Mental Retardation

Type of psychology: Developmental psychology
Fields of study: Childhood and adolescent disorders; organic disorders
Mental retardation occurs about three times per thousand births and usually indicates
an intelligence quotient (IQ) of less than 70. Variations in severity may allow some individuals
to be virtually independent and capable of retaining simple jobs, whereas
more severely affected persons may require lifetime institutional care. The causes of
mental retardation are numerous, with many having a clear-cut underlying genetic
basis, others implicating environmental factors, and still others with no known cause.
Key concepts
• congenital
• Down syndrome
• fetal alcohol syndrome
• fragile X syndrome
• idiopathic
• intelligence quotient (IQ)
• mental retardation
• phenylketonuria
• teratogens
The term“mental retardation” conjures up different meanings for different
people. A useful definition is provided by the American Association on Mental
Retardation: “Mental retardation is a particular state of functioning that
begins in childhood and is characterized by limitation in both intelligence
and adaptive skills.” Mental retardation reflects the “fit” between the capabilities
of individuals and the structure and expectations of their environment.
It is characterized by significantly subaverage intellectual functioning,
existing concurrently with related limitations in two or more of the
following skill areas: communication, home living, community use, health
and safety, leisure, self-care, social skills, self-direction, functional academics,
and work. It is evident that deficits in intelligence and adaptive skills will
be related to the complexity of the society in which the individual lives.
Categorizations of the severity of mental retardation have been established
based on IQ scores. The four levels of severity are mild retardation
(IQ range 50-70), moderate retardation (IQ range 35-50), severe retardation
(IQ range 20-35), and profound (IQ range less than 20). Rather than
use a classification based on the severity level, a classification based on the
type and intensity of support needed also is now in practice: intermittent,
limited, extensive, or pervasive. Persons with mild retardation usually are capable
of living with some degree of independence in the community and
can usually work successfully at simple jobs. The great majority—85 percent—
of cases of mental retardation fall into this category. The remaining
15 percent of cases are at the moderate, severe, and profound levels, with
only approximately 1 percent to 2 percent at the profound level. These last three levels are sometimes grouped together as severe. Profoundly affected
individuals require constant care and supervision.
Several causes of mental retardation are becoming known, although in
many cases it may not be possible to ascribe mental retardation to a specific
cause. Just because a disorder is congenital (present at birth) does not necessarily
imply that the disorder is genetic. Agents that are capable of affecting
the developing fetus such as alcohol, mercury, infections, maternal
phenylketonuria, and many other substances may lead to mental retardation.
Many single-gene disorders and chromosomal abnormalities produce
mental retardation as part of their syndromes, or disorders characterized by
multiple effects. A large-scale study of severely mentally retarded patients institutionalized
in Wisconsin, summarized by Sarah Bundey in 1997, indicated
that 11.8 percent of the cases were caused by chromosomal abnormality,
6.5 percent by single-gene defects, 16.3 percent by multiple congenital
anomaly syndromes, 14.7 percent by central nervous system malformations
such as hydrocephalus, 32.1 percent by central nervous system dysfunction
due to perinatal or unidentified prenatal causes including cerebral palsy, 8.5
percent by infectious disease, 3.9 percent by postnatal brain damage, and
1.2 percent by infantile psychosis; 4.3 percent were unclassified. It was noted
that the number of patients with Down syndrome was low because they were
admitted less frequently. Other surveys have shown that Down syndrome accounts
for about one-third of mentally retarded patients.