Mental Retardation and Developmental Disabilities

Developmental disability is an umbrella term that broadly refers to a set of severe and chronic physical or mental impairments characterized by an absence or delay in reaching certain developmental milestones that typify the normally developing person. Mental retardation (MR) is a developmental disability that is exemplified by the presence of deficits in both cognitive and adaptive functioning. Individuals with MR have significantly limited patterns of personal functioning, though these patterns will vary from person to person, depending on the severity and type of deficits. As with any developmental disability, the limits in functioning of an MR individual are manifest by infancy or early childhood and are lifelong in nature.

The Nature of Mental Retardation and Developmental Disabilities

Diagnostic Criteria

The diagnostic criteria for MR are listed in the Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition, Text Revision (DSM—IV—TR) under the heading of “Disorders Usually First Diagnosed in Infancy, Childhood, and Adolescence” and can also be found in the American Association on Mental Retardation’s (AAMR) Mental Retardation: Definition, Classification, and Systems of Supports, Tenth Edition. (Note: The AAMR has changed its name to the American Association on Intellectual and Developmental Disabilities.)

To warrant a diagnosis of MR, both of the following symptoms must be present in an individual prior to age 18: below average intellectual functioning and impairment in at least two areas of adaptive functioning.

Intellectual functioning is assessed using standardized instruments that measure an individual’s intelligence quotient (IQ), such as the Wechsler Adult Intelligence Scale—Third Edition and the Stanford-Binet Intelligence Scales, Fifth Edition. Standardized IQ scores exist along a normal continuum, where a mean IQ score of 100 (standard deviation of 15) reflects an individual with average intelligence. To be considered for an MR diagnosis, individuals must have an IQ score approximately two standard deviations below the mean. This means that an IQ score of approximately 70 or below is needed to demonstrate significantly subpar intellectual functioning. Moreover, intellectual functioning is also coded to reflect the severity of impairment: mild (50-55 to approximately 70), moderate (35-10 to 50-55), severe (20-25 to 35-40), and profound (20-25 or below). Unspecified is the code given when there is a strong suspicion of MR, but actual IQ scores cannot be determined due to factors that interfere with IQ testing, such as uncooperativeness or extremely impaired functioning.

Adaptive functioning refers to activities needed to successfully navigate day-to-day demands of life. When individuals exhibit adaptive functioning deficits, they display consistent ineffectiveness in completing these daily activities. Levels of adaptive functioning are considered in light of the chronological age of an individual and are measured using standardized assessment instruments, such as the Vineland Adaptive Behavior Scales, Second Edition and the AAMR Adaptive Behavior Scale, Second Edition. The broad areas of adaptive functioning considered for an MR diagnosis include daily living skills, communication skills, and social and interpersonal skills. Daily living skills refer to a set of behaviors that center on self-care, household chores, work or academic involvement, and the ability to access community resources. Communication skills involve the ability to accurately understand others and express oneself. Social and interpersonal skills are the set of skills needed to successfully interact with others, cope with daily stressors, and make use of free time. In addition to these three areas, deficits in fine and gross motor skills, such as toilet training and walking, may also be assessed in children.

In addition to intellectual functioning and adaptive functioning, the AAMR definition of MR goes a step further and integrates a skills-based component to shift the focus away from the solely limitations-based definition given by the DSM-IV-TR. For example, the AAMR definition of MR addresses the context of the individual’s limitations, strengths, and available environmental supports. Both the DSM-IV-TR and the AAMR suggest that before a diagnosis of MR is rendered, it is imperative to rule out medical conditions, cultural and language considerations, and physical limitations (e.g., hearing or visual impairments) that may interfere with normal development or current functioning. In addition, developmental issues should be taken into account when assessing adaptive functioning; for example, it would be inappropriate to measure occupational performance when assessing the adaptive functioning of a child.


A variety of genetic, chromosomal, and environmental causal sources have been linked to MR. Genetic and chromosomal disorders are thought to account for approximately half of MR cases, with the other half due to environmental or unknown causes. Examples of genetic and chromosomal types of disorders include the following: metabolic disorders (e.g., phenylketonuria, galactosemia) and chromosomal abnormalities (Down syndrome, Klinefelter’s syndrome, fragile X). Environmental causes include chronic and severe neglect of basic needs (e.g., food, affection), teratogen exposure in utero (e.g., fetal alcohol syndrome, Food and Drug Administration pregnancy category D- and X-classed drugs), fetal and infant malnutrition, and birth complications (e.g., hypoxia, premature delivery).


Most reliable sources, such as the Centers for Disease Control and Prevention and the AAMR, put the prevalence rate of MR at about 1% to 3% of the general population. The incidences of MR caused by genetic and chromosomal factors occur similarly across different socioeconomic categories. Environmental causes (e.g., lead poisoning and malnutrition) appear to occur more commonly in individuals of lower socioeconomic status. In addition, MR seems to occur at a higher frequency in males (1.5 males to 1 female). Mild MR is more common, representing approximately 85% of all individuals in this group. Moderate MR occurs at 10%, severe MR corresponds to approximately 3% of all MR cases, and profound MR occurs in approximately 1% of such individuals. In general, people with MR have the same rates of chronic physical and mental health disorders as the general population. The exceptions are due to the problematic symptoms specific to certain disorders, such as metabolic disorders, and with the neurological abnormalities and physical limitations present in individuals with profound MR.

Changes and Challenges

Treatment Interventions

Interventions for MR individuals directly target adaptive living skills and take into account both the limitations and strengths of the individual. Interventions typically address increasing self-determination, targeting problematic behaviors, increasing social skills, and assisting caregivers, families, and employers in modifying the individual’s environment to maximize success. This is best accomplished by coordinating several services, such as therapy, vocational rehabilitation, social services, schools, and caregivers. Interventions are highly dependent on the individual’s level of disability; for instance, a treatment plan for an individual with mild MR will be different from that for an individual with more extensive limitations. A typical example of an individual with mild MR might be a day school program that places academic lessons in the context of daily living, such as focusing on social skills, reading bus schedules, calculating a simple budget, cooking, and household upkeep. Vocational training is also a recommended treatment consideration and should be based on the individual’s strengths.

In addition, healthcare providers are increasingly aware that psychological disorders occur at a much higher rate in the MR population than previously thought and warrant psychological services. Rates of depression, anxiety, conduct problems, or impulse control issues are common, and, consistent with the symptomatology of these disorders, injurious behaviors are sometimes present. Interventions targeting these behaviors are important, as is teaching beneficial coping mechanisms (e.g., time-out/taking a break, redirecting attention). Likewise, the appropriate use of psychotropic medication can be a viable treatment avenue. Insight-oriented therapy may have some utility with individuals with mild MR. Therapeutic interventions should focus on modeling healthy emotional expression, managing interpersonal and professional relationships, and managing stressful situations.

Caregiver issues

According to the AAMR, trends indicate a four-fold decrease in the number of MR individuals in state institutions over the past 30 years. This transition out of institutions into residential settings has been beneficial for many MR individuals, as it promotes self-determination, yet this trend has increased stresses for caregivers and families in several ways. The additional emotional and social supports needed to care for MR individuals can be difficult to understand by all members of the family, particularly for siblings. Financial hardship induced by the unique needs of MR individuals can be taxing. Though many MR individuals are recipients of Medicaid benefits that decrease the financial burden of health care, there is often difficulty finding physicians who will take Medicaid patients, because reimbursement rates are lower than those provided by other types of health insurance. Physical demands are placed on caregivers and families. Activities such as lifting or transporting the person with MR or dealing with injurious or other challenging behaviors can increase stress and fatigue. Nonprofit organizations that assist families with these demands are invaluable, as they help with issues such as finding community resources, transportation, and respite care. Many of these organizations, however, have extensive waiting lists. Paid caregivers are typically reimbursed for their services at a low rate, generating high turnover and poor continuity of care. In general, stress and fatigue can prompt an increase in abusive and neglect-hil behaviors among caregivers. Such behaviors can be either passive (e.g., failure to respond to basic needs) or active (e.g., physical violence, verbal abuse). Regardless, attending to the psychological needs of the caregivers of MR individuals and providing social supports are important intervention considerations.


  1. American Association on Mental Retardation. (2002). Mental retardation: Definition, classification, and systems of supports (10th ed.). Washington, DC: Author.
  2. American Psychiatric Association. (2000). Diagnostic and statistical manual of mental disorders (4th ed., Text rev.). Washington, DC: Author.
  3. American Psychological Association. (2005). Guidelines on effective behavioral treatment for persons with mental retardation and developmental disabilities. A resolution by APA Division 33.
  4. Centers for Disease Control and Prevention. (2005, October 25). Developmental disabilities: Mental retardation. Retrieved from

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