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Mental deficiency is the old term for mental retardation and its more modern term intellectual disabilities and is of historical interest only. It has fallen out of use because of its historical and negative connotations and no longer figures in the current literature.
Attempts to develop a classification and nomenclature of the different levels of intellectual deficiency led to a number of taxonomies and terms that built on common usage in various countries, but which were informed by the development of IQ tests
So for example one scheme categorised people into groups by IQ:
- Borderline deficiency - (70-80)
- Moron - (50-69) - itwas defined by the American Association for the Study of the Feeble-minded in 1910, following work by Henry H. Goddard, as the term for an adult with a mental age between eight and twelve; mild mental retardation is now the term for this condition. Alternative definitions of these terms based on IQ were also used. This group was known in UK law from 1911 to 1959/60 as "feeble-minded."
- Imbecile - (20-49)
- Idiot - (below 20) indicated the greatest degree of intellectual disability, where the mental age is two years or less, and the person cannot guard himself or herself against common physical dangers. The term was gradually replaced by the term profound mental retardation.
Other categorizations included
- Feeblemindedness (Below 70 - North American : 50-69 - British)
- Subnormal - (75-80)
- Cretin is the oldest and probably comes from an old French word for Christian. The implication was that people with significant intellectual or developmental disabilities were "still human" (or "still Christian") and deserved to be treated with basic human dignity. This term has not been used in any serious or scientific endeavor since the middle of the 20th century and is now always considered a term of abuse. "Cretinism" is also used as an obsolescent term to refer to the condition of congenital hypothyroidism, in which there is some degree of mental retardation.
Usage has changed over the years, and differed from country to country, which needs to be borne in mind when looking at older books and papers. For example, "mental retardation" now covers the whole field, but used to apply to what is now the mild MR group. "Feeble-minded" used to mean mild MR in the UK, and once applied in the US to the whole field. "Borderline MR" is not currently defined, but the term may be used to apply to people with IQs in the 70s. People with IQs of 70 to 85 used to be eligible for special consideration in the US public education system on grounds of mental retardation.
Along with the changes in terminology, and the downward drift in acceptability of old terms, institutions of all kinds have had to repeatedly change their names. This affects the names of schools, hospitals, societies, government departments and academic journals. For example, the Midlands Institute of Mental Subnormality became the British Institute of Mental Handicap and is now the British Institute of Learning Disability. This phenomenon is shared with mental health and motor disabilities, and seen to a lesser degree in sensory disabilities.