In Canada, dual diagnosis usually refers to an individual with a mental illness and a co-occurring developmental disability.1
An individual with a developmental disability has significantly below average intellectual functioning, which is also accompanied by considerable limitations in their adaptive functioning or life skills. People with persistent developmental disorders (such as autism) are often included under the umbrella of developmental disabilities, even if their IQ approaches average. Approximately 1 to 3 percent of Canadians have a developmental disability.2
Prevalence rates for dual diagnosis are difficult to identify because studies apply differing criteria in defining developmental disabilities and mental health issues. For example, definitions of developmental disability often use varying criteria in terms of severity of the disability or IQ level, and similarly, there is an inconsistency in how mental health problems are defined in this population. It is generally agreed, however, that individuals with developmental disabilities are three to four times more likely to develop emotional, behavioural and psychiatric difficulties than the general population.3
1In the United States, however, and sometimes even in Canada, dual diagnosis is the term used to describe concurrent disorder, the co-occurrence of substance use and mental illness. For more information, see Concurrent Disorders.
2Centre for Addiction and Mental Health, “Common Questions about Dual Diagnosis,” Journal of Addiction and Mental Health, May-June 2002, 13.