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.

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.

Various factors have been found to be more associated with a diagnosis of AD than other axis I disorders, including:

- younger age

- more identified psychosocial and environmental problems

- increased suicidal behaviour, more likely to be rated as improved by the time of discharge from mental healthcare

- less frequent previous psychiatric history

- shorter length of treatment

Those exposed to repeated trauma are at greater risk, even if that trauma is in the distant past. Age can be a factor due to young children having fewer coping resources; children are also less likely to assess the consequences of a potential stressor.

A stressor is generally an event of a serious, unusual nature that an individual or group of individuals experience. The stressors that cause adjustment disorders may be grossly traumatic or relatively minor, like loss of a girlfriend/boyfriend, a poor report card, or moving to a new neighborhood. It is thought that the more chronic or recurrent the stressor, the more likely it is to produce a disorder. The objective nature of the stressor is of secondary importance. Stressors' most crucial link to their pathogenic potential is their perception by the patient as stressful. The presence of a causal stressor is essential before a diagnosis of adjustment disorder can be made.

There are certain stressors that are more common in different age groups:

Adulthood:

- Marital conflict

- Financial conflict

- Health issues with Oneself/Partner or Dependent children

- Personal tragedy (Death/personal loss)

- Loss of job or unstable employment conditions (e.g. Corporate takeover/redundancy)

Adolescence and childhood:

- Family conflict/parental separation

- School problems/changing schools

- Sexuality issues

- Death/illness/trauma in the family

In a study conducted from 1990 to 1994 on 89 psychiatric outpatient adolescents, 25% had attempted suicide in which 37.5% had misused alcohol, 87.5% displayed aggressive behaviour, 12.5% had learning difficulties, and 87.5% had anxiety symptoms.

Depressed mood can be the result of a number of infectious diseases, nutritional deficiencies, neurological conditions and physiological problems, including hypoandrogenism (in men), Addison's disease, Cushing's syndrome, hypothyroidism, Lyme disease, multiple sclerosis, Parkinson's disease, chronic pain, stroke, diabetes, and cancer.

Studies have shown that those who fall into minorities due to either their gender identity or sexual orientation (such as those that identify as LGBT), are more prone to depression.

Bipolar disorder can cause suicidal ideation that leads to suicidal attempts. Individuals whose bipolar disorder begins with a depressive or mixed affective episode seem to have a poorer prognosis and an increased risk of suicide. One out of two people with bipolar disorder attempt suicide at least once during their lifetime and many attempts are successfully completed. The annual average suicide rate is 0.4 percent, which is 10–20 times that of the general population. The standardized mortality ratio from suicide in bipolar disorder is between 18 and 25. The lifetime risk of suicide has been estimated to be as high as 20 percent in those with bipolar disorder.

According to the DSM IV-TR, the development of the emotional or behavioral symptoms of this diagnosis have to occur within three months of the onset of the identifiable stressor(s). Some emotional signs of adjustment disorder are:

However, the stress-related disturbance does not only exist as an exacerbation of a pre-existing axis I or axis II disorder and cannot be diagnostic as axis 1 disorder.

Suicidal behavior is prominent among people with AD of all ages, and up to one-fifth of adolescent suicide victims may have an adjustment disorder. Bronish and Hecht (1989) found that 70% of a series of patients with AD attempted suicide immediately before their index admission and they remitted faster than a comparison group with major depression. Asnis et al. (1993) found that AD patients report persistent ideation or suicide attempts less frequently than those diagnosed with major depression. According to a study on 82 AD patients at a clinic, Bolu et al. (2012) found that 22 (26.8%) of these patients were admitted due to suicide attempt, consistent with previous findings. In addition, it was found that 15 of these 22 patients chose suicide methods that involved high chances of being saved. Henriksson et al. (2005) states statistically that the stressors are of one-half related to parental issues and one-third in peer issues.

A lifelong condition with periods of partial or full recovery in between recurrent episodes of relapse, bipolar disorder is considered to be a major health problem worldwide because of the increased rates of disability and premature mortality. It is also associated with co-occurring psychiatric and medical problems and high rates of initial under- or misdiagnosis, causing a delay in appropriate treatment interventions and contributing to poorer prognoses. After a diagnosis is made, it remains difficult to achieve complete remission of all symptoms with the currently available psychiatric medications and symptoms often become progressively more severe over time.

Compliance with medications is one of the most significant factors that can decrease the rate and severity of relapse and have a positive impact on overall prognosis. However, the types of medications used in treating BD commonly cause side effects and more than 75% of individuals with BD inconsistently take their medications for various reasons.

Of the various types of the disorder, rapid cycling (four or more episodes in one year) is associated with the worst prognosis due to higher rates of self-harm and suicide. Individuals diagnosed with bipolar who have a family history of bipolar disorder are at a greater risk for more frequent manic/hypomanic episodes. Early onset and psychotic features are also associated with worse outcomes, as well as subtypes that are nonresponsive to lithium.

Early recognition and intervention also improve prognosis as the symptoms in earlier stages are less severe and more responsive to treatment. Onset after adolescence is connected to better prognoses for both genders, and being male is a protective factor against higher levels of depression. For women, better social functioning prior to developing bipolar disorder and being a parent are protective towards suicide attempts.

"See Adverse Childhood Experiences Study"

Adverse childhood experiences (ACEs) are potentially traumatic events that can have negative, lasting effects on health and well-being. Adverse childhood experiences range from abuse to neglect to living in a household where the mother is treated violently or there is a parent with a mental illness. Kaiser Permanente and the Centers for Disease Control and Prevention's 1998 study on adverse childhood experiences determined that traumatic experiences during childhood are a root cause of many social, emotional, and cognitive impairments that lead to increased risk of unhealthy behaviors, risk of violence or re-victimization, chronic health conditions, low life potential and premature mortality. As the number of adverse experiences increases, the risk of problems from childhood through adulthood also rises. Nearly 30 years of study following the initial study has confirmed this. Many states, health providers, and other groups now routinely screen parents and children for ACEs.

Environmental factors associated with the development of schizophrenia include the living environment, drug use, and prenatal stressors.

Maternal stress has been associated with an increased risk of schizophrenia, possibly in association with reelin. Maternal Stress has been observed to lead to hypermethylation and therefore under-expression of reelin, which in animal models leads to reduction in GABAergic neurons, a common finding in schizophrenia. Maternal nutritional deficiencies, such as those observed during a famine, as well as maternal obesity have also been identified as possible risk factors for schizophrenia. Both maternal stress and infection have been demonstrated to alter fetal neurodevelopment through pro-inflammatory proteins such as IL-8 and TNF.

Parenting style seems to have no major effect, although people with supportive parents do better than those with critical or hostile parents. Childhood trauma, death of a parent, and being bullied or abused increase the risk of psychosis. Living in an urban environment during childhood or as an adult has consistently been found to increase the risk of schizophrenia by a factor of two, even after taking into account drug use, ethnic group, and size of social group. Other factors that play an important role include social isolation and immigration related to social adversity, racial discrimination, family dysfunction, unemployment, and poor housing conditions.

It has been hypothesized that in some people, development of schizophrenia is related to intestinal tract dysfunction such as seen with non-celiac gluten sensitivity or abnormalities in the intestinal flora. A subgroup of persons with schizophrenia present an immune response to gluten different from that found in people with celiac, with elevated levels of certain serum biomarkers of gluten sensitivity such as anti-gliadin IgG or anti-gliadin IgA antibodies.

Traumatic experiences during childhood causes stress that increases an individual’s allostatic load and thus affects the immune system, nervous system, and endocrine system. Early childhood trauma is often associated with adverse health outcomes including depression, hypertension, autoimmune diseases, lung cancer, and premature mortality Effects of early childhood trauma on brain development includes a negative impact on emotional regulation and impairment of development of social skills. Research has shown that children raised in traumatic or risky family environments tend to have excessive internalizing (e.g., social withdrawal, anxiety) or externalizing (e.g., aggressive behavior), and suicidal behavior. Recent research has found that physical and sexual abuse are associated with mood and anxiety disorders in adulthood, while personality disorders and schizophrenia are linked with emotional abuse as adults.

About half of those with schizophrenia use drugs or alcohol excessively.

Amphetamine, cocaine, and to a lesser extent alcohol, can result in a transient stimulant psychosis or alcohol-related psychosis that presents very similarly to schizophrenia. Although it is not generally believed to be a cause of the illness, people with schizophrenia use nicotine at much higher rates than the general population.

Alcohol abuse can occasionally cause the development of a chronic, substance-induced psychotic disorder via a kindling mechanism. Alcohol use is not associated with an earlier onset of psychosis.

Cannabis can be a contributory factor in schizophrenia, potentially causing the disease in those who are already at risk. The increased risk may require the presence of certain genes within an individual or may be related to preexisting psychopathology. Early exposure is strongly associated with an increased risk. The size of the increased risk is not clear, but appears to be in the range of two to three times greater for psychosis. Higher dosage and greater frequency of use are indicators of increased risk of chronic psychoses.

Other drugs may be used only as coping mechanisms by individuals who have schizophrenia, to deal with depression, anxiety, boredom, and loneliness.

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.

While the cause of conduct disorder is complicated by an intricate interplay of biological and environmental factors, identifying underlying mechanisms is crucial for obtaining accurate assessment and implementing effective treatment. These mechanisms serve as the fundamental building blocks on which evidence-based treatments are developed. Despite the complexities, several domains have been implicated in the development of conduct disorder including cognitive variables, neurological factors, intraindividual factors, familial and peer influences, and wider contextual factors. These factors may also vary based on the age of onset, with different variables related to early (e.g., neurodevelopmental basis) and adolescent (e.g., social/peer relationships) onset.

The development of conduct disorder is not immutable or predetermined. A number of interactive risk and protective factors exist that can influence and change outcomes, and in most cases conduct disorder develops due to an interaction and gradual accumulation of risk factors. In addition to the risk factors identified under cause, several other variables place youth at increased risk for developing the disorder, including child physical abuse and prenatal alcohol abuse and maternal smoking during pregnancy. Protective factors have also been identified, and most notably include high IQ, being female, positive social orientations, good coping skills, and supportive family and community relationships.

However, a mere correlation between a particular risk factor and a later developmental outcome (such as conduct disorder) cannot be taken as definitive evidence for a causal link. Co-variation between two variables can arise, for instance, if they represent age-specific expressions of similar underlying genetic factors. For example, the tendency to smoke during pregnancy (SDP) is subject to substantial genetic influence (D'Onofrio et al., 2007), as is conduct disorder. Thus, the genes that dispose the mother to SDP may also dispose the child to CD following mitotic transmission. Indeed, Rice et al. (2009) found that in mother-fetus pairs that were not genetically related (by virtue of in-vitro fertilisation), no link between SDP and later conduct problems arose. Thus, the distinction between causality and correlation is an important consideration.

Individual differences in women's estrogen cycles may be related to the expression of BPD symptoms in female patients. A 2003 study found that women's BPD symptoms were predicted by changes in estrogen levels throughout their menstrual cycles, an effect that remained significant when the results were controlled for a general increase in negative affect.

Schizophrenia disorders in children are rare. Boys are twice as likely to be diagnosed with childhood schizophrenia. There is often an disproportionately large number of males with childhood schizophrenia, because the age of onset of the disorder is earlier in males than females by about 5 years. People have been and still are reluctant to diagnose schizophrenia early on, primarily due to the stigma attached to it.

While very early-onset schizophrenia is a rare event, with prevalence of about 1:10,000, early-onset schizophrenia is manifests more often with an estimated prevalence of 0.5 %.

Psychotherapy, more specifically, cognitive behavioral therapy (CBT), is the most widely used form of treatment for Somatic symptom disorder. In 2016, a randomized 12-week study suggested steady and significant improvement in health anxiety measures with cognitive behavioral therapy compared to the control group.

CBT can help in some of the following ways:

- Learn to reduce stress

- Learn to cope with physical symptoms

- Learn to deal with depression and other psychological issues

- Improve quality of life

- Reduce preoccupation with symptom

Moreover, brief psychodynamic interpersonal psychotherapy (PIT) for patients with multisomatoform disorder has shown its long-term efficacy for improving the physical quality of life in patients with multiple, difficult-to-treat, medically unexplained symptoms.

Antidepressant medication has also been used to treat some of the symptoms of depression and anxiety that are common among people who have somatic symptom disorder. Medications will not cure somatic symptom disorder, but can help the treatment process when combined with CBT.

In the opinion of Allen Frances, chair of the DSM-IV task force, the DSM-5's somatic symptom disorder brings with it a risk of mislabeling a sizable proportion of the population as mentally ill. “Millions of people could be mislabeled, with the burden falling disproportionately on women, because they are more likely to be casually dismissed as ‘catastrophizers’ when presenting with physical symptoms.”

There is no known single cause or causes of schizophrenia, however, it is a heritable disorder.

Several environmental factors, including perinatal complications and prenatal maternal infections could cause schizophrenia. These factors in a greater severity or frequency could result in an earlier onset of schizophrenia. Maybe a genetic predisposition is a important factor too, familial illness reported for childhood-onset schizophrenic patients.

There is a strong correlation between child abuse, especially child sexual abuse, and development of BPD. Many individuals with BPD report a history of abuse and neglect as young children, but causation is still debated. Patients with BPD have been found to be significantly more likely to report having been verbally, emotionally, physically, or sexually abused by caregivers of either gender. They also report a high incidence of incest and loss of caregivers in early childhood. Individuals with BPD were also likely to report having caregivers of both sexes deny the validity of their thoughts and feelings. Caregivers were also reported to have failed to provide needed protection and to have neglected their child's physical care. Parents of both sexes were typically reported to have withdrawn from the child emotionally and to have treated the child inconsistently. Additionally, women with BPD who reported a previous history of neglect by a female caregiver and abuse by a male caregiver were significantly more likely to experience sexual abuse by a non-caregiver.

It has been suggested that children who experience chronic early maltreatment and attachment difficulties may go on to develop borderline personality disorder. Writing in the psychoanalytic tradition, Otto Kernberg argues that a child's failure to achieve the developmental task of and failure to overcome splitting might increase the risk of developing a borderline personality. A child's inability to tolerate delayed gratification at age four does not predict later development of BPD.

Trichotillomania is an impulse control disorder which causes an individual to pull out their hair from various parts of their body without a purpose. The cause for trichotillomania remains unknown. Like OCD trichotillomania isn’t a nervous condition but stress can trigger this habit. For some people pulling their hair out of boredom is normal, but that isn’t the case for someone that is dealing with trichotillomania. Emotions do not affect the behavior but these behaviors are more prevalent in those that suffer with depression.

Estimates of prevalence and gender distribution have varied widely via discrepancies in diagnosis and reporting. In American psychiatry, BDD gained diagnostic criteria in the "DSM-IV", but clinicians' knowledge of it, especially among general practitioners, is constricted. Meanwhile, shame about having the bodily concern, and fear of the stigma of vanity, makes many hide even having the concern.

Via shared symptoms, BDD is commonly misdiagnosed as social-anxiety disorder, obsessive-compulsive disorder, major depressive disorder, or social phobia. Correct diagnosis can depend on specialized questioning and correlation with emotional distress or social dysfunction. Estimates place the Body Dysmorphic Disorder Questionnaire's sensitivity at 100% (0% false negatives) and specificity at 92.5% (7.5% false positives).

The obsessive–compulsive spectrum is a model of medical classification where various psychiatric, neurological and/or medical conditions are described as existing on a spectrum of conditions related to obsessive–compulsive disorder (OCD). "The disorders are thought to lie on a spectrum from impulsive to compulsive where impulsivity is said to persist due to deficits in the ability to inhibit repetitive behavior with known negative consequences, while compulsivity persists as a consequence of deficits in recognizing completion of tasks." OCD is a mental disorder characterized by obsessions and/or compulsions. An obsession is defined as "a recurring thought, image, or urge that the individual cannot control". Compulsion can be described as a "ritualistic behavior that the person feels compelled to perform". The model suggests that many conditions overlap with OCD in symptomatic profile, demographics, family history, neurobiology, comorbidity, clinical course and response to various pharmacotherapies. Conditions described as being on the spectrum are sometimes referred to as "obsessive–compulsive spectrum disorders".

Intellectual disability in children can be caused by genetic or environmental factors. The individual could have a natural brain malformation or pre or postnatal damage done to the brain caused by drowning or a traumatic brain injury, for example. Nearly 30 to 50% of individuals with intellectual disability will never know the cause of their diagnosis even after thorough investigation.

Prenatal causes of intellectual disability include:

- Congenital infections such as cytomegalovirus, toxoplasmosis, herpes, syphilis, rubella and human immunodeficiency virus

- Prolonged maternal fever in the first trimester

- Exposure to anticonvulsants or alcohol

- Untreated maternal phenylketonuria (PKU)

- Complications of prematurity, especially in extremely low-birth-weight infants

- Postnatal exposure to lead

Single-gene disorders that result in intellectual disability include:

- Fragile X syndrome

- Neurofibromatosis

- Tuberous sclerosis

- Noonan's syndrome

- Cornelia de Lange's syndrome

These single-gene disorders are usually associated with atypical physical characteristics.

About 1/4 of individuals with intellectual disability have a detectable chromosomal abnormality. Others may have small amounts of deletion or duplication of chromosomes, which may go unnoticed and therefore, undetermined.