Intellectual disability is a broad concept encompassing various intellectual or cognitive deficits, including mental retardation (MR), deficits too mild to properly qualify as MR, various specific conditions (such as specific learning disability), and problems acquired later in life through acquired brain injuries or neurodegenerative diseases like dementia. Intellectual disabilities may appear at any age.
Intellectual disability is also increasingly being used as a synonym for the term mental retardation as used in standard medical references.[note 1]
Mental retardation 
Mental retardation, also known as general learning disability,' is a generalized disorder characterized by significantly impaired cognitive functioning and deficits in two or more adaptive behaviors that appears before adulthood. It has historically been defined as an Intelligence Quotient (IQ) score under 70, but the definition now includes both a component relating to mental functioning and one relating to individuals' functional skills in their environment, so IQ is not the only factor.
Although the clinical term[note 1] mental retardation is a subtype of intellectual disability the latter is now preferred by most advocates in most English-speaking countries as a euphemism for mental retardation and is replacing it in United States official documents following the passing of Rosa's Law on October 6, 2010. Some bodies are using developmental disability which also has an established wider meaning.
By contrast, people with cognitive impairment have, or had, normal IQ, but show confusion, forgetfulness and difficulty concentrating; cognitive impairment is typical of brain injuries, side effects from medications, and dementia.
Specific learning disability 
A specific learning disability is a classification including several disorders in which a person has difficulty learning in a typical manner, usually caused by an unknown factor or factors, but sometimes caused by stroke or other medical problems. Specific learning disabilities include dyslexia and dyspraxia and other disorders of psychological development. Unlike other forms of intellectual disability, it is not indicative of general intelligence level, and many experts consequently do not consider it to be a true intellectual disability. Rather, people with a specific learning disability have trouble performing specific types of cognitive skills or if taught in conventional ways. A specific learning disability cannot be cured or fixed, but the effects can be mitigated by the use of different learning strategies.
Individuals with specific learning disabilities face some challenges throughout life. Social support can be a crucial component for students with specific learning disabilities in the school system. With the right support and intervention, people with specific learning disabilities can succeed in school and be successful later in life. Conversely, many individuals who have struggled at school or who were considered 'lazy' or 'stupid' at school, may have had unrecognised specific learning disability instead of character flaws or low IQ.
Acquired brain injuries 
An acquired brain injury (ABI) is brain damage caused by events after birth, rather than as part of a genetic or congenital disorder. It usually affects cognitive, physical, emotional, social or independent functioning and can result from either traumatic brain injury or nontraumatic injury such as stroke, infection or substance abuse. Most definitions of ABI exclude neurodegenerative disorders.
People with a brain injury may have difficulty controlling, coordinating and communicating their thoughts and actions. They may or may not retain their intellectual abilities, depending on the type and extent of the injury. However, the intellectual abilities of a person with a brain injury are likely to be interfered with by the resulting thought coordination and communication difficulties, which can make it difficult for them to express themselves in a manner intelligible to others. This may give the impression of a damaged intelligence even in people with normal intellectual capacity.
Neurodegenerative diseases 
Neurodegeneration is the umbrella term for the progressive loss of structure or function of neurons, including death of neurons. Many neurodegenerative diseases including Parkinson’s, Alzheimer’s, and Huntington’s occur as a result of neurodegenerative processes. As research progresses, many similarities appear which relate these diseases to one another on a sub-cellular level. Discovering these similarities offers hope for therapeutic advances that could ameliorate many diseases simultaneously.
Dementia is a serious loss of cognitive ability in a previously unimpaired person, beyond what might be expected from normal aging. Both dementia and mental retardation are defined by neurologists as having an IQ that is two standard deviations below median (below about 70, when 100 is the median); the difference between these two classifications for intellectual disability is whether the low IQ represents a lifelong condition (mental retardation), or a condition that is acquired later (dementia).
Dementia may be static, the result of a unique global brain injury, or progressive, resulting in long-term decline due to damage or disease in the body. In the early stages of Alzheimer's disease, whose symptoms of dementia are called mild cognitive impairment, the person typically loses 8 to 10 IQ points per year, with the result that a person of previously normal intelligence usually becomes intellectually disabled in less than five years.
Although dementia is far more common in the geriatric population, it may occur in any stage of adulthood. The overwhelming factor emerging from genetic studies of the dementias and other central nervous system neurodegenerative conditions is abnormalities of protein handling.
- Standard diagnostic references include Diagnostic and Statistical Manual of Mental Disorders (DSM-IV) and International Statistical Classification of Diseases and Related Health Problems (ICD).
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- Special Education Support Service [http://www.sess.ie/categories/general-learning-disabilities General Learning Disabilities
- Ansberry, Clare (2010-11-20). "Erasing a Hurtful Label From the Books". New York: Wall Street Journal. Retrieved 2010-12-04. "Decades-long quest by disabilities advocates finally persuades state, federal governments to end official use of 'retarded'"
- Lawyer, Liz (2010-11-26). "Rosa's Law to remove stigmatized language from law books". Ithaca, New York: The Ithaca Journal. Retrieved 2010-12-04. "The resolution ... urges a change from the old term to "developmental disability""
- "International Statistical Classification of Diseases and Related Health Problems 10th Revision (ICD-10) Version for 2010". World Health Organization. Retrieved 12 March 2013.
- Jessica L. Schultz; Davis, Larry E.; Molly K. King (2005). Fundamentals Of Neurologic Disease. Demos Medical Publishing. p. 115. ISBN 1-888799-84-6.
- Dalton, A. J.; Janicki, Matthew P. (1999). Dementia, aging, and intellectual disabilities: a handbook. New York: Brunner/Mazel. p. 12. ISBN 0-87630-916-3.