628 CHAPTER 14
verbally, which can heighten aggressive or impulsive tendencies. Unfortunately, peo-
ple with mental retardation are more likely than average to be exploited or abused
by others. Table 14.2 lists additional facts about mental retardation.Understanding Mental Retardation
There are many neurological paths to mental retardation, some of which arise be-
cause the fetus is exposed to certain types of substances (such as drugs or a virus) or
to other stimuli (such as radiation); such harmful substances and stimuli are referred
to as teratogens. Mental retardation may also arise from particular complications
during labor (such as occurs when a newborn receives insuffi cient oxygen during
birth) or from exposure to high levels of lead prenatally or during childhood (which
can occur when young children eat lead-based paint chips or inhale lead-based dust
from sanded paint; prior to 1978, lead was an ingredient used in paints).Neurological Factors: Teratogens and Genes
One type of teratogen is environmental toxins, to which a fetus typically is exposed
through the placenta after the toxin has entered the mother’s bloodstream. Examples
of environmental toxins include synthetic chemicals such as methyl mercury,Table 14.2 • Mental Retardation Facts at a Glance
Prevalence- Approximately 1% of the general population has mental retardation; however, prevalence
estimates vary depending on the survey method used and the particular population studied.
Onset- Severe and profound retardation are generally identifi ed at birth, although in some cases,
mental retardation is caused by a medical condition later in childhood, such as head trauma. - Mild retardation is sometimes not diagnosed until relatively late in childhood, although the
onset may have been earlier.
Comorbidity- Compared to the general population, people with mental retardation are three to four times
more likely to have an additional psychological disorder. - Among the most common comorbid disorders are major depressive disorder and attention-
defi cit/hyperactivity disorder. - Symptoms of mental retardation may affect the presentation of symptoms of a comorbid
disorder; people with severe or profound mental retardation, for instance, may not be able to
report feelings of hopelessness or depressed mood.
Course- The diagnosis is typically lifelong for moderate to profound mental retardation, but benefi cial
environmental factors can improve adaptive functioning for those with mild mental retardation to
the point where they no longer meet all the criteria for the disorder. - Educational opportunities, support, and stimulation can improve the level of functioning.
Gender Differences- Mental retardation occurs more frequently in males, with a male-to-female ratio of 1.5 to 1.
Cultural Differences- Although the criteria for mental retardation used in other countries are similar to those used in
the United States, they are not always the same; such differences may account for the higher
prevalence rates in some other countries, such as 4.5% in France (Oakland et al., 2003).
Source: Unless otherwise noted, the source for information is American Psychiatric Association, 2000.
Teratogens
Substances or other stimuli that are harmful
to a fetus.