March 1, 2011 ◆ Volume 83, Number 5
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American Family Physician 515
The Changing Prevalence of the
Autism Spectrum Disorders
CATHERINE E. RICE, PhD
National Center on Birth Defects and
Developmental Disabilities, Centers for Disease
Control and Prevention, Atlanta, Georgia
Autism spectrum disorders are a group of
developmental disabilities characterized by
atypical development in socialization, com-
munication, and behavior. The term autism
spectrum disorders describes three of the five
pervasive developmental disorders, including
autistic disorder, Asperger syndrome, and
pervasive developmental disorder—not oth-
erwise specified. Symptoms of autism spec-
trum disorders typically are present before
three years of age and often are accompanied
by unusual patterns in learning, attention,
and sensory processing.1 Based on the most
recent report by the Autism and Devel-
opmental Disabilities Monitoring (ADDM)
Network, an average of one in 110 children
was identified with autism spectrum disor-
ders, which translates to about 1 percent of
children.2 For every girl, four or five boys
were affected by the autism spectrum dis-
orders. Less than one-half of the children
with autism spectrum disorders also had an
intellectual disability, and 47 percent were
diagnosed with autistic disorder by eight
years of age.
We know that the number of persons
receiving services for autism spectrum dis-
orders has increased substantially since the
early 1990s,3,4 but are there really more
children today with the cluster of behaviors
that make up the autism spectrum disorders
than there were in the past? Debates on
this issue typically have been dichotomized
to indicate that increases in the preva-
lence of autism spectrum disorders must
be explained by changes in identification
patterns or by a true increase in symptoms
among children born in more recent times.
We do know that the way autism spectrum
disorders are defined and identified makes a
difference in the prevalence estimates. From
the time autism was first described in the
1940s until the 1980s, the term primarily
referred to more severely affected persons
with autistic disorder and was thought to be
rare, affecting approximately one in every
2,000 children (0.05 percent).5 Current esti-
mates on the prevalence of autism spectrum
disorders based on expanded definitions
from the Diagnostic and Statistical Manual
of Mental Disorders, 4th ed., are approxi-
mately 10 to 20 times higher than those
using earlier criteria. Some of the most
recent studies have documented a preva-
lence of more than 1 percent of children in
multiple areas of the world,2,6-8 with symp-
toms identified in 2.7 percent of children.9
One effort to retrospectively apply more
modern criteria to an older study estimated
a prevalence of about four per 1,000 chil-
dren, indicating that older studies underes-
timated prevalence because they considered
a more strict interpretation of what autism
and related disorders look like.10
Few studies have tracked the same pop-
ulation over time to evaluate changes in
the prevalence of the autism spectrum dis-
orders. The ADDM Network reported a
57 percent increase overall in the prevalence
of identified autism spectrum disorders
among eight-year-old children from 2002 to
2006.2 On average, the prevalence increased
across all sex, racial/ethnic, and cognitive
functioning subgroups. Improved identifi-
cation of some subgroups was a factor but
could not completely explain how the preva-
lence increased by more than 50 percent in
such a short time. These findings raise the
question of what else may be contributing
to the increase in autism spectrum disorder
The search for a single cause account-
ing for the change in prevalence is likely to
obscure our view of a much more compli-
cated situation involving multiple potential
biologic and environmental factors. Some
progress has been reported in quantifying
the effect of single factors such as reduction
in age at diagnosis, inclusion of milder cases,
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520 American Family Physician
Volume 83, Number 5 ◆ March 1, 2011
and a shift in diagnostic patterns.11,12 The progressive
increases in the prevalence of autism spectrum disorders
underscore the need to better understand the risk factors
and causes of these conditions. One study estimated that
about 5 percent of the increased prevalence of autism
spectrum disorders in a California service system was
caused by older parental age,13 but this appears to be
only one small part of the picture. Although immuni-
zation has been the subject of great debate, studies on
immunization practices have not shown a correlation
with changes in the prevalence of autism spectrum
disorders.14,15 Efforts are needed to understand the
multiple, possibly overlapping factors involved in the
increasing prevalence of autism spectrum disorders.
In addition, improved understanding of concomitant
changes in the prevalence of other childhood develop-
mental disabilities and health conditions would help us
gain a broader understanding of the changing landscape
of health and development.16,17
The fact that some increases in the prevalence of autism
spectrum disorders are accounted for by improved iden-
tification and awareness is important and positive news
for the many persons who may have better-informed
intervention options and earlier access to services. How-
ever, we still have work to do to improve early identifica-
tion. The ADDM Network found that although parents
or professionals noted concerns about a child’s develop-
ment before two years of age 70 to 95 percent of the time,
the median age at diagnosis of the autism spectrum dis-
orders was four years, six months. A significant lag exists
between the time that earliest concerns are expressed
and when autism spectrum disorders are diagnosed,
contributing to potentially significant delays in inter-
vention. It is important to monitor the early and ongo-
ing development of every child and to refer for further
assessment and intervention if delays are suspected.18,19
Family physicians are particularly important in helping
families navigate the complex array of health care and
intervention needs. As we continue to explore the com-
plex etiology of autism, concerted efforts are essential
to address the many needs of affected persons and to
provide coordinated support services that improve daily
functioning and long-term life outcomes.
The findings and conclusions in this report are those of the author and do
not necessarily represent the official position of the Centers for Disease
Control and Prevention.
Address correspondence to Catherine E. Rice, PhD, at firstname.lastname@example.org.
Reprints are not available from the author.
Author disclosure: Dr. Rice has served as a consultant to Western Psy-
chological Services on autism assessment training.
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Am Fam Physician.?2010;81(4):453-460.?■