Prevalence and Definitions of Mental Retardation

Individuals classified as mentally retarded represent 10.3% of the students ages 6–21 served under IDEA (U.S. Department of Education, 2002a) or about .9% of the population in general. Although this number includes all individuals with mental retardation served under IDEA, as many as 85% have mild or moderate mental retardation, as opposed to severe disabilities discussed in chapter 4 (Drew & Hardman, 2004).

Although mental retardation is commonly used, other terms are used to describe this condition, including intellectual disability, cognitive disability, mental deficiency, mental subnormality, mentally handicapped, or intellectually challenged. Mental retardation is also referred to as one type of the more general term, developmental disability (Beirne-Smith, Patton, & Kim, 2006).

The definition of the American Association on Mental Retardation (AAMR, 2002) states that:

Mental retardation is a disability characterized by significant limitations both in intellectual functioning and in adaptive behavior as expressed in conceptual, social, and practical adaptive skills. This disability originates before age 18. (p. 1)

This definition also includes five assumptions to be used in applying the definition: (1) consideration of the context of community, peers, and culture; (2) consideration of cultural and linguistic diversity; (3) consideration of strengths as well as weaknesses; (4) the necessity of developing a profile of needed supports; (5) the expectation that the individual’s functioning will improve over time with appropriate supports (AAMR, 2002; see also Drew & Hardman, 2004).

According to the American Psychological Association (APA), mild mental retardation represents the upper range of functioning within the mental retardation classification with IQ scores between 55 to 70. Scores between 35 and 54 are considered moderate mental retardation, scores of 20 to 34 are severe, and scores below 20 are associated with profound mental retardation (Jacobson & Mulick, 1996).

The American Association on Mental Retardation (AAMR, 2002) does not employ a classification system based on IQ level. Rather the AAMR definition suggested that individuals could be evaluated relative to a system of services and supports. These include support areas (e.g., human development, teaching and education, home and community living), relevant support activities (e.g., individual’s interest, activities and settings for participation), and levels and intensity of supports (intermittent, limited, extensive or pervasive). These levels were intended to replace previous classification systems of mild to severe mental retardation, although these latter terms are still widely used (Berine-Smith, Patton, & Kim, 2006)

Causes of Mental Retardation

The vast majority of causes of mental retardation are unknown, and some speculate that known causes account for only 10% to 15% of the cases of retardation (Beirne-Smith, Patton, & Kim, 2006). The causes of mild mental retardation are more difficult to determine than causes for severe and profound mental retardation. Known causes can be classified into genetic factors, brain factors, and environmental factors.

Genetic Factors

Genetic disorders or damage to genetic matter can cause mental retardation. Disorders can include chromosomal abnormalities and genetic transmission of traits through families. Down syndrome is an example of a genetic disorder. It is sometimes referred to as Trisomy 21, because the 21st pair of chromosomes divides into three (trisomy) instead of a single pair of chromosomes. Down syndrome represents 5% to 6% of individuals with mental retardation, and is associated with some specific characteristics, including intellectual functioning in the mild to moderate ranges, short stature, upward slanting of the eyes, and a susceptibility to heart defects or upper respiratory infections (Beirne-Smith, Patton, & Kim, 2006).

Recent research into the hereditability of different traits, and other types of chromosomal abnormalities, has added to the knowledge base surrounding causes of some types of mental retardation. Medical tests can detect the presence of some genetic abnormalities, including Down syndrome, during early pregnancy.

Brain Factors

Brain factors refer to defects in the brain or central nervous system. These can occur during prenatal development, perinatally (during the birth process), or postnatally (after the baby is born). Brain factors may be congenital (present at birth) or may appear later in life. Prenatal factors include exposure to rubella (German measles) and syphilis (Beirne-Smith, Patton, & Kim, 2006). Exposure to alcohol during prenatal development can lead to fetal alcohol syndrome (FAS), which may result in retardation (Connor, Sampson, Bookstein, Barr, & Streissguth, 2001). Infections such as meningitis and encephalitis, which cause inflammations to the brain, may result in brain damage. Some forms of retardation are associated with cranial malformations that result in microcephaly or hydrocephaly. Microcephaly is associated with a very small skull, while hydrocephaly is often characterized by an enlarged head due to an interference in the flow of cerebral spinal fluid in the head. Finally, it is known that anoxia, or lack of oxygen to the brain, any time including during birth causes brain damage and may result in mental retardation, depending upon the extent of the damage (Beirne-Smith, Patton, & Kim, 2006).

Environmental Influences

Environmental influences refer to factors such as poor nutrition during prenatal development that can influence the development of the brain and result in retardation (Cohen, 2000). It has been seen that many premature and low-birth-weight babies may have mental retardation. The ingestion of lead, often through lead-based paint, can also cause retardation. Although factors such as poverty and lack of early sensory stimulation are associated with retardation, it is more difficult to prove that such environmental factors always lead to mental retardation. Future research may uncover additional important factors related to causes and prevention of mental retardation (Beirne-Smith, Patton, & Kim, 2006).

Issues in Identification and Assessment of Mental Retardation

Both intellectual functioning and adaptive behavior are assessed in making determinations regarding mental retardation. Individually administered intelligence tests are used in most states to assess intellectual functioning, and usually contain, for example, measures of vocabulary, common knowledge, short term memory, and ability to solve mazes and jigsaw puzzles. Adaptive behavior scales assess how well individuals are able to perform daily living skills, self-help care, communication skills, and social skills. Although there is variability across states, in most states an individual should be functioning at least two standard deviations below average (approximately the second percentile) on both measures to be classified as mentally retarded (Jacobson & Mulick, 1996).

Characteristics of Mental Retardation

The most common features associated with retardation include slower pace of learning, lack of age-appropriate adaptive behavior and social skills, and below-average language and academic skills. Many individuals with mental retardation exhibit poor motor coordination, which can be improved by working with occupational therapists, physical therapists, or adaptive physical educators (Mohan, Singh, & Mandal, 2001). However, most students with mild and moderate mental retardation have the ability to learn to read, write, and do mathematics, up to the sixth-grade level, or higher in some cases. Following is an autobiographical statement written by Kirstin Palson, an individual with mental retardation who was institutionalized as a child. The statement was included in a book of poetry she wrote.

From “About the Author”:

My name is Kirstin Ann Palson. I was born in Boston, Massachusetts, in 1952 with complications. My diagnosis was mental retardation plus cerebral palsy, due to brain damage at birth. It was difficult those early years. Because of my behavior problems, I was sent to the Wrentham State School when I was seven years old. Those were horrendous times. At the age of fourteen I came out of Wrentham. I have overcome my handicaps, graduated from High School at twenty-two, and worked as a volunteer library aid in two elementary schools. For two years after graduation, I had a struggle getting employment. Finally I got a full time job in a company and worked five years. The company moved out of town and I struggled with unemployment and job search for six and a half months. I have gained employment in another company full time. I got both jobs on my own.

I have a great love for words. When I was growing up, it was the reading of children’s stories by Mom at bedtime which gave me the ability for reading and loving books. During the past years I have given books of my poetry to family members as gifts, especially at Christmas. My love for poetry is still with me and will remain forever more. K. A. P. (1986).

Intellectual and Cognitive Functioning

Individuals with mental retardation exhibit deficits in intellectual functioning. In addition, these individuals usually function substantially below their age peers in related areas, including metacognitive abilities, memory, attention, thinking, and problem-solving abilities. Like students with learning disabilities, individuals with mental retardation often have difficulty generalizing learned information to novel situations (Mastropieri, Scruggs, & Carter, 1997).

Social and Adaptive Behavior

By most definitions, individuals with mild mental retardation have less well-developed adaptive behavior than their peer counterparts, including such behavior as using the telephone or dressing appropriately. They may appear socially immature, exhibit inappropriate social behavior, or have difficulty making and maintaining friendships. Some individuals may become easily frustrated when they experience difficulty and then may act inappropriately, drawing negative attention to themselves. On the other hand, some individuals with mental retardation have particularly amiable dispositions and are well liked by others (Drew & Hardman, 2004).

Some individuals with mental retardation tend to have an external “locus of control,” meaning they see their lives as being controlled and influenced by factors outside of themselves (e.g., fate, chance, other people; Ezell & Klein, 2003). This external locus of control may hinder their development of self-reliance. A related problem is “outerdirectness”; that is, looking to external cues or modeling behavior of others rather than relying on their own judgments (Zigler, Bennett-Gates, Hodapp, & Henrich, 2002).


Both receptive and expressive language are problem areas for individuals with mental retardation. There is usually a direct relationship with severity of retardation and all aspects of language development (Vicari, Caselli, Gagliardi, Tonucci, & Volterra, 2002). Communication skills are typically less well-developed and can result in misunderstandings of directions (Cascella, 2004). Students may exhibit difficulties with comprehension of abstract vocabulary and concepts (Vicari et al., 2002).

Academic Skills

Individuals with mental retardation may have difficulty learning basic skills of reading, writing, and mathematics (Young, Moni, Jobling, & van Kraayenoord, 2004). The rate of learning new information may be very slow, and students may require repetition and concrete, meaningful examples on all learning activities.

Classroom Adaptations for Students with Mental Retardation


Careful preparation can greatly enhance the successful inclusion of students with mental retardation. First, have an open, accepting classroom environment so that students feel welcome as genuine class members. Provide students with the same materials—desks, lockers, mailboxes—as the other students. Involve students in daily activities. Meet with them privately and preteach the daily routine. Show them where materials are kept and how things in the class proceed. This will help build their confidence before they come in for the first time in front of the general education peers. More information for preparing classmates is given in the In the Classroom feature.

Monitor Peer Relationships

Although peers can be good friends and strong supporters of students with mental retardation, teachers also should be aware that some students may try to take advantage of students with mental retardation. For example, in one sixth-grade class, several boys bullied a boy with mental retardation and consistently took away part of his lunch. In another example, high school students who had been smoking cigarettes in the girls’ restroom handed their cigarettes to a girl with mental retardation when a teacher entered the restroom. Careful monitoring can decrease the likelihood of either negative situation occurring and increase the likelihood that peer relations will be positive and productive.

Instructional Modifications

Many of the modifications described in the learning disabilities section may also be helpful for students with mental retardation. However, additional modifications probably will be required if the general education experience is to be successful.

  • Prioritize objectives for students with mental retardation in general education classes, and teach directly to these prioritized objectives.
  • Adapt materials to the needs of students, by reducing reading, writing, and language requirements and simplifying work sheets.
  • Adapt instruction by employing clear, organized presentations, providing concrete, meaningful examples and activities, providing frequent reviews, and encouraging independent thinking.
  • Communicate with families to further your understanding and obtain additional information on how students work best. The Diversity in the Classroom feature describes a model of “Person-Centered Planning” for working with Asian American families.
  • Adapt evaluation using individual testing, portfolio assessments, tape or video recordings.
  • Use specialized curriculum when necessary. Some students with mental retardation may require an alternative, more functional curriculum. Such a curriculum may include communication, community living, domestic skills, socialization, self-help, and vocational and leisure skills. Additionally, some students may benefit from a life-skills curriculum, which emphasizes transition to adulthood. This curriculum could include education in home and family, community involvement, employment, emotional-physical health, and personal responsibility and relationships (see Cronin & Patton, 1993).

In the Classroom: Getting Classmates Ready for Students with Disabilities

  • Prepare general education students for the arrival of students with disabilities by asking a special educator to talk about disabilities and explain strengths and limitations of individuals with disabilities.
  • Encourage students to ask questions, and set a model of open acceptance.
  • Tell students about their roles as possible peer tutors and helpers. Provide models of how peers can assist, but make it clear that they should also encourage independent functioning. They should not try to do everything for students with disabilities.
  • Explain that all classmates, even if they are not peer tutors or helpers, can encourage students with disabilities to be active participants and members of the class.