Mental retardation is caused by many factors; many of these are known, but others remain unidentified (The Arc, 2005). The link between the identification of specific causes of mental retardation and the development and implementation of preventive measures is clear. When a cause is identified, ways to prevent the debilitating effects of cognitive disabilities have often followed soon after. But it takes action for solutions actually to prevent or reduce the impact of the condition.


According to The Arc, a parent organization advocating for individuals with mental retardation, several hundred causes of mental retardation have been discovered, but for about one-third of those affected the cause is unknown (The Arc, 2005). Of those known causes, three conditions—explained later in this section—are the most common reasons for mental retardation:

  1. Down syndrome
  2. Fragile X syndrome
  3. Fetal alcohol syndrome

Many different systems for organizing the causes of mental retardation can be applied. Sometimes they are divided into four groups: socioeconomic and environmental factors, injuries, infections and toxins, and biological causes. AAMR divide them instead into three groups by time of onset—that is, by when the event or cause first occurred (AAMR, 2002):

  1. Prenatal: causes that occur before birth
  2. Perinatal: causes that occur during the birth process
  3. Postnatal: causes that happen after birth or during childhood

Prenatal causes exert their effects before birth. Examples include genetic and heredity, toxins taken by the pregnant mother, disease, and neural tube defects. Genetics and heredity include conditions such as fragile X syndrome and Down syndrome, as well as phenylketonuria (PKU). Prenatal toxins include alcohol, tobacco, and drug exposure resulting from the behavior of the mother during pregnancy. Diseases and infection, such as HIV/AIDS, can devastate an unborn baby. Neural tube disorders, such as anencephaly (where most of the child's brain is missing at birth) and spina bifida (incomplete closure of the spinal column), are also prenatal causes of mental retardation.

Perinatal causes occur during the birthing process. They include birth injuries due to oxygen deprivation (anoxia or asphyxia), umbilical cord accidents, obstetrical trauma, and head trauma. They also include low birth weight.

Postnatal causes occur after birth. The environment is a major factor in many of these situations. Child abuse and neglect, environmental toxins, and accidents are examples of postnatal causes. An additional reason for being identified as having mental retardation is societal biases, particularly toward diverse students.

Now let's turn our attention to some major causes of mental retardation across the three periods of onset. In particular, let's think about some genetic causes, both prenatal and postnatal toxins, low birth weight, and child abuse. Finally, we will briefly return to the situation of Black youngsters and their risk for being identified as having mental retardation.

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).

Low Birth Weight

Low birth weight is a major risk factor for disabilities and is definitely associated, with poverty and with little or no access to prenatal care (Children's Defense Fun. [CDF], 2004). Medical advances of the 1980s have greatly increased the likelihoood that infants born weighing less than 2 pounds will survive. These premature, very small infants make up less than 1.4 percent of all newborns and are at great risk for disabilities, including mental retardation (Allen, 2002). However, babies born between 3 and 5 pounds also are at greater risk for disabilities than many doctors and parents believe. Babies with moderately low birth weight represent 5 to 7 percent of all births, but they represent 18 to 37 percent of children with cerebral palsy and 7 to 12 percent of children with cerebral palsy who also have mental retardation. Whereas about 5 percent of White babies have moderately low birth weight, between 10 and 12 percent of African American babies are born early and have low birth weight.

Child Abuse and Neglect

Abused children have lower IQs and reduced response rates to cognitive stimuli (CDF, 2001, 2004). In one of the few studies of its kind, Canadian researchers compared abused children with those not abused, and the results of abuse became clear (Youth Record, 1995). The verbal IQ scores were very different between the two groups of otherwise matched peers: The abused children had an average total IQ score of 88, whereas the average overall IQ of their nonabused peers was 101; and the more abuse, the lower the IQ score. The link between child abuse and impaired intellectual functioning is now definite, but the reasons for the damage are not known. Rather than resulting from brain damage, the disruption in language development caused by the abusive situation may be the source of permanent and profound effects on language ability and cognition. Or the abuse may itself be a result of the frustration often associated with raising children with disabilities. Remember, the connection between neglect and mental retardation has long been recognized and is part of the early history and documentation of this field.

Discrimination and Bias

It is important to remember that many subjective reasons account for students' placement in special education. There is little doubt that poverty and its risk factors are clearly linked to disabilities (CDF, 2004; National Research Council, 2002). It is also true that culturally and linguistically diverse children are overrepresented in some categories of special education (Hosp & Reschly, 2002, 2003; U.S. Department of Education, 2005a). This situation is particularly true for Black students, who are almost three times more likely to be identified as having mental retardation than their White peers (National Alliance of Black School Educators & ILIAD Project, 2002). Specifically, a definite relationship exists between poverty and three other factors: ethnicity, gender, and mental retardation (Oswald et al., 2001). However, this relationship may be somewhat different from what one might initially suspect: The risk factors of poverty (limited access to health care, poor living conditions) do not entirely explain this disproportionate representation (Ford et al., 2002; Neal et al., 2003). Rather, "the increased rate of identification among students of color may be attributable to systemic bias" (Oswald et al., 2001, p. 361). Black students who live in a predominantly White neighborhood are more likely to be identified as having mental retardation than those who live in a neighborhood with more diversity. One conclusion is that students are more vulnerable to discrimination when they represent a minority. Many strategies can be undertaken to reduce mistakes in the identification process, including pre-referral intervention, appropriate and meaningful curricula, and instruction anchored in culturally relevant examples.


Many cases of mental retardation can be prevented by directly addressing the cause. According to The Are, because of advances in research over the last 30 years, many cases of mental retardation are prevented (The Are, 2005). For example, each year 9,000 cases of mental retardation are prevented via the measles and Hib vaccines. 1,250 cases via newborn screening for phenylketonuria (PKU) and congential hypothyroidism, and 1,000 cases via the anti-RH immune globulin. Even more cases are preventable. Most of these strategies (as seen in the table below) are simple and obvious, but the effects can be significant. For example, in the case of child abuse, teachers now have a legal (and, many believe, a moral) responsibility to report suspected cases so that further damage to the child might be avoided.

Education and access are at the heart of many prevention measures. For example, education about the prevention of HIV/AIDS can be effective with all adolescents, including those with mental retardation (Johnson, Johnson, & Jefferson-Aker, 2001). Public education programs can also help pregnant women understand the importance of staying healthy. Other prevention strategies involve testing the expectant mother, analyzing the risk factors of the family (genetic history of disabilities or various conditions), and taking action when necessary; screening infants; protecting children from disease through vaccinations; creating positive, nurturing, and rich home and school environments; and implementing safety measures. Note that not all of these strategies are biological or medical. It is important to look at all aspects of the child and the environment.

The importance of immunization programs to protect children and their mothers from disease cannot be overemphasized (The Arc, 2005; CDF, 2004). The incidence of disabilities, including mental retardation, has been greatly reduced by immunization against viruses such as rubella, meningitis, and measles. However, immunization is still not provided universally. Despite more federal and state programs to assist families in protecting their children, only some 78 percent of two-year-olds had received all recommended immunizations in 2002. Why is this so? Some families do not have access to immunizations because a health care facility is unavailable or is too far from home, or because the immunizations are too expensive. Some families ignore or are uninformed about the risks of skipping vaccinations, and other families avoid immunizations for religious reasons or believe that the risk of getting the disease from the vaccination itself is greater than the risk of being unprotected. As a result, easily preventable cases of mental retardation due to infection still occur.

People must not underestimate the importance of prenatal care. For example, FAS and FAE are 100 percent preventable (CDC, 2004b; Davis & Davis, 2003). Pregnant mothers who do not drink alcohol prevent this condition in their children! Staying healthy also means taking proper vitamins and eating well, and there are good examples of why this is essential. For example, folic acid reduces the incidence of neural tube defects. By eating citrus fruits and dark, leafy vegetables (or taking vitamin supplements), one receives the benefits of folic acid—a trace B vitamin that contributes to the prevention of conditions such as spina bifida and anencephaly. Here's proof that such prevention measures make a difference: In 1992 the U.S. Public Health Service recommended that all women thinking about becoming pregnant take folic acid daily, either through diet or supplements, and mandated that cereal be fortified with folic acid. Since then, the prevalence of spina bifida has dropped by 31 percent and that of anencephaly by 16 percent (CDC, 2004b). Think of the difference yet to be made if all potential moms ate well and planned ahead!

We also noted that prematurity (being born before 37 weeks of pregnancy) and low birth weight are risk factors for mental retardation and other disabilities. Unfortunately, relatively little is known about how to prevent many of these cases (Alexander & Slay, 2002). It is known that it is important for as many unborn babies as possible to reach full-term and that good prenatal care is an important part of making that happen.

Couples can take certain actions before the woman becomes pregnant to reduce the risk of biologically caused mental retardation. For example, gene therapy may become universally available to families who know they are at risk for having offspring with PKU (Eisensmith, Kuzmin, & Krougliak, 1999). Some couples have medical tests before deciding to conceive a child. These tests, combined with genetic counseling, help couples determine whether future children are at risk for certain causes of mental retardation. In one study, the majority of women who received genetic counseling either because of their age or because of an abnormal blood test indicated that they would avoid or terminate the pregnancy if a test was positive for a disability (Roberts, Stough, & Parrish, 2002). Tay-Sachs disease, for example, is a cause of mental retardation that can be predicted through genetic testing. Other couples take tests for defects after they find out that the woman is pregnant. These tests can determine, in utero, the presence of approximately 270 defects. It is possible that prenatal gene therapy, now in experimental phases, will one day correct such abnormalities before babies are born (Ye et al., 2001).

Overcoming Challenges

Although some conditions or causes of mental retardation cannot be prevented. at least at the present time, the impact of the condition can be reduced substantially. For example, we have seen that PKU is a genetic reason for mental retardation but that it takes factors in the individual's environment for damage to be devastating. Infant screening can detect the problem. Here's how it works: In a procedure developed by Robert Guthrie in 1957, a few drops of the newborn's blood are taken from the heel to determine whether the infant has the inherited genetic disorder that prevents metabolizing phenylalanine, a naturally occurring amino acid found in milk. This test, which costs 3 cents, makes it possible to change any affected baby's diet before the disastrous effects of PKU can begin to mount. Guthrie developed the test because his son and his niece had PKU, and he wanted to prevent the condition from affecting others. In the past, PKU was responsible for 1 percent of all severe cases of mental retardation, nearly all of which are now identified and the severity of the problem substantially reduced (Schettler et al., 2000).

Prevention of Mental Retardation

For Pregnant Women For Children For Society
Obtain early prenatal medical care. Guarantee universal infant screening. Eliminate the risks of child poverty.
Seek genetic counseling. Ensure proper nutrition. Make early intervention programs universally available.
Maintain good health. Place household chemicals out of reach. Provide parent education and support.
Avoid alcohol, drugs, and tobacco. Use automobile seatbelts, safety seats, and cycle helmets. Protect children from abuse and neglect.
Obtain good nutrition. Provide immunizations. Remove environmental toxins.
Prevent premature births. Prevent or treat infections. Provide family planning services.
Take precautions against injuries and accidents. Have quick and easy access to health care. Provide public education about prevention techniques.
Prevent or immediately treat infections. Prevent lead poisoning. Have universal access to health care.
Avoid sexually transmitted diseases. Guarantee proper medical care for all children. Vaccinate all children.
  Provide early intervention programs.  
  Eliminate child abuse and neglect.