depression and perceived poor intimate relationships are internal risk factors associated with this type of depression. It is important to know these risk factors in order to take steps to recognize and help prevent this illness.

Thought insertion is defined by the ICD-10 as feeling as if one's thoughts are not one's own, but rather belong to someone else and have been inserted into one's mind. The person experiencing thought insertion will not necessarily know where the thought is coming from, but is able to distinguish between their own thoughts and those inserted into their minds. However, patients do not experience all thoughts as inserted, only certain ones, normally following a similar content or pattern. This phenomenon is classified as a delusion. A person with this delusional belief is convinced of the veracity of their beliefs and is unwilling to accept such diagnosis.

Thought insertion is a common symptom of psychosis and occurs in many mental disorders and other medical conditions. However, thought insertion is most commonly associated with schizophrenia. Thought insertion, along with thought broadcasting, thought withdrawal, thought blocking and other first rank symptoms, is a primary symptom and should not be confused with the delusional explanation given by the respondent. Although normally associated with some form of psychopathology, thought insertion can also be experienced in those considered nonpathological, usually in spiritual contexts, but also in culturally influenced practices such as mediumship and automatic writing.

Examples of thought insertion:

"She said that sometimes it seemed to be her own thought 'but I don't get the feeling that it is'. She said her 'own thoughts might say the same thing', 'but the feeling isn't the same', 'the feeling is that it is somebody else's'"

"I look out the window and I think that the garden looks nice and the grass looks cool, but the thoughts of Eamonn Andrews come into my mind. There are no other thoughts there, only his. He treats my mind like a screen and flashes thoughts onto it like you flash a picture"

"The subject has thoughts that she thinks are the thoughts of other people, somehow occurring in her own mind. It is not that the subject thinks that other people are making her think certain thoughts as if by hypnosis or psychokinesis, but that other people think the thoughts using the subject's mind as a psychological medium."

The most frequent cause of the syndrome is brain damage to the frontal lobe. Brain damage leading to the dysexecutive pattern of symptoms can result from physical trauma such as a blow to the head or a stroke or other internal trauma.

It is important to note that frontal lobe damage is not the only cause of the syndrome. It has been shown that damage, such as lesions, in other areas of the brain may indirectly affect executive functions and lead to similar symptoms. There is not one specific pattern of damage that leads to DES, as multiple affected brain structures and locations have led to the symptoms. This is one reason why the term frontal lobe syndrome is not preferred.

DES often occurs with other disorders, which is known as comorbidity. Many studies have examined the presence of DES in patients with schizophrenia. Results of schizophrenic patients on the "Behavioural Assessment of the Dysexecutive Syndrome (BADS)" test (discussed below) are comparable to brain injured patients. Further, results of BADS have been shown to correlate with phases of schizophrenia. Patients in the chronic phase of the disorder have significantly lower scores than those who are acute. This is logical due to the similarities in executive disruptions that make everyday life difficult for those with schizophrenia and symptoms that form DES.

Patients with Alzheimer's disease have been shown to exhibit impairment in executive functioning as well. The effects of DES symptoms on the executive functions and working memory, such as attentiveness, planning and remembering recently learned things, are some of the earliest indicators of Alzheimer's.

Studies have also indicated that chronic alcoholism (see Korsakoff's syndrome) can lead to a mild form of DES according to results of BADS.

Multiple complex developmental disorder is likely to be caused by a number of different various genetic factors. Each individual with MCDD is unique from one another and displays different symptoms. Various neuropsychological disorders can also be found in family members of people with MCDD.

A person afflicted with circumstantiality has slowed thinking and invariably talks at length about irrelevant and trivial details (i.e. circumstances). Eliciting information from such a person can be difficult since circumstantiality makes it hard for the individual to stay on topic. In most instances however, the relevant details are eventually achieved.

The disorder is often associated with schizophrenia and obsessive-compulsive disorder.

This type of depression often occurs due to biological reasons. Since symptoms are due to an internal phenomena, prevalence rates tend to be higher in older adults and more prevalent among women. Although endogenous depression has been associated with increased age, there has been few attempts to evaluate this fully. More research is needed to indicate factual prevalence rates on this type of depression in society.

Some debates have pervaded the field of psychology since its genesis. Perhaps one of the most salient ones deals with the nature of personality. Personality psychology studies one's distinctive style of cognition, behavior, and affect. However, this concept elicits discord among psychologists as some have insisted that it does not exist, while others struggle with issues of measurement.

Ideas of reference and delusions of reference describe the phenomenon of an individual's experiencing innocuous events or mere coincidences and believing they have strong personal significance. It is "the notion that everything one perceives in the world relates to one's own destiny".

In psychiatry, delusions of reference form part of the diagnostic criteria for psychotic illnesses such as schizophrenia, delusional disorder, bipolar disorder (during the elevated stages of mania), as well as schizotypal personality disorder. To a lesser extent, it can be a hallmark of paranoid personality disorder. Such symptoms can also be caused by intoxication, especially with hallucinogens or stimulants like methamphetamine.

ODD has an estimated lifetime prevalence of 10.2% (11.2% for males, 9.2% for females).

Negative parenting practices and parent–child conflict may lead to antisocial behavior, but they may also be a reaction to the oppositional and aggressive behaviors of children. Factors such as a family history of mental illnesses and/or substance abuse as well as a dysfunctional family and inconsistent discipline by a parent or guardian can lead to the development of behavior disorders.

Insecure parent–child attachments can also contribute to ODD. Often little internalization of parent and societal standards exists in children with conduct problems. These weak bonds with their parents may lead children to associate with delinquency and substance abuse.

Family instability and stress can also contribute to the development of ODD. Although the association between family factors and conduct problems is well established, the nature of this association and the possible causal role of family factors continues to be debated.

Low socioeconomic status is associated with poor parenting, specifically with inconsistent discipline and poor parental monitoring, which are then associated with an early onset of aggression and antisocial behaviors.

Externalizing problems are reported to be more frequent among minority-status youth, a finding that is likely related to economic hardship, limited employment opportunities, and living in high-risk urban neighborhoods.