Disorders usually first diagnosed in
infancy, childhood, or adolescence.
Autistic Disorder--

Autism
is a brain development disorder characterized by impairments in social interaction and communication, and restricted
and repetitive behavior, all exhibited before a child is three years old. These characteristics distinguish autism from milder
autism spectrum disorders (ASD).

Heritability contributes a large fraction of the risk of a child's developing the disorder, although the genetics of autism are
complex, and it is generally unclear which genes are responsible.[1] In rare cases, autism is strongly associated with agents that
cause birth defects.[2] Other proposed causes, such as the exposure of children to vaccines, are controversial and the vaccine
hypotheses are unsupported by convincing scientific evidence.[3][4] Most recent reviews estimate a prevalence of one to two
cases per 1,000 people for autism, and about six per 1,000 for ASD, with ASD averaging a 4.3:1 male-to-female ratio. The
number of people known to have autism has increased dramatically since the 1980s, at least partly due to changes in diagnostic
practice; the question of whether prevalence has increased is unresolved.[5]

Autism affects many parts of the brain; how this occurs is poorly understood. Parents usually notice signs in the first year or two
of their child's life. Early intervention may help children gain self-care and social skills, although few of these interventions are
supported by scientific studies; there is no cure.[6] With severe autism, independent living is unlikely; with milder autism, there
are some success stories for adults,[7] and an autistic culture has developed, with some seeking a cure and others believing
that autism is a condition rather than a disorder.[8]

Autism is a developmental disorder of the human brain that first shows signs during infancy or childhood and follows a steady
course without remission or relapse.[9] Impairments result from maturation-related changes in various systems of the brain.[10]
Autism is one of the five pervasive developmental disorders (PDD) or autism spectrum disorders (ASD), which are characterized
by widespread abnormalities of social interactions and communication, and severely restricted interests and highly repetitive
behavior.[9]

Of the other four autism spectrum disorders, Asperger's syndrome is closest to autism in signs and likely causes; Rett syndrome
and childhood disintegrative disorder share several signs with autism, but may have unrelated causes; pervasive developmental
disorder not otherwise specified (PDD-NOS) is diagnosed when the criteria are not met for a more specific disorder.[11] Unlike
autism, Asperger's has no substantial delay in language development.[12] The terminology of autism can be bewildering, with
autism, Asperger's and PDD-NOS sometimes called the autistic disorders,[1] whereas autism itself is often called autistic
disorder, childhood autism, or infantile autism. In this article, autism refers to the classic autistic disorder, while other sources
sometimes use autism to refer to autistic disorders or even ASD. ASD, in turn, is a subset of the broader autism phenotype
(BAP), which describes individuals who may not have ASD but do have autistic-like traits, such as avoiding eye contact.[13]

Autism's manifestations cover a wide spectrum, ranging from individuals with severe impairments—who may be silent, mentally
disabled, and locked into hand flapping and rocking—to less impaired individuals who may have active but distinctly odd social
approaches, narrowly focused interests, and verbose, pedantic communication.[14] Sometimes the syndrome is divided into low-
, medium- and high-functioning autism (LFA, MFA, and HFA), based on IQ thresholds,[15] or on how much support the individual
requires in daily life; these subdivisions are not standardized and are controversial. Autism can also be divided into syndromal
and non-syndromal autism, where the former is associated with severe or profound mental retardation or a congenital syndrome
with physical symptoms, such as tuberous sclerosis.[16] Although individuals with Asperger's tend to perform better cognitively
than those with autism, the extent of the overlap between Asperger's, HFA, and non-syndromal autism is unclear.[17]

Some studies have reported diagnoses of autism in children due to a loss of language or social skills, as opposed to a failure to
make progress. Several terms are used for this phenomenon, including regressive autism, setback autism, and developmental
stagnation. The validity of this distinction remains controversial; it is possible that regressive autism is a specific subtype.[18][19]

Characteristics

Autism is distinguished by a pattern of symptoms rather than one single symptom. The main characteristics are impairments in
social interaction, impairments in communication, restricted interests and repetitive behavior. Other aspects, such as atypical
eating, are also common but are not essential for diagnosis.[20]

Social development

Autistic people have social impairments and often lack the intuition about others that many people take for granted. Noted
autistic Temple Grandin described her inability to understand the social communication of neurotypicals as leaving her feeling
"like an anthropologist on Mars".[21]

Social impairments become apparent early in childhood and continue through adulthood. Autistic infants show less attention to
social stimuli, smile and look at others less often, and respond less to their own name. Autistic toddlers have more striking social
deviance; for example, they have less eye contact and anticipatory postures and are less likely to use another person's hand or
body as a tool.[19] Three- to five-year-old autistic children are less likely to exhibit social understanding, approach others
spontaneously, imitate and respond to emotions, communicate nonverbally, and take turns with others. However, they do form
attachments to their primary caregivers.[22] They display moderately less attachment security than usual, although this feature
disappears in children with higher mental development or less severe ASD.[23] Older children and adults with ASD perform
worse on tests of face and emotion recognition.[24]

Contrary to common belief, autistic children do not prefer to be alone. Making and maintaining friendships often proves to be
difficult for those with autism. For them, the quality of friendships, not the number of friends, predicts how lonely they are.[25]

There are many anecdotal reports, but few systematic studies, of aggression and violence in individuals with ASD. The limited
data suggest that in children with mental retardation, autism is associated with aggression, destruction of property, and
tantrums. Dominick et al. interviewed the parents of 67 children with ASD and reported that about two-thirds of the children had
periods of severe tantrums and about one third had a history of aggression, with tantrums significantly more common than in
children with a history of language impairment.[26]

Communication

About a third to a half of individuals with autism do not develop enough natural speech to meet their daily communication needs.
[27] Differences in communication may be present from the first year of life, and may include delayed onset of babbling, unusual
gestures, diminished responsiveness, and the desynchronization of vocal patterns with the caregiver. In the second and third
years, autistic children have less frequent and less diverse babbling, consonants, words, and word combinations; their gestures
are less often integrated with words. Autistic children are less likely to make requests or share experiences, and are more likely
to simply repeat others' words (echolalia)[18][28] or reverse pronouns.[29] Autistic children may have difficulty with imaginative
play and with developing symbols into language.[18][28] They are more likely to have problems understanding pointing; for
example, they may look at a pointing hand instead of the pointed-at object.[19][28]

In a pair of studies, high-functioning autistic children aged 8–15 performed equally well, and adults better than individually
matched controls at basic language tasks involving vocabulary and spelling. Both autistic groups performed worse than controls
at complex language tasks such as figurative language, comprehension and inference. As people are often sized up initially
from their basic language skills, these studies suggest that people speaking to autistic individuals are more likely to
overestimate what their audience comprehends.[30]

Repetitive behavior

Autistic individuals display many forms of repetitive or restricted behavior, which the Repetitive Behavior Scale-Revised (RBS-R)
categorizes as follows:

* Stereotypy is apparently purposeless movement, such as hand flapping, head rolling, body rocking, or spinning a plate.
* Compulsive behavior is intended and appears to follow rules, such as arranging objects in a certain way.
* Sameness is resistance to change; for example, insisting that the furniture not be moved or refusing to be interrupted.
* Ritualistic behavior involves the performance of daily activities the same way each time, such as an unvarying menu or
dressing ritual.
* Restricted behavior is limited in focus, interest, or activity, such as preoccupation with a single television program.
* Self-injury includes movements that injure or can injure the person, such as biting oneself. Dominick et al. reported that self-
injury at some point affected about 30% of children with ASD.[26]

No single repetitive behavior is associated with autism, but only autism appears to have an elevated pattern of occurrence and
severity of these behaviors.[31]

Other symptoms

Autistic individuals may have symptoms that are independent of the diagnosis, but that can affect the individual or the family.[20]
As many as 10% of individuals with ASD show unusual abilities, ranging from splinter skills such as the memorization of trivia to
the extraordinarily rare talents of prodigious autistic savants.[32]

Unusual responses to sensory stimuli are more common and prominent in autistic children, although there is no good evidence
that sensory symptoms differentiate autism from other developmental disorders.[33] The responses may be more common in
children: a pair of studies found that autistic children had impaired tactile perception while autistic adults did not. The same two
studies also found that autistic individuals had more problems with complex memory and reasoning tasks such as Twenty
Questions; these problems were somewhat more marked among adults.[30] Several studies have reported associated motor
problems that include poor muscle tone, poor motor planning, and toe walking; ASD is not associated with severe motor
disturbances.[34]

Atypical eating behavior occurs in about three-quarters of children with ASD, to the extent that it was formerly a diagnostic
indicator. Selectivity is the most common problem, although eating rituals and food refusal also occur;[26] this does not appear
to result in malnutrition. Some children with autism also have gastrointestinal (GI) symptoms, but there is a lack of published
rigorous data to support the theory that autistic children have more or different GI symptoms than usual.[35]

Sleep problems are known to be more common in children with developmental disabilities, and there is some evidence that
children with ASD are more likely to have even more sleep problems than those with other developmental disabilities; autistic
children may experience problems including difficulty in falling asleep, frequent nocturnal awakenings, and early morning
awakenings. Dominick et al. found that about two-thirds of children with ASD had a history of sleep problems.[26]

Causes of autism

Although many genetic and environmental causes of autism have been proposed, its theory of causation is still incomplete.[36]
Some researchers argue this is because autism is not a single disorder, but rather a triad of core aspects (social impairment,
communication difficulties, and repetitive behaviors) that have distinct causes but often co-occur.[37]

Genetic factors are the most significant cause for autism spectrum disorders. Early studies of twins estimated heritability to be
more than 90%; in other words, that genetics explains more than 90% of autism cases.[1] This may be an overestimate; new twin
data and models with structural genetic variation are needed.[38] When only one identical twin is autistic, the other often has
learning or social disabilities. For adult siblings, the risk of having one or more features of the broader autism phenotype might
be as high as 30%,[39] much higher than the risk in controls.[40]

The genetics of autism is complex.[1] Linkage analysis has been inconclusive; many association analyses have had inadequate
power.[38] For each autistic individual, mutations in more than one gene may be implicated. Mutations in different sets of genes
may be involved in different autistic individuals. There may be significant interactions among mutations in several genes, or
between the environment and mutated genes. By identifying genetic markers inherited with autism in family studies, numerous
candidate genes have been located, most of which encode proteins involved in neural development and function.[41] However,
for most of the candidate genes, the actual mutations that increase the risk for autism have not been identified. Typically, autism
cannot be traced to a Mendelian (single-gene) mutation or to a single chromosome abnormality such as fragile X syndrome or
22q13 deletion syndrome.[16][42]

The large number of autistic individuals with unaffected family members may result from copy number variations (CNVs)—
spontaneous alterations in the genetic material during meiosis that delete or duplicate genetic material. Sporadic (non-inherited)
cases have been examined to identify candidate genetic loci involved in autism. Using array comparative genomic hybridization
(array CGH), a technique for detecting CNVs, one study found them in 10% of families with one affected child.[44] Some of the
altered loci had been identified in previous studies of inherited autism; many were unique to the sporadic cases examined in this
study. Hence, a substantial fraction of autism may be highly heritable but not inherited: that is, the mutation that causes the
autism is not present in the parental genome. The fraction of autism traceable to a genetic cause may grow to 30–40% as the
resolution of array CGH improves.[43] The Autism Genome Project database contains genetic linkage and CNV data that
connect autism to genetic loci and suggest that every human chromosome may be involved.[45]

Teratogens (agents that cause birth defects) related to the risk of autism include exposure of the embryo to thalidomide,
valproic acid, or misoprostol, or to rubella infection in the mother. These cases are rare.[46] All known teratogens appear to act
during the first eight weeks from conception, and though this does not exclude the possibility that autism can be initiated or
affected later, it is strong evidence that autism arises very early in development.[2] Other possible contributors to autism include
gastrointestinal or immune system abnormalities, allergies, and the exposure of children to drugs, vaccines,[4] infection, certain
foods,[47] or heavy metals; the evidence for these risk factors is anecdotal and has not been confirmed by reliable studies,[3]
and extensive further searches are underway.[46] Parents may first become aware of autistic symptoms in their child around the
time of a routine vaccination. Although there is overwhelming scientific evidence showing no causal association between the
measles-mumps-rubella vaccine and autism, and there is no convincing evidence that the vaccine preservative thiomersal helps
cause autism, parental concern has led to a decreasing uptake of childhood immunizations and the increasing likelihood of
measles outbreaks[48] such as the measles cases in Britain during summer 2007.[49]

Mechanism

Despite extensive investigation, how autism occurs is not well understood. Its mechanism can be divided into two areas: the
pathophysiology of brain structures and processes associated with autism, and the neuropsychological linkages between brain
structures and behaviors.[10]

Pathophysiology

Autism appears to result from developmental factors that affect many or all functional brain systems, as opposed to localized
physical damage.[42] Neuroanatomical studies and the associations with teratogens strongly suggest that autism's mechanism
includes alteration of brain development soon after conception.[2] Many major structures of the human brain have been
implicated. Consistent abnormalities have been found in the development of the cerebral cortex; and in the cerebellum and
related inferior olive, which have a significant decrease in the number of Purkinje cells. Brain weight and volume and head
circumference tend to be greater in autistic children; the effects of these are unknown.[50] It may be due to poorly regulated
growth of neurons.[10]

Interactions between the immune system and the nervous system begin early during embryogenesis, and successful
neurodevelopment depends on a balanced immune response. Several symptoms consistent with a poorly regulated immune
response have been reported in autistic children. It is possible that aberrant immune activity during critical periods of
neurodevelopment is part of the mechanism of some forms of ASD.[51] Given the lack of data in this area, it is still hard to draw
conclusions about the role of immune factors in autism.[52]

Several neurotransmitter abnormalities have been detected in autism, notably increased blood levels of serotonin. Whether
these lead to structural or behavioral abnormalities is unclear.[10]

The mirror neuron system (MNS) theory of autism hypothesizes that distortion in the development of the MNS interferes with
imitation and leads to autism's core features of social impairment and communication difficulties. The MNS operates when an
animal performs an action or observes another animal of the same species perform the same action. The MNS may contribute to
an individual's understanding of other people by enabling the modeling of their behavior via embodied simulation of their
actions, intentions, and emotions.[53] Several studies have tested this hypothesis by demonstrating structural abnormalities in
MNS regions of individuals with ASD, delay in the activation in the core circuit for imitation in individuals with Asperger's, and a
correlation between reduced MNS activity and severity of the syndrome in children with ASD.[54]

The underconnectivity theory of autism hypothesizes that autism is marked by underfunctioning high-level neural connections
and synchronization, along with an excess of low-level processes.[55] Evidence for this theory has been found in functional
neuroimaging studies on autistic individuals[30] and by a brain wave study that suggested that adults with ASD have local
overconnectivity in the cortex and weak functional connections between the frontal lobe and the rest of the cortex.[56] Other
evidence suggests the underconnectivity is mainly within each hemisphere of the cortex and that autism is a disorder of the
association cortex.[57]

Neuropsychology

Two major categories of cognitive theories have been proposed about the links between autistic brains and behavior.

The first category focuses on deficits in social cognition. Hyper-systemizing hypothesizes that autistic individuals can
systematize—that is, they can develop internal rules of operation to handle internal events—but are less effective at
empathizing by handling events generated by other agents.[15] It extends the extreme male brain theory, which hypothesizes
that autism is an extreme case of the male brain, defined psychometrically as individuals in whom systemizing is better than
empathizing.[58] This in turn is related to the earlier theory of mind, which hypothesizes that autistic behavior arises from an
inability to ascribe mental states to oneself and others. The theory of mind is supported by autistic children's atypical responses
to the Sally-Anne test for reasoning about others' motivations,[59] and is mapped well from the mirror neuron system theory of
autism.[54]

The second category focuses on nonsocial or general processing. Executive dysfunction hypothesizes that autistic behavior
results in part from deficits in flexibility, planning, and other forms of executive function. A strength of the theory is predicting
stereotyped behavior and narrow interests;[60] a weakness is that executive function deficits are not found in young autistic
children.[24] Weak central coherence theory hypothesizes that a limited ability to see the big picture underlies the central
disturbance in autism. One strength of this theory is predicting special talents and peaks in performance in autistic people.[61] A
related theory—enhanced perceptual functioning—focuses more on the superiority of locally oriented and perceptual
operations in autistic individuals.[62] The latter two theories map well from the underconnectivity theory of autism.

Neither category is satisfactory on its own; social cognition theories poorly address autism's rigid and repetitive behaviors, while
the nonsocial theories have difficulty explaining social impairment and communication difficulties.[37] A combined theory based
on multiple deficits may prove to be more useful.[8]

Screening

Parents are usually the first to notice unusual behaviors in their child.[63] As postponing treatment may affect long-term
outcome, any of the following signs is reason to have a child evaluated by a specialist without delay:

* No babbling by 12 months.
* No gesturing (pointing, waving goodbye, etc.) by 12 months.
* No single words by 16 months.
* No two-word spontaneous phrases (not including echolalia) by 24 months.
* Any loss of any language or social skills, at any age.[20]

The American Academy of Pediatrics recommends that all children be screened for ASD at the 9-, 18-, and 30-month well-child
doctor visits, using autism-specific formal screening tests.[64] In contrast, the UK National Screening Committee recommends
against screening for ASD in the general population, because screening tools have not been fully validated and interventions
lack sufficient evidence for effectiveness.[65]

Genetic screening for autism is generally still impractical. As genetic tests are developed several ethical, legal, and social issues
will emerge. Commercial availability of tests may precede adequate understanding of how to use test results, given the
complexity of autism's genetics.[66]

Diagnosis

Autism is defined in the DSM-IV-TR as exhibiting at least six symptoms total, including at least two symptoms of qualitative
impairment in social interaction, at least one symptom of qualitative impairment in communication, and at least one symptom of
restricted and repetitive behavior. Sample symptoms include lack of social or emotional reciprocity, stereotyped and repetitive
use of language or idiosyncratic language, and persistent preoccupation with parts of objects. Onset must be prior to age three
years, with delays or abnormal functioning in either social interaction, language as used in social communication, or symbolic or
imaginative play. The disturbance must not be better accounted for by Rett syndrome or childhood disintegrative disorder.[67]
ICD-10 uses essentially the same definition.[9]

Several diagnostic instruments are available. Two are commonly used in autism research: the Autism Diagnostic Interview-
Revised (ADI-R) is a semistructured parent interview, and the Autism Diagnostic Observation Schedule (ADOS) uses
observation and interaction with the child. The Childhood Autism Rating Scale (CARS) is used widely in clinical environments to
assess severity of autism based on observation of children.[19]

A pediatrician commonly performs a preliminary investigation by taking developmental history and physically examining the child.
If warranted, diagnosis and evaluations are conducted with help from ASD specialists, observing and assessing cognitive,
communication, family, and other factors using standardized tools, and taking into account any associated medical conditions. A
differential diagnosis for ASD at this stage might also consider mental retardation, hearing impairment, and a specific language
disorder[68] such as Landau-Kleffner syndrome.[69] In the UK the National Autism Plan for Children recommends at most 30
weeks from first concern to completed diagnosis and assessment, though few cases are handled that quickly in practice.[68]
ASD can sometimes be diagnosed by age 14 months,[70] but a 2006 U.S. study found the average age of first evaluation by a
qualified professional was 48 months and of formal ASD diagnosis was 61 months, reflecting an average 13-month delay, all far
above recommendations.[71]

Underdiagnosis and overdiagnosis are problems in marginal cases, and much of the recent increase in the number of reported
ASD cases is likely due to changes in diagnostic practices. The increasing popularity of drug treatment options and the
expansion of benefits has given providers incentives to diagnose ASD, resulting in some overdiagnosis of children with
uncertain symptoms. Conversely, the cost of screening and diagnosis and the challenge of obtaining payment can inhibit or
delay diagnosis.[72] It is particularly hard to diagnose autism among the visually impaired, partly because some of its diagnostic
criteria depend on vision, and partly because autistic symptoms overlap with those of common blindness syndromes.[73]

The symptoms of autism and ASD begin early in childhood but are occasionally missed. Adults may seek retrospective
diagnoses to help them or their friends and family understand themselves, to help their employers make adjustments, or in some
locations to claim disability living allowances or other benefits.[74]

Autism therapies

The goal of treatment is to manage and improve symptoms and functioning. No single treatment is best and treatment is typically
tailored to the child's needs. Intensive, sustained special education programs and behavior therapy early in life can help
children acquire self-care, social, and job skills.[75][76] Among the available approaches, applied behavior analysis (ABA) has
demonstrated efficacy in promoting social and language development and in reducing behaviors that interfere with learning and
cognitive functioning;[76] ABA focuses on teaching tasks one-on-one using the behaviorist principles of stimulus, response and
reward.[77] Several programs are based on ABA. Some focus on discrete trial teaching; more-comprehensive ones use multiple
assessment and intervention methods individually and dynamically.[78] Cognitive therapies based on comprehensive programs
in treatment centers are a common alternative: for example, TEACCH focuses on structuring the physical environment and using
visual supports for language development tasks.[76] A 2005 California study found that early intensive behavior analytic
treatment, a form of ABA, was substantially more effective for preschool children with autism than the mixture of methods
provided in many programs,[77], but a 2007 British study found that home-based early intensive behavioral interventions,
another ABA form, was no more effective than nursery-based eclectic programs.[79] The limited research on the effectiveness
of adult residential programs shows mixed results.[80]

Medications are often used to treat problems associated with ASD. More than half of U.S. children diagnosed with ASD are
prescribed psychoactive drugs or anticonvulsants, with the most common drug classes being antidepressants, stimulants, and
antipsychotics.[81] In the United States, the antipsychotic risperidone is approved for treating symptomatic irritability in autistic
children and adolescents.[82] Other drugs are prescribed off-label, which means they have not been approved for treating ASD.
For example, serotonin reuptake inhibitors and dopamine blockers can sometimes reduce some symptoms.[10] However, there
is scant reliable research about the effectiveness or safety of drug treatments for adolescents and adults with ASD.[83] A
person with ASD may respond atypically to medications, the medications can have adverse side effects, and no known
medication relieves autism's core symptoms of social and communication impairments.[63]

Many other therapies and interventions are available. Few are supported by scientific studies.[6][24][84][85] Treatment
approaches lack empirical support in quality-of-life contexts, and many programs focus on success measures that lack
predictive validity and real-world relevance.[25] Scientific evidence appears to matter less to service providers than program
marketing, training availability, and parent requests.[86] Even if they do not help, conservative treatments such as changes in
diet are probably harmless aside from their bother and cost.[47] Dubious invasive treatments are a much more serious matter:
for example, in 2005, botched chelation therapy killed a 5-year-old autistic boy.[48]

Treatment is expensive;[87] indirect costs are more so. A U.S. study estimated the average additional lifetime cost due
exclusively to autism to be $3.2 million in 2003 U.S. dollars for an autistic individual born in 2000, with about 10% medical care,
30% nonmedical care such as child care and education, and 60% the lost economic productivity of individuals and their parents.
[88] A British study estimated an average lifetime cost of ₤2.4 million in 1997–1998 British pounds.[89] Legal rights to treatment
are complex, vary with location and age, and require advocacy by caregivers.[85] Publicly supported programs are often
inadequate or inappropriate for a given child, and unreimbursed out-of-pocket medical or therapy expenses are associated with
likelihood of family financial problems.[90] After childhood, key treatment issues include residential care, job training and
placement, sexuality, social skills, and estate planning.[85]

Prognosis

No cure is known for autism. Most children with autism lack social support, meaningful relationships, future employment
opportunities or self-determination.[25] Although core difficulties remain, the severity of symptoms often becomes less marked in
later childhood.[91] Few high-quality studies address long-term prognosis. Some adults show modest improvement in some
symptoms, but some decline; no study has focused on autism after midlife.[92] Acquiring language before age 6, having IQ
above 50, and having a marketable skill all predict better outcomes; independent living is unlikely with severe autism.[93] A 2004
British study of 68 adults who were diagnosed before 1980 as autistic children with IQ above 50 found that 12% achieved a high
level of independence as adults, 10% had some friends and were generally in work but required some support, 19% had some
independence but were generally living at home and needed considerable support and supervision in daily living, 46% needed
specialist residential provision from facilities specializing in ASD with a high level of support and very limited autonomy, and 12%
needed high-level hospital care.[7] A 2005 Swedish study of 78 adults that did not exclude low IQ found worse prognosis; for
example, only 4% achieved independence.[94] Changes in diagnostic practice and increased availability of effective early
intervention make it unclear whether these findings can be generalized to recently diagnosed children.[5]

Epidemiology

Estimates of the prevalence of autism vary widely depending on diagnostic criteria, age of children screened, and geographical
location.[95] Most recent reviews tend to estimate a prevalence of 1–2 per 1,000 for autism and close to 6 per 1,000 for ASD;[5]
PDD-NOS is the vast majority of ASD, Asperger's is about 0.3 per 1,000 and the atypical forms childhood disintegrative disorder
and Rett syndrome are much rarer.[96] 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 ASD; these higher figures could be associated with broadening diagnostic
criteria.[97]

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.[98]

About 10–15% of autism cases have an identifiable Mendelian (single-gene) condition, chromosome abnormality, or other
genetic syndrome,[39] and ASD is associated with several genetic disorders.[99] Autism is associated with mental retardation: 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 ASD 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.[100] ASD is also associated with epilepsy, with variations in risk of epilepsy due to age, cognitive level, and type of
language disorder.[101] Boys are at higher risk for autism than girls. The ASD sex ratio averages 4.3:1 and is greatly modified
by cognitive impairment: it may be close to 2:1 with mental retardation 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.[5]
Phobias, depression and other psychopathological disorders have often been described along with ASD but this has not been
assessed systematically.[102]

Autism's incidence, 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.[5]

Reports of autism cases grew dramatically in the U.S. in 1996–2005. It is unknown how much, if any, growth came from changes
in autism's prevalence.

Reports of autism cases grew dramatically in the U.S. in 1996–2005. It is unknown how much, if any, growth came from changes
in autism's prevalence.

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,
[103] though as-yet-unidentified contributing environmental risk factors cannot be ruled out.[3] A widely cited 2002 pilot study
concluded that the observed increase in autism in California cannot be explained by changes in diagnostic criteria,[104] 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.[105] 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 on genetics.[46]

* References provided upon request.