Mental Retardation: Types, Classification, Causes And Prevention

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What is Mental Retardation? Mental Retardation is a disorder in which an individual’s intellectual functioning is sub-average or well below average having an IQ of around 70 or less.

Types and Classifications of Mental Retardation

There are 4 types of mental retardation;


Individuals with mental retardation under this type have 55-70 IQ. About 85% of people with retardation are affected by this type.

Affected individuals cannot be determined from normal children until they attend school.

They are slow learners but can develop academic skills equivalent to 6th-grade level.

As adults, they can work and live in the community if assisted during occurrences of unusual social or economic problems.

A few individual with mild mental retardation may marry and have kids.

*Mildly retarded individuals are classified as “educable”. They are capable of learning basic academic areas such as reading and math.


Individuals with moderate mental retardation have IQ that range from 35 to 54. About 10% of people with mental retardation are affected by this type.

Academically, people with moderate retardation can progress to about the 2nd-grade level.

During adolescence period, they usually have good self-care skills – they can eat, dress and go to bathroom themselves.

Likewise, as adolescents, they can perform simple tasks.

Adults with moderate retardation can work at unskilled or semiskilled jobs with supervision.

*Moderately retarded individuals are classified as “trainable”. They are not likely to acquire sufficient skills in the 3 Rs to operate at the grade 4 level and up.


People with severe mental retardation have IQ that falls from 20 to 34. Up to 4% individuals with mental retardation are affected by this type.

People affected by this type of retardation learn to talk and develop basic self-care skills during childhood.

Adults with severe mental retardation can perform simple tasks with close supervision.

Severely retarded individuals usually live with their families or in group homes.

*Severely retarded people are categorized as”severe”.  They learn basic self-care skills.


Profoundly retarded individuals have IQ of below 20. Up to 2% of mentally retarded individuals fall under this type.

People affected by this type of mental retardation require constant care.

Profoundly retarded people can understand some language but they have little ability to talk.

These individuals usually have a neurological condition that accounts for their retardation.

*Profoundly retarded people are categorized as “custodial”. They are unstable and totally dependent of care because they can’t acquire any basic skill.

Causes of Mental Retardation

Genetic Causes

Genetic causes of mental retardation include single-gene defects like Fragile X Syndrome and chromosomal disorders such as Down syndrome.

Fragile X Syndrome

Fragile X Syndrome is the most common inherited cause of mental retardation affecting 10% of cases. This single-gene defect is more likely to cause retardation in males than females.

Down syndrome

Down syndrome occurs when people inherit all or part of an extra copy of a pair of a chromosome known together as chromosome 21.

Chromosomal Disorders

Chromosomal disorder occurs in about 7 out of every 1,000 infants. This disorder involves an abnormal number of chromosomes or changes in the structure of a chromosome. Chromosomal disorders are not necessarily inherited

Inborn Error of Metabolism

Inborn Error of metabolism is another genetic cause of mental disorder which involves inheritance of a defective gene unable to produce proteins or enzyme needed for critical cell functions.

Congenital Hypothyroidism

Congenital Hypothyroidism is another common cause of mental retardation and occurs 1 in every 4,000 births. Newly born babies affected by this disorder are unable to produce needed thyroxine, a hormone secreted by the thyroid gland.

Prevention of Mental Retardation

There are three ways to prevent mental retardation

Newborn Screening Programs

Newborn screening program is a way of identifying inherited conditions that may lead to retardation and is conducted through blood test for congenital hypothyroidism and phenylketonuria or PKU. Newly born babies can’t metabolize the amino acid phenylalanine found in foods with protein. Upon detection, these infants will be given a low-phenylalanine diet that prevents retardation.

Adult Screening Tests

Adult screening tests can be conducted to identify carriers of other conditions before couples conceive a child. People with family history of mental retardation are strongly recommended to undergo genetic counseling to evaluate their own risks and need for screening. A specialized laboratory test known as amniocentesis can detect Down syndrome and other related disorders in the early stages of pregnancy.

Proper Prenatal Care

No drugs and alcohol during pregnancy. Routine immunization against measles and other diseases are highly recommended. Lastly, magnesium sulfate treatments for pregnant women with babies with very low birth weight can reduce the risk of mental retardation in the infant.

See also

20 Most Bizarre Congenital Defects/Abnormalities

Common and Rare Birth Defects or Congenital Disorders and Their Causes

Deformities Caused by Bizarre Diseases or Physical Anomalies

Extremely Bizarre Birth Defects and Physical Deformities

Morbid and Bizarre Cutaneous Conditions


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