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Dual diagnosis: Overview

The term dual diagnosis is used to describe a situation where someone has both a developmental disability (also called intellectual disability) and a mental health problem.


People with developmental disabilities are at increased risk for developing mental health problems throughout their lives. 

  • 1–3 per cent of the population has a developmental disability.
  • 30–40 per cent of people with a developmental disability develop a psychiatric disorder or mental health problem that requires treatment.

Estimated rates of co-occurring developmental disability and psychiatric disorder vary depending on the types of problems included as the "second" diagnosis; however, it is clear that many people with developmental disabilities have serious mental health problems:

  • Psychotic disorders have been suggested to occur three times more often in adults with developmental disabilities compared to the general population (Cooper et al., 2007; Turner, 1989). Rates of psychotic disorder may be inflated in some studies due to challenges in making the diagnosis, particularly by clinicians with less training in developmental disabilities (Lunsky et al., 2006; Robertson et al., 2000).
  • One large study in the U.K. reported that mood disorders (e.g., major depression, bipolar disorder, dysthymia) were three times more common in people with developmental disabilities than in the general population (Richards et al., 2001). Comparative studies have also reported higher rates in individuals with developmental disabilities than in the general population.
  • Anxiety disorders (e.g., posttraumatic stress disorder, separation anxiety, social phobia, generalized anxiety disorder) are common in people with developmental disabilities, particularly those with specific genetic syndromes (Harris, 2006).

Challenging behaviour in developmental disability

Challenging behaviour, sometimes called "disruptive behaviour" or "problem behaviour," refers to behaviour that may deny or limit the person's access to ordinary community facilities, or behaviour that, because of its intensity, frequency or duration, puts the physical safety of the person or others at risk (Emerson & Emerson, 1987).

  • This behaviour may be due to an underlying psychiatric disorder, but it may also have other biological, psychological or social causes.
  • The overall prevalence of disruptive behaviour in adults with developmental disabilities has been estimated at 22.5 per cent (Cooper et al., 2007).

Relationship between developmental disability and mental health problems 

The relationship between developmental disabilities and mental health problems is complex. Mental health problems present in unique ways in people with developmental disabilities, and how they present can change over time.

Several factors have been suggested.

Biological factors


Some genetic disorders that cause developmental disability can predispose a person to having specific mental health problems. For example, someone who has fragile X syndrome is at increased risk for developing social anxiety, and someone with 22q11 deletion is at increased risk for developing schizophrenia.

At least nine genetic conditions have been shown to overlap with mental health problems and challenging behaviours (Dykens et al., 2000).

Brain chemistry

Research indicates that chemical processes in the brain are involved in the development of mental health problems. Recent research has also pointed to abnormalities in brain structure as a possible factor in the development of mental health problems, particularly schizophrenia. Because people with developmental disabilities have physical brain structures and chemical processes that differ from those in a neurotypically developing person, they may be predisposed to having mental health problems.

Medical vulnerabilities

Individuals with developmental disabilities have a higher incidence of medical conditions compared to the general population (Kerr, 2006). Medical problems are often missed in people with developmental disabilities either because of communication impairments or because they are misattributed as "behaviour problems."

Seizures are more common among people who have developmental disabilities than in the general population. There is a complex relationship between epilepsy and mental health problems among people with developmental disabilities that may reflect the underlying brain disorder causing the seizures rather than the epilepsy itself (Deb & Hunter, 1991a, b). Seizure disorders are important to consider because they may explain a person's problematic behaviour.

Psychological factors

The temperament a person is born with (e.g., a tendency to internalize feelings) may increase a person's risk of developing mental health problems. Some psychological risk factors interact with life stressors and biological predispositions by reducing a person's ability to cope with stressors and be resilient. Psychological risk factors include:

  • poor social skills
  • poor coping and self-soothing skills
  • poor problem-solving skills
  • problems with communication
  • low self-esteem.


Although stress does not cause mental health problems, it can trigger them or make them worse. People with developmental disabilities experience a lot of stress in their lives and may have an even more difficult time coping with stress than do people in the general population.

Social factors

Negative life events have been tied to the development of mental health problems in individuals with developmental disabilities (Hulbert-Williams & Williams, 2008). Research suggests that early childhood trauma and losses, such as the death or separation of parents, or adult events, such as the death of a family member or loss of a job or day activity, can be precursors to a mental health problem. Conflicts with family members, residents or staff may be an important area of concern for individuals with dual diagnosis.

Other environmental risk factors include poverty and lack of social support. People with developmental disabilities are more likely to experience poverty and poorer housing conditions and live in high-crime areas compared to the general population (Robinson & Rathbone, 1999).

Physical, sexual and psychological abuse

Individuals with developmental disabilities are at increased risk for maltreatment and neglect. Those admitted to psychiatric hospitals, as well as those living in the community, should be screened for a history of maltreatment.

Transition from adolescence to adulthood

In the general population, the transition from adolescence to young adulthood (16–25 years) is a high-risk period for developing mental health problems. This risk is higher in young adults with developmental disabilities (Masi, 1998). Young adulthood is the most likely time for psychiatric hospitalization in this population (Lunsky & Balogh, 2010).

People with developmental disabilities are affected by the stresses of puberty, as well as by the specific stresses associated with transitioning from the child to adult service system, which often means losing school supports and not getting sufficient services. This transition may also be a period of increased stress on family and parents.

Evidence summary

Cooper, S., Smiley, E., Morrison, J., Williamson, A. & Allan, L. (2007). Mental ill-health in adults with intellectual disabilities: Prevalence and associated factors. British Journal of Psychiatry, 190(1), 27–35.

Deb, S. & Hunter, D. (1991a). Psychopathology of people with mental handicap and epilepsy: II. Psychiatric illness. British Journal of Psychiatry, 159, 826–830.

Deb, S. & Hunter, D. (1991b). Psychopathology of people with mental handicap and epilepsy: III. Personality disorder. British Journal of Psychiatry, 159, 830–834.

Dykens, E. M., Hodapp, R. M. & Finucane, B. M. (2000). Genetics and mental retardation syndromes: A new look at behavior and interventions. Baltimore, MD, US: Paul H Brookes.

Hulbert-Williams, L. & Hastings, R.P. (2008). Life events as a risk factor for psychological problems in individuals with intellectual disabilities: A critical review. Journal of Intellectual Disability Research, 52, 883–895.

Kerr, M. (2006). Improving the general health of people with intellectual disabilities. Directions in Psychiatry, 26, 235–240.

Lunsky, Y., Bradley, E., Durbin, J., Koegl, C., Canrinus, M. & Goering, P. (2006). The clinical profile and service needs of hospitalized adults with mental retardation and a psychiatric diagnosis. Psychiatric Services, 57(1), 77–83.

Masi, G. (1998). Psychiatric illness in mentally retarded adolescents: clinical features. Adolescence, 33, 425–434.

Robinson, E.G. & Rathbone, G.N. (1999). Impact of race, poverty, and ethnicity on services for persons with mental disabilities: Call for cultural competence. Mental Retardation, 37, 333–338.

Whiteman, N. & Roan-Yager, L. (2007). Building a Joyful Life with Your Child Who Has Special Needs. London, UK: Jessica Kingsley.

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