What is the difference between mental illness, developmental disability, and intellectual disability?

Intellectual Disabilities and Physical Disabilities, both fall under the umbrella term of Developmental Disabilities. These are chronic conditions that appear at birth or in childhood, but certainly before age 22, and sometimes, but not always, occur together. Cerebral Palsy, for example is a physical disability, which in and of itself does not effect intellectual functioning, though 20-30% of individuals with Cerebral Palsy also have a cognitive disability. Down Syndrome is the most common cause of intellectual disability, though levels of impairment vary widely.

In order for someone to be diagnosed with an Intellectual Disability, the person must have a well below average IQ (70 points or lower) and have severe limitations on daily functioning skills, which include Conceptual skills, such as language and literacy; Social skills, such as social responsibility and problem solving; and Practical skills, such as personal care, use of money, and occupational skills.

Mental illness, also known as mental health disorder or behavioral health disorder, is not the same as Intellectual Disability. Mental health disorders affect mood, thought processes or behavior and can manifest in anyone at any time in their life. Mental Illness does not directly impact cognitive abilities, but can change a person’s perceptions and thought processes and affect a person’s everyday functioning and ability to relate to others. When mental illness and intellectual disability occur together, the descriptive term used is “dual diagnosis.”

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