Abnormal Psychology

Childhood Disorders 629

polychlorinated biphenyls (PCBs), and mixtures of chemicals such as those found

in pesticides. Exposure to these toxins in the fi rst trimester of pregnancy can affect

important early developmental processes of the central nervous system (Lanphear,

Vorhees, & Bellinger, 2005). Mental retardation may also arise because of a variety

of genetic abnormalities, listed in Table 14.3.

The cognitive and behavioral defi cits observed in people with mental retardation

arise because the brain does not process information appropriately, often because of

abnormal brain structure. In fact, using evidence from autopsies and neuroimaging,

researchers report that people with some forms of mental retardation have larger

heads, which may suggest larger brains (Herbert, 2005), which in turn could re-

sult from any number of factors, including larger fl uid-fi lled ventricles in the center

of the brain, bulkier white matter (myelin around the axons), or greater numbers of

glial cells.

In contrast, with other forms of mental retardation, people have smaller than

normal heads. For example, this occurs in people who suffer from fetal alcohol syn-

drome. This syndrome is a set of birth defects caused by the mother’s alcohol use

during pregnancy (alcohol is a teratogen). Moreover, smaller head size may arise be-

cause the sizes of some specifi c brain areas are reduced. In fetal alcohol syndrome,

the cerebellum, the basal ganglia, and the corpus callosum are affected (Riley &

McGee, 2005), all of which are involved in coordination and smooth motor move-

ment. The frontal lobes are also affected, specifi cally the portions involved in plan-

ning, carrying out tasks, and controlling impulsive behavior. All of these activities

are diffi cult for many children with fetal alcohol syndrome.

Although severe or profound mental retardation often involves global abnormal-

ities, each case of mild or moderate mental retardation may present a unique profi le

of specifi c impaired abilities, related to the particular cause of the retardation.

Psychological Factors: Problem Behaviors

Individuals with mental retardation often show two types of problematic behav-

iors that are not specifi cally mentioned in the DSM-IV-TR criteria: (1) stereotyped

behaviors (also referred to as stereotypies), which are repetitive behaviors that don’t

serve a function, such as hand fl apping, slight but fast fi nger and hand motions,

The characteristic facial features associated with

fetal alcohol syndrome include small eyes, a

proportionately large smooth space between the

upper lip and the nose, and a thin upper lip.

David Young-Wolff/Photo Edit

Cause of Mental Retardation Genetic Abnormality

• Down syndrome • Abnormality in chromosome 21


• Rett’s disorder (females only) • Abnormality in X chromosome (which is lethal for male fetuses)


• Fragile X ( the most common
  cause of inherited mental
  retardation)
  
  
  • Repetition of a piece of genetic code on the X chromosome
    that becomes progressively more severe in each
    generation
  
  
  
  


• Prader-Willi and Angelman
  Syndromes
  
  
  • Deletion on chromosome 15 that has different consequences
    depending on which parent’s genes contribute the deletion
  
  
  
  


• Phenylketonuria (PKU) • A genetically based defect in an enzyme, phenylalanine
  hydroxylase, that leads to a failure to convert phenylalanine
  to tyrosine. Unconverted phenylalanine is toxic to brain cells,
  leading to mental retardation. Mental retardation can be pre-
  vented if PKU is identifi ed (through a blood test at birth) and
  the individual adheres to a diet that restricts phenylalanine.


• Congenital hypothyroidism • Inadequate production of thyroid hormone caused by a
  genetic mutation. The fetus gets thyroid hormone from the
  mother, but after birth the defi ciency leads to defects in the
  developing brain. If hypothyroidism is not detected within
  the fi rst 3 months of life, the damage is not reversible even
  with thyroid hormone replacement.

Table 14.3 • Genetic Causes of Mental Retardation

Stereotyped behaviors

Repetitive behaviors—such as body

rocking—that do not serve a function; also

referred to as stereotypies.