INCIDENCE OF AUTISM
is a very common disorder on the autism spectrum and it affects people equally regardless of ethnicity,
intelligence, geographical location or socioeconomic background.
Autism appears to be three to four times more likely to affect boys
gender differences in Autism
There is debate over whether boys are more likely
to be diagnosed simply because girls generally have more developed
social skills at a given age and will perform better on diagnostic
tests. Studies have found much higher prevalence in boys at the
high-functioning end of the autism spectrum, while the ratios
appear to be closer to 1:1 at the low-functioning end. In addition,
a study published in 2006 suggested that men over 40 are more likely
than younger men to father a child with autism, and that the ratio
of autism incidence in boys and girls is closer to 1:1 with older
Theories on increase of autism
There was a worldwide increase in reported cases
of autism over the decade to 2006, which may echo the pattern following
the description of schizophrenia in the twentieth century. There
are several theories about the apparent sudden increase. Many epidemiologists
argue that the rise in the incidence of autism is largely attributable
to a broadening of the diagnostic concept, reclassifications and
public awareness. However, some researchers indicate that the existence
of an as yet unidentified contributing environmental risk factor
cannot be ruled out.
The question of whether the rise in incidence is real or an artifact
of improved diagnosis
and a broader concept of autism remains controversial. Dr. Chris
Johnson, a professor of pediatrics at the University of Texas Health
Sciences Center at San Antonio and co-chair of the American Academy
of Pediatrics Autism Expert Panel, sums up the state of the issue
by saying, “There is a chance we’re seeing a true rise, but right
now I don't think anybody can answer that question for sure.”For
more information, see the Is
there an autism epidemic? fact sheet.
Incidence of autism
Estimates of the prevalence of autism vary widely
depending on diagnostic criteria, age of children screened, and
geographical location. Most recent reviews tend to estimate
a prevalence of 1–2 per 1,000 for autism and close to 6 per 1,000
for Autism Spectrum Disorder; PDD-NOS is the vast majority of Autism Spectrum Disorder, Asperger's is about
0.3 per 1,000 and the atypical forms childhood disintegrative disorder
Syndrome are much rarer. A 2006 study of nearly 57,000
British nine- and ten-year-olds reported a prevalence of 3.89 per
1,000 for autism and 11.61 per 1,000 for Autism Spectrum Disorder; these higher figures
could be associated with broadening diagnostic criteria.
The risk of autism is associated with several
prenatal and perinatal risk factors. A 2007 review of risk factors
found associated parental characteristics that included advanced
maternal age, advanced paternal age, and maternal place of birth
outside Europe or North America, and also found associated obstetric
conditions that included low birth weight and gestation duration,
and hypoxia during childbirth.
About 10–15% of autism cases have an identifiable
Mendelian (single-gene) condition, chromosome abnormality, or other
genetic syndrome, and Autism Spectrum Disorder is associated with
several genetic disorders. Autism is associated with intellectual
a 2001 British study of 26 autistic children found about 30% with
intelligence in the normal range (IQ above 70), 50% with mild to
moderate retardation, and about 20% with severe to profound retardation
(IQ below 35). For Autism Spectrum Disorders other than autism the
association is much weaker: the same study reported about 94% of
65 children with PDD-NOS or Asperger's had normal intelligence.
Autism Spectrum Disorder is also associated with epilepsy,
with variations in risk of epilepsy due to age, cognitive level,
and type of language disorder. Boys are at higher risk for autism
than girls. The Autism Spectrum Disorder sex ratio averages 4.3:1
and is greatly modified by cognitive impairment: it may be close
to 2:1 with intellectual
disability and more than 5.5:1 without. Recent
studies have found no association with socioeconomic status, and
have reported inconsistent results about associations with race
or ethnicity. Phobias, depression and other psychopathological disorders
have often been described along with Autism Spectrum Disorder but
this has not been assessed systematically.
Autism's incidence rate, despite its advantages
for assessing risk, is less useful in autism epidemiology, as the
disorder starts long before it is diagnosed, and the gap between
initiation and diagnosis
is influenced by many factors unrelated to risk. Attention is focused
mostly on whether prevalence is increasing with time. Earlier prevalence
estimates were lower, centering at about 0.5 per 1,000 for autism
during the 1960s and 1970s and about 1 per 1,000 in the 1980s, as
opposed to today's 1–2 per 1,000.
The number of reported cases of autism increased
dramatically in the 1990s and early 2000s. This increase is largely
attributable to changes in diagnostic practices, referral patterns,
availability of services, age at diagnosis, and public awareness,
though as-yet-unidentified contributing environmental risk factors
cannot be ruled out. A widely cited 2002 pilot study concluded
that the observed increase in autism in California cannot be explained
by changes in diagnostic criteria, but a 2006 analysis found
that special education data poorly measured prevalence because so
many cases were undiagnosed, and that the 1994–2003 U.S. increase
was associated with declines in other diagnostic categories, indicating
that diagnostic substitution had occurred. It is unknown whether
autism's prevalence increased during the same period. An increase
in prevalence would suggest directing more attention and funding
toward changing environmental factors instead of continuing to focus
Should research focus on genetic or environmental causes?
The answer to this question has significant ramifications
on the direction of research, since a real increase would focus
more attention and research funding on the search for environmental
factors, while the alternative would focus more attention to genetics.
On the other hand, it is conceivable that certain environmental
factors (such as chemicals, infections, medicines, vaccines, diet
and societal changes) may have a particular impact on people with
a specific genetic constitution.
Autism - a disorder or a label?
One of the more popular theories is that there
is a connection between “geekdom” and autism. This is hinted, for
instance, by a Wired Magazine article in 2001 entitled “The Geek
Syndrome”, which is a point argued by many in the autism rights
movement. This article, many professionals assert, is just one example
of the media’s application of mental disease labels to what is actually
variant normal behavior – they argue that shyness, lack of athletic
ability or social skills, and intellectual interests, even when
they seem unusual to others, are not in themselves signs of autism,
Asperger’s syndrome or other Autism Spectrum Disorder. Others
assert that children who in the past would have simply been accepted
as a little different or even labeled ‘gifted’ are now being labeled
with mental disease diagnoses.
Due to the recent publicity surrounding autism and Autism Spectrum Disorders, an increasing number of adults are choosing to seek diagnoses
of high-functioning autism or Asperger’s syndrome in light of symptoms
they currently experience or experienced during childhood. Since
the cause of autism is thought to be at least partly genetic, a
proportion of these adults seek their own diagnosis specifically
as follow-up to their children’s diagnoses. Because autism falls
into the Pervasive Developmental Disorder category, an individual’s
symptoms must have been present before age seven in order to make
a strict differential diagnosis.
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* Wing L, Potter D (2002). "The epidemiology of autism spectrum
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9. ^ Byrd RS, Sage AC, Keyzer J et al. (2002). "Report to the
legislature on the principal findings of the epidemiology of autism
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PMID 16585296. Lay summary (2006-04-03).
Click here for the full
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