In addition to genetics, some environmental factors might play a role in causing ADHD. Alcohol intake during pregnancy can cause fetal alcohol spectrum disorders which can include ADHD or symptoms like it. Children exposed to certain toxic substances, such as lead or polychlorinated biphenyls, may develop problems which resemble ADHD. Exposure to the organophosphate insecticides chlorpyrifos and dialkyl phosphate is associated with an increased risk; however, the evidence is not conclusive. Exposure to tobacco smoke during pregnancy can cause problems with central nervous system development and can increase the risk of ADHD.

Extreme premature birth, very low birth weight, and extreme neglect, abuse, or social deprivation also increase the risk as do certain infections during pregnancy, at birth, and in early childhood. These infections include, among others, various viruses (measles, varicella zoster encephalitis, rubella, enterovirus 71). There is an association between long term but not short term use of acetaminophen during pregnancy and ADHD. At least 30% of children with a traumatic brain injury later develop ADHD and about 5% of cases are due to brain damage.

Some studies suggest that in a small number of children, artificial food dyes or preservatives may be associated with an increased prevalence of ADHD or ADHD-like symptoms but the evidence is weak and may only apply to children with food sensitivities. The United Kingdom and the European Union have put in place regulatory measures based on these concerns. In a minority of children, intolerances or allergies to certain foods may worsen ADHD symptoms.

Research does not support popular beliefs that ADHD is caused by eating too much refined sugar, watching too much television, parenting, poverty or family chaos; however, they might worsen ADHD symptoms in certain people.

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.

An 8-year follow up of children diagnosed with ADHD (combined type) found that they often have difficulties in adolescence, regardless of treatment or lack thereof. In the USA, fewer than 5% of individuals with ADHD get a college degree, compared to 28% of the general population aged 25 years and older. The proportion of children meeting criteria for ADHD drops by about half in the three years following the diagnosis and this occurs regardless of treatments used. ADHD persists into adulthood in about 30–50% of cases. Those affected are likely to develop coping mechanisms as they mature, thus compensating to some extent for their previous symptoms. Children with ADHD have a higher risk of unintentional injuries, and greater mortality rates.

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

Externalizing disorders are frequently comorbid or co-occurring with other disorders. Individuals who have the co-occurrence of more than one externalizing disorder have homotypic comorbidity, whereas individuals who have co-occurring externalizing and Internalizing disorders have heterotypic comorbidity. It is not uncommon for children with early externalizing problems to develop both internalizing and further externalizing problems across the lifespan.

Impulsive behavior, and especially impulsive violence predisposition has been correlated to a low brain serotonin turnover rate, indicated by a low concentration of 5-hydroxyindoleacetic acid (5-HIAA) in the cerebrospinal fluid (CSF). This substrate appears to act on the suprachiasmatic nucleus in the hypothalamus, which is the target for serotonergic output from the dorsal and median raphe nuclei playing a role in maintaining the circadian rhythm and regulation of blood sugar. A tendency towards low 5-HIAA may be hereditary. A putative hereditary component to low CSF 5-HIAA and concordantly possibly to impulsive violence has been proposed. Other traits that correlate with IED are low vagal tone and increased insulin secretion. A suggested explanation for IED is a polymorphism of the gene for tryptophan hydroxylase, which produces a serotonin precursor; this genotype is found more commonly in individuals with impulsive behavior.

IED may also be associated with lesions in the prefrontal cortex, with damage to these areas, including the amygdala, increasing the incidence of impulsive and aggressive behavior and the inability to predict the outcomes of an individual's own actions. Lesions in these areas are also associated with improper blood sugar control, leading to decreased brain function in these areas, which are associated with planning and decision making. A national sample in the United States estimated that 16 million Americans may fit the criteria for IED.

ADHD often precedes the onset of ODD, and approximately half of children with ADHD, Combined Type also have ODD. ODD is a risk factor for CD and frequently precedes the onset of CD symptoms. Children with an early onset of CD symptoms, with at least one symptom before age 10 years, are at risk for more severe and persistent antisocial behavior continuing into adulthood. Youth with early-onset conduct problems are particularly at risk for ASPD (note that an onset of CD prior to age 15 is part of the diagnostic criteria for ASPD), whereas CD is typically limited to adolescence when youth's CD symptoms begin during adolescence.

Socioeconomic status has also been looked at as a potential cause for personality disorders. There is a strong association with low parental/neighborhood socioeconomic status and personality disorder symptoms. In a recent study comparing parental socioeconomic status and a child's personality, it was seen that children who were from higher socioeconomic backgrounds were more altuistic, less risk seeking, and had overall higher IQs. These traits correlate with a low risk of developing personality disorders later on in life. In a study looking at female children who were detained for disciplinary actions found that psychological problems were most negatively associated with socioeconomic problems. Furthermore, social disorganization was found to be inversely correlated with personality disorder symptoms.

Two epidemiological studies of community samples approximated the lifetime prevalence of IED to be 4–6%, depending on the criteria set used. A Ukrainian study found comparable rates of lifetime IED (4.2%), suggesting that a lifetime prevalence of IED of 4–6% is not limited to American samples. One-month and one-year point prevalences of IED in these studies were reported as 2.0% and 2.7%, respectively. Extrapolating to the national level, 16.2 million Americans would have IED during their lifetimes and as many as 10.5 million in any year and 6 million in any month.

Among a "clinical" population, a 2005 study found the lifetime prevalence of IED to be 6.3%.

Prevalence appears to be higher in men than in women.

Of US subjects with IED, 67.8% had engaged in direct interpersonal aggression, 20.9% in threatened interpersonal aggression, and 11.4% in aggression against objects. Subjects reported engaging in 27.8 high-severity aggressive acts during their worst year, with 2–3 outbursts requiring medical attention. Across the lifespan, the mean value of property damage due to aggressive outbursts was $1603.

A study in the March 2016 "Journal of Clinical Psychiatry" suggests a relationship between infection with the parasite "Toxoplasma gondii" and psychiatric aggression such as IED.

Currently, genetic research for the understanding of the development of personality disorders is severely lacking. However, there are a few possible risk factors currently in discovery. Researchers are currently looking into genetic mechanisms for traits such as aggression, fear and anxiety, which are associated with diagnosed individuals. More research is being conducted into disorder specific mechanisms.

The rate in school age children is thought to be about 1.5%, compared with an estimated 5.3% for ADHD.

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.

Once the patient and family have been educated about the nature, management and treatment of the disorder and a decision has been made to treat, the European ADHD Guidelines group recommends medication rather than behavioral training as the first treatment approach; and the UK's National Institute for Health and Clinical Excellence recommends medication as first line treatment for those with hyperkinesis/severe ADHD, and the provision of group parent-training in all cases of ADHD.

Complications of late Parkinson's disease may include a range of impulse-control disorders, including eating, buying, compulsive gambling and sexual behavior. One study found that 13.6% of Parkinson's patients exhibited at least one form of ICD. There is a significant co-occurrence of pathological gambling and personality disorder, and is suggested to be caused partly by their common "genetic vulnerability". The degree of heritability to ICD is similar to other psychiatric disorders including substance abuse disorder. There has also been found a genetic factor to the development of ICD just as there is for substance abuse disorder. The risk for subclinical PG in a population is accounted for by the risk of alcohol dependence by about 12-20% genetic and 3-8% environmental factors. There is a high rate of comorbidity between ADHD and other impulse-control disorders.

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.

An internalizing disorder is one type of emotional and behavioral disorder, along with externalizing disorders, and low incidence disorders. One who suffers from an internalizing disorder will keep their problems to themselves, or internalize the problems.

Kleptomania is characterized by an impulsive urge to steal purely for the sake of gratification.

In the U.S. the presence of kleptomania is unknown but has been estimated at 6 per 1000 individuals. Kleptomania is also thought to be the cause of 5% of annual shoplifting in the U.S. If true, 100,000 arrests are made in the U.S. annually due to kleptomaniac behavior.

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.

Some treatments for internalizing disorders include antidepressants, electroconvulsive therapy, and psychotherapy.

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.

The prevalence of excoriation disorder is not well understood.

Estimates of prevalence of the condition range from 1.4 to 5.4% in the general population. One U.S. telephone survey found that 16.6% of respondents "picked their skin to the point of noticeable tissue damage" and that 1.4% would qualify as meeting the requirements of excoriation disorder. Another community survey found a rate of 5.4% had excoriation disorder. A survey of college students found a rate of 4%. One study found that among non-disabled adults, 63% of individuals engaged in some form of skin picking and 5.4% engaged in serious skin picking. Lastly, a survey of dermatology patients found that 2% suffered from excoriation disorder.

In some patients excoriation disorder begins with the onset of acne in adolescence, but the compulsion continues even after the acne has gone away. Skin conditions such as keratosis pilaris, psoriasis, and eczema can also provoke the behavior. In patients with acne, the grooming of the skin is disproportionate to the severity of the acne. Certain stressful events including marital conflicts, deaths of friends or family, and unwanted pregnancies have been linked to the onset of the condition. If excoriation disorder does not occur during adolescence another common age of onset is between the ages of 30 to 45. Additionally, many cases of excoriation disorder have been documented to begin in children under the age of 10. One small survey of patients with excoriation disorder found that 47.5% of them had an early onset of excoriation disorder that began before age 10. Traumatic childhood events may initiate the behavior.

Excoriation disorder is statistically more common in females than in males.

Excoriation disorder has a high rate of comorbidity with other psychiatric conditions, especially with mood and anxiety disorders . One survey of patients with excoriation disorder found that 56.7% also had a DSM-IV Axis-I disorder and 38% had alcohol- or drug-abuse problems. Studies have shown the following rates of psychiatric conditions found in patients with excoriation disorder: trichotillomania (38.3%), substance abuse (38%), major depressive disorder (approximately 31.7% to 58.1%), anxiety disorders (approximately 23% to 56%), obsessive-compulsive disorder (approximately 16.7% to 68%), and body dysmorphic disorder (approximately 26.8% to 44.9%). There are also higher rates of excoriation disorder in patients in psychiatric facilities; a study of adolescent psychiatric inpatients found that excoriation disorder was present in 11.8% of patients. It is also present at high rates with some other conditions: 44.9% of patients with body dysmorphic disorder also have excoriation disorder; 8.9% of patients with OCD have excoriation disorder; and 8.3% of patients with trichotillomania have excoriation disorder.

Skin picking is also common in those with certain developmental disabilities; for example, Prader–Willi syndrome and Smith–Magenis syndrome. Studies have shown that 85% of people with Prader–Willi syndrome also engage in skin-picking. Children with developmental disabilities are also at an increased risk for excoriation disorder.

Excoriation disorder also correlates with "social, occupational, and academic impairments, increased medical and mental health concerns (including anxiety, depression, obsessive–compulsive disorder) ... and financial burden". Excoriation disorder also has a high degree of comorbidity with occupational and marital difficulties.

Substance abuse is often present, and individuals with excoriation disorder are twice as likely to have first-degree relatives who have substance abuse disorders than those without the condition.

Some cases of body-focused repetitive behaviors also suggest a hereditary factor.

There are a number of factors that could potentially contribute to the development of feeding and eating disorders of infancy or early childhood. These factors include:

- Physiological – a chemical imbalance effecting the child's appetite could cause a feeding or eating disorder.

- Developmental – developmental abnormalities in oral-sensory, oral-motor, and swallowing can impact the child's eating ability and elicit a feeding or eating disorder.

- Environmental – simple issues such as inconsistent meal times can cause a feeding or eating disorder. Giving the child food that they are not developmentally acquired for can also cause these disorders. Family dysfunction and sociocultural issues could also play a role in feeding or eating disorders.

- Relational – when the child is not securely attached to the mother, it can cause feeding interactions to become disturbed or unnatural. Other factors, such as parental emotional unavailability and parental eating disorders, can cause feeding and eating disorders in their children.

- Psychological and behavioral – these factors include one involving the child's temperament. Characteristics such as being anxious, impulsive, distracted, or strong-willed personality types are ones that could affect the child's eating and cause a disorder. The individual could have learned to reject food due to a traumatic experience such as choking or being force fed.

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.

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).

There have been many different theories regarding the causes of excoriation disorder including biological and environmental factors.

A common hypothesis is that excoriation disorder is often a coping mechanism to deal with elevated levels of turmoil, arousal or stress within the individual, and that the individual has an impaired stress response. A review of behavioral studies found support in this hypothesis in that skin-picking appears to be maintained by automatic reinforcement within the individual.

In contrast to neurological theories, there are some psychologists who believe that picking behavior can be a result of repressed rage felt toward authoritarian parents. A similar theory holds that overbearing parents can cause the behavior to develop in their children.