Risk factors for mental illness include genetic inheritance, such as parents having depression, or a propensity for high neuroticism or "emotional instability".

In depression, parenting risk factors include parental unequal treatment, and there is association with high cannabis use.

In schizophrenia and psychosis, risk factors include migration and discrimination, childhood trauma, bereavement or separation in families, and abuse of drugs, including cannabis, and urbanicity.

In anxiety, risk factors may include family history (e.g. of anxiety), temperament and attitudes (e.g. pessimism), and parenting factors including parental rejection, lack of parental warmth, high hostility, harsh discipline, high maternal negative affect, anxious childrearing, modelling of dysfunctional and drug-abusing behaviour, and child abuse (emotional, physical and sexual).

Environmental events surrounding pregnancy and birth have also been implicated. Traumatic brain injury may increase the risk of developing certain mental disorders. There have been some tentative inconsistent links found to certain viral infections, to substance misuse, and to general physical health.

Social influences have been found to be important, including abuse, neglect, bullying, social stress, traumatic events and other negative or overwhelming life experiences. For bipolar disorder, stress (such as childhood adversity) is not a specific cause, but does place genetically and biologically vulnerable individuals at risk for a more severe course of illness. The specific risks and pathways to particular disorders are less clear, however. Aspects of the wider community have also been implicated, including employment problems, socioeconomic inequality, lack of social cohesion, problems linked to migration, and features of particular societies and cultures.

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.

Correlations of mental disorders with drug use include cannabis, alcohol and caffeine, use of which appears to promote anxiety. For psychosis and schizophrenia, usage of a number of drugs has been associated with development of the disorder, including cannabis, cocaine, and amphetamines. There has been debate regarding the relationship between usage of cannabis and bipolar disorder.

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.

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.

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.

Another theory is that there may be shared risk factors that can lead to both substance abuse and mental illness. Mueser hypothesizes that these may include factors such as social isolation, poverty, lack of structured daily activity, lack of adult role responsibility, living in areas with high drug availability, and association with people who already misuse drugs.

Other evidence suggests that traumatic life events, such as sexual abuse, are associated with the development of psychiatric problems and substance abuse.

As it has already been mentioned, the organic personality disorder is included in a wide group of personality and behavioural disorders. This mental health disorder can be caused by disease, brain damages or dysfunctions in specific brain areas in frontal lobe. The most common reason for this profound change in personality is the traumatic brain injury (TBI). Children, whose brain areas have injured or damaged, may present Attention Deficit Hyperactivity Disorder (ADHD), oppositional defiant disorder (ODD) and organic personality disorder. Moreover, this disorder is characterised as "frontal lobe syndrome". This characteristic name shows that the organic personality disorder can usually be caused by lesions in three brain areas of frontal lobe. Specifically, the symptoms of organic personality disorder can also be caused by traumatic brain injuries in orbitofrontal cortex, anterior cingulate cortex and dorsolateral prefrontal cortex. It is worth to be mentioned that organic personality disorder may also be caused by lesions in other circumscribed brain areas.

The scientific study of the causes of developmental disorders involves many different theories. Some of the major differences between these theories involves whether or not environment disrupts normal development, or if abnormalities are pre-determined.

Normal development occurs with a combination of contributions from both the environment and genetics. The theories vary in the part each factor has to play in normal development, thus affecting how the abnormalities are caused.

One theory that supports environmental causes of developmental disorders involves stress in early childhood. Researcher and child psychiatrist Bruce D. Perry, M.D., Ph.D, theorizes that developmental disorders can be caused by early childhood traumatization. In his works he compares developmental disorders in traumatized children to adults with post-traumatic stress disorder, linking extreme environmental stress to the cause of developmental difficulties. Other stress theories suggest that even small stresses can accumulate to result in emotional, behavioral, or social disorders in children.

A 2017 study tested all 20,000 genes in about 4,300 families with children with rare developmental difficulties in the UK and Ireland in order to identify if these difficulties had a genetic cause.They found 14 new developmental disorders caused by spontaneous genetic mutations not found in either parent (such as a fault in the CDK13 gene). They estimated that about one in 300 children are born with spontaneous genetic mutations associated with rare developmental disorders.

The alleviation of dysphoria theory suggests that people with severe mental illness commonly have a negative self-image, which makes them vulnerable to using psychoactive substances to alleviate these feelings. Despite the existence of a wide range of dysphoric feelings (anxiety, depression, boredom, and loneliness), the literature on self-reported reasons for use seems to lend support for the experience of these feelings being the primary motivator for drug and alcohol misuse.

Dependent personality disorder occurs in about 0.6% of the general population. The disorder is diagnosed more often in females than males; however, research suggests that this is largely due to behavioural differences in interviews and self-reporting rather than a difference in prevalence between the sexes. A 2004 twin study suggests a heritability of 0.81 for developing dependent personality disorder. Because of this, there is significant evidence that this disorder runs in families. Children and adolescents with a history of anxiety disorders and physical illnesses are more susceptible to acquiring this disorder.

Avolition, as a symptom of various forms of psychopathology, is the decrease in the motivation to initiate and perform self-directed purposeful activities. Such activities that appear to be neglected usually include routine activities, including hobbies, going to work and/or school, and most notably, engaging in social activities. A person experiencing avolition may stay at home for long periods of time, rather than seeking out work or peer relations.

Organic personality disorder (OPD) is not included in the wide variety of group of personality disorders. For this reason, the symptoms and diagnostic criteria of the organic personality disorder are different from those of the mental health disorders, which are included in this various group of personality disorders. According to the Tenth Revision of the International Statistical Classification of Diseases and Related Health Problems ICD-10, it defines the organic personality disorder as the personality change, which can be caused by traumatic brain injury (TBI) that means there are specific brain areas of patients, which have been injured after a very strong accident. Moreover, according to the ICD-10, the organic personality disorder is associated with a "significant alteration of the habitual patterns of premorbid behaviour". Furthermore, organic personality disorder is associated with "personality change due to general medical condition". There are crucial influences on emotions, impulses and personal needs because of this disorder. Thus, all these definitions about the organic personality disorder support that this type of disorder is associated with changes in personality and behaviour.

Implications from avolition often result in social deficits. Not being able to initiate and perform purposeful activities can have many implications for a person with avolition. By disrupting interactions with both familiar and unfamiliar people, it jeopardizes the patient's social relations. When part of a severe mental illness, avolition has been reported, in first person accounts, to lead to physical and mental inability to both initiate and maintain relationships, as well as work, eat, drink or even sleep.

Clinically, it may be difficult to engage an individual experiencing avolition in active participation of psychotherapy. Patients are also faced with the stresses of coping with and accepting a mental illness and the stigma that often accompanies such a diagnosis and its symptoms. Regarding schizophrenia, the American Psychiatric Association reported in 2013 that there currently are "no treatments with proven efficacy for primary negative symptoms" (such as avolition). Together with schizophrenia's chronic nature, such facts added to the outlook of never getting well, might further implicate feelings of hopelessness and similar in patients as well as their friends and family.

Attention deficit hyperactivity disorder is a neurodevelopmental disorder that is characterised by significant levels of hyperactivity, inattentiveness, and impulsiveness. There are three subtypes of ADHD: predominantly inattentive, predominantly hyperactive, and combined (which presents as both hyperactive and inattentive subtypes).

Prevalence estimates for muscle dysmorphia have greatly varied, ranging from 1% to 54% of men. Samples of gym members, weightlifters, and bodybuilders show higher prevalence than do samples from the general population. Rates even higher have been found in users of anabolic steroids. The disorder is rare in women but does occur, especially in sexual-assault victims. Crossing cultures, muscle dysmorphia is known to occur in China, South Africa, and Latin America. Yet this may be mediated substantially by exposure to Western ideals of muscularity, as populations less exposed tend to have lower prevalence.

Traumatic grief or complicated mourning are conditions where both trauma and grief coincide. There are conceptual links between trauma and bereavement since loss of a loved one is inherently traumatic. If a traumatic event was life-threatening, but did not result in death, then it is more likely that the survivor will experience post-traumatic stress symptoms. If a person dies, and the survivor was close to the person who died, then it is more likely that symptoms of grief will also develop. When the death is of a loved one, and was sudden or violent, then both symptoms often coincide. This is likely in children exposed to community violence.

For C-PTSD to manifest, the violence would occur under conditions of captivity, loss of control and disempowerment, coinciding with the death of a friend or loved one in life-threatening circumstances. This again is most likely for children and stepchildren who experience prolonged domestic or chronic community violence that ultimately results in the death of friends and loved ones. The phenomenon of the increased risk of violence and death of stepchildren is referred to as the Cinderella effect.

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.

Dependent personality disorder (DPD), formerly known as asthenic personality disorder, is a personality disorder that is characterized by a pervasive psychological dependence on other people. This personality disorder is a long-term condition in which people depend on others to meet their emotional and physical needs, with only a minority achieving normal levels of independence.

The cause of dependent personality disorder is unknown. A study in 2012 estimated that between 55% and 72% of the risk of the condition is inherited from one's parents. The difference between a "dependent personality" and a "dependent personality disorder" is somewhat subjective, which makes diagnosis sensitive to cultural influences such as gender role expectations.

Neurodevelopmental disorders are in their multitude associated with widely varying degrees of difficulty, depending on which there are different degrees of mental, emotional, physical, and economic consequences for individuals, and in turn families, groups and society.

Neurocognitive disorders can have numerous causes: genetics, brain trauma, stroke, and heart issues. The main causes are neurodegenerative diseases such as Alzheimer's disease, Parkinson's disease, and Huntington's disease because they affect or deteriorate brain functions. Other diseases and conditions that cause NDCs include vascular dementia, frontotemporal degeneration, Lewy body disease, prion disease, normal pressure hydrocephalus, and dementia/neurocognitive issues due to HIV infection. They may also include dementia due to substance abuse or exposure to toxins.

Neurocongnitive disorder may also be caused by brain trauma, including concussions and Traumatic Brain Injuries, as well as post-traumatic stress and alcoholism. This is referred to as amnesia, and is characterized by damage to major memory encoding parts of the brain such as the hippocampus. Difficulty creating recent term memories is called anterograde amnesia and is caused by damage to the hippocampus part of the brain, which is a major part of the memory process. Retrograde amnesia is also caused by damage to the hippocampus, but the memories that were encoded or in the process of being encoded in long term memory are erased

Functional neurological symptom disorder can mimic many other conditions. Some alternative diagnoses for FND include:

- Hemiplegic migraine

- Multiple sclerosis

- Motor neurone disease

- Parkinson's

- Autoimmune disorders

- Ehlers–Danlos syndrome

- Stroke

- Vitamin B12 deficiency or pernicious anaemia

- Myasthenia gravis

Dissociative disorder not otherwise specified (DDNOS) is a mental health diagnosis for pathological dissociation that matches the DSM-5 criteria for a dissociative disorder, but does not fit the full criteria for any of the specifically identified subtypes, which include dissociative identity disorder, dissociative amnesia, and depersonalization/derealization disorder. The International Statistical Classification of Diseases and Related Health Problems (ICD-10) refers to the diagnosis as "Other dissociative and conversion disorders".

Examples of DDNOS include chronic and recurrent syndromes of mixed dissociative symptoms, identity disturbance due to prolonged and intense coercive persuasion, disorders similar to dissociative identity disorder, acute dissociative reactions to stressful events, and dissociative trance.

DDNOS is the most common dissociative disorder and is diagnosed in 40% of dissociative disorder cases. It is often co-morbid with other mental illnesses such as complex posttraumatic stress disorder, major depressive disorder, generalized anxiety disorder, personality disorders, substance abuse disorders and eating disorders.