Mental Retardation: Causes and Prevention (page 2)

By — Pearson Allyn Bacon Prentice Hall
Updated on Jul 20, 2010

Genetic Causes

Today, more than 500 genetic causes associated with mental retardation, many of them rare biological conditions, have been identified (The Arc, 2001). For example, fragile X syndrome is an inherited disability caused by a mutation on the X chromosome, and it was identified in 1991. It is now recognized as the most commonly known inherited cause of mental retardation, affecting about 1 in 4,000 males and 1 in 8,000 females (Crawford, Acuna, & Sherman, 2001). A common associated condition is recurrent otitis media (middle ear infection) with resulting hearing and language problems. Cognitive disabilities can be severe. Many of these individuals are challenged by limited attention span, hyperactivity, stereotypic behaviors (such as hand flapping or hand biting), and an inability to relate to others in typical ways. It is believed that almost half of individuals with fragile X syndrome have coexisting autism (Abbeduto et al., 2004; Demark, Feldman, & Holden, 2003). Many of these individuals also have repetitive speech patterns (Belser & Sudhalter, 2001).

Another example of a genetic cause for mental retardation due to a chromosomal abnormality is Down syndrome (a chromosomal disorder wherein the individual has too few or too many chromosomes). The nucleus of each human cell normally contains 23 pairs of chromosomes (a total of 46). In the most common type of Down syndrome, trisomy 21, the 21st set of chromosomes contains three chromosomes rather than the normal pair. Certain identifiable physical characteristics, such as an extra flap of skin over the innermost corner of the eye (an epicanthic fold), are usually present in cases of Down syndrome. The degree of mental retardation varies, depending in part on how soon the disability is identified, the adequacy of the supporting medical care, and the timing of the early intervention. ~e great majority of people with Down syndrome have a high incidence of medical problems (National Down Syndrome Society [NDSS], 2005). For example, about half have congenital heart problems, and these individuals have a 15 to 20 times greater risk of developing leukemia. Although people with Down syndrome have intellectual disabilities, they have fewer adaptive behavior challenges than many of their peers with mental retardation (Chapman & Hesketh, 2000). These individuals do, however, have a higher prevalence of obesity, despite typically consuming fewer calories (Roizen, 2001). Possibly their reduced food consumption explains why individuals with Down syndrome are less active and less likely to spend time outdoors than their brothers and sisters. Teachers should help increase these students' opportunities for recreation and social outlets by creating exciting reasons to exercise and play with friends.

Some genetic causes of disabilities are not so definite but rather result from interplay between genes and the environment. Phenylketonuria (PKU), also hereditary, occurs when a person is unable to metabolize phenylalanine, which builds up in the body to toxic levels that damage the brain. If untreated, PKU eventually causes mental retardation. Changes in diet (eliminating certain foods that contain this amino acid, such as milk) can control PKU and prevent mental retardation, though cognitive disabilities can be seen in both treated and untreated individuals with this condition. Because of the devastating effects of PKU, it is critical that the diet of these individuals be strictly controlled. Here, then, is a condition rooted in genetics, but it is an environmental factor (a protein in milk) that becomes toxic to the individuals affected and causes the mental retardation. And both prompt diagnosis and parental vigilance are crucial to minimizing retardation. Now let's look at some toxins that do not have a hereditary link.


Poisons that lurk in the environment, toxins, are both prenatal and postnatal causes of mental retardation, as well as of other disabilities. Many believe that the increased rates of attention deficit hyperactivity disorder, learning disabilities, and even autism are due to some interplay of genetics, environmental factors, and social factors (Office of Special Education Programs, 2000; Schettler et al., 2000). Clearly, exposures to toxins harm children and are a real source of disabilities. Here are two reasons why toxins deserve special attention:

  1. Toxic exposures are preventable.
  2. Toxins abound in our environment.

Let's think about how toxins can harm children. Mothers who drink, smoke, or take drugs place their unborn children at serious risk for premature birth, low birth weight, and mental retardation (The Arc, 2001). One well-recognized cause of birth defects is fetal alcohol syndrome (FAS), which is strongly linked to mental retardation and results from the mother's drinking alcohol during pregnancy. FAS is recognized by Congress as the most common known cause of mental retardation. It costs the U.S. taxpayers 5.4 billion dollars in 2003 alone, and the costs in quality of life to the individuals affected and their families are immeasurable (U.S. Senate Appropriations Committee, 2004). The average IQ of people with FAS is 79, very close to the cutoff score for mental retardation (Bauer, 1999). This means that almost half of those with FAS qualify for special education because of cognitive disabilities. This group's average adaptive behavior score is 61, indicating a strong need for supports. These data explain why some 58 percent of individuals with FAS have mental retardation and why some 94 percent require supplemental assistance at school. Unfortunately, most of these people are not free of other problems in the areas of attention, verbal learning, and self-control (Centers for Disease Control [CDC], 2004a). Estimates are that some 5,000 babies with FAS are born each year. An additional 50,000 show fewer symptoms and have what is considered the less serious condition fetal alcohol effects (FAE), which, like FAS, is caused by mothers drinking alcohol during pregnancy (Davis & Davis, 2003).

Toxins abound in our environment. All kinds of hazardous wastes are hidden in neighborhoods and communities. One toxin that causes mental retardation is lead. Two major sources of lead poisoning can be pinpointed. One is exhaust fumes from leaded gasoline, which is no longer sold in the United States. The other source is lead-based paint, which is no longer manufactured. Unfortunately, however, it remains on the walls of older apartments and houses. Children can get lead poisoning from a paint source by breathing lead directly from the air or by eating paint chips. For example, if children touch paint chips or household dust that contains lead particles and then put their fingers in their mouths or touch their food with their hands, they ingest the lead. And lead is not the only source of environmental toxins that government officials should be worried about; other concerns include mercury found in fish, pesticides, and industrial pollution from chemical waste (Schettler et al., 2000).

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