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Guide · Autism

What is autism? Meaning, levels and how it's diagnosed

Autism is a lifelong difference in how a person's brain develops, affecting how they communicate, relate to others and experience the world around them. Here is what the word means, what the diagnostic manuals say, how common it is in Australia, what causes it, and how a diagnosis is made.

Updated 5 October 2026 · About 10 minutes to read · All guides

In short

  • Autism is a lifelong neurodevelopmental condition: a difference in how the brain develops that shapes social communication, patterns of behaviour and interests, and how a person responds to sensory input.
  • A diagnosis of autism spectrum disorder needs differences in social communication and interaction plus restricted or repetitive behaviours, present from early childhood and causing real difficulty in daily life.
  • "Spectrum" means autistic people differ widely from one another, not that autism runs from mild to severe. Asperger's and PDD-NOS stopped being separate diagnoses in 2013.
  • The ABS counted 290,900 autistic Australians in 2022, 1.1% of the population, up from 0.8% in 2018. Worldwide, about 1 in 100 children are autistic.
  • Autism is strongly heritable and is not caused by vaccines or parenting. In Australia it is diagnosed by paediatricians, psychiatrists, psychologists or a team, following the national guideline.

What autism is

Autism is a difference in how the brain develops. It is lifelong, present from early childhood even when nobody notices until much later, and it shapes three things: how a person communicates and relates to others, how they handle routine, change and interests, and how they respond to what they see, hear, touch and feel. Healthdirect calls it "a lifelong neurodevelopmental disorder". Autistic people are neurodivergent: their brains work differently from what is typical, which is not in itself a problem.

The formal definition: DSM-5-TR

Australian clinicians diagnose against one of two manuals. The American DSM-5-TR calls it autism spectrum disorder and requires:

  • A. Social communication and interaction. Persistent differences in all three of: back-and-forth interaction; non-verbal communication such as eye contact and gesture; and developing and understanding relationships.
  • B. Restricted, repetitive behaviours and interests. At least two of four: repetitive movements, speech or use of objects; insistence on sameness and routines; intense, narrow interests; and over- or under-reaction to sensory input.
  • C. Present in early development, although it "may not become fully manifest until social demands exceed limited capacities".
  • D. Causing significant difficulty in social, work, study or other important areas of life.
  • E. Not better explained by intellectual disability or global developmental delay.

The formal definition: ICD-11

The World Health Organization's ICD-11, code 6A02, defines autism spectrum disorder by "persistent deficits in the ability to initiate and to sustain reciprocal social interaction and social communication, and by a range of restricted, repetitive, and inflexible patterns of behaviour, interests or activities that are clearly atypical or excessive for the individual's age and sociocultural context". Onset is in the developmental period and the differences must impair daily life. It adds that "individuals along the spectrum exhibit a full range of intellectual functioning and language abilities".

What "spectrum" means

The spectrum is often pictured as a line from "mild" to "severe". That is not what the manuals describe. A person can have strong differences in one area and few in another: one autistic person speaks fluently and hides their differences at work; another has no spoken language and an intellectual disability. Healthdirect puts it plainly: "spectrum" means autism affects people differently.

Levels, Asperger's and "high-functioning"

Support levels 1, 2 and 3

The DSM-5-TR lets a clinician add a level describing support needs: level 1 "requiring support", level 2 "requiring substantial support" and level 3 "requiring very substantial support". Levels are rated separately for social communication and for restricted, repetitive behaviours, so a report can give two. The manual says they "should not be used to determine eligibility for and provision of services". A level is a snapshot of support needs at assessment, not a measure of how autistic someone is. Our autism levels guide goes deeper.

What happened to Asperger's and PDD-NOS

Until 2013 the DSM listed separate diagnoses, including autistic disorder, Asperger's disorder and pervasive developmental disorder not otherwise specified (PDD-NOS). The DSM-5, published that year, folded them into one diagnosis, autism spectrum disorder, and said anyone with a well-established DSM-IV diagnosis of any of them should now be given that diagnosis. The ICD-11 did the same: Asperger syndrome "has been discontinued as a valid diagnosis". Some people still use the word for themselves, which is their call; a report written today will say autism spectrum disorder.

Why "high-functioning" isn't a diagnosis

"High-functioning autism" has never been a diagnostic term. It usually means autistic without an intellectual disability, and it hides real need. A Western Australian study of 2,225 autistic children found that, for the 1,184 without intellectual disability, everyday adaptive skills fell well below what their IQ would predict, and the gap stayed large with age. The authors called the term "an inaccurate clinical descriptor" that should be abandoned.

How common is autism?

The Australian Bureau of Statistics counted 290,900 autistic Australians in 2022, 1.1% of the population, in its Survey of Disability, Ageing and Carers, up from 205,200 (0.8%) in 2018. Rates are highest in young people: 3.1% of people under 25, peaking at 4.4% among 10 to 14 year olds. Males were recorded at 1.6% and females at 0.7%, a narrower gap than in 2018.

Internationally, a 2022 systematic review of 71 studies from 34 countries found a median prevalence of 100 per 10,000 children, about 1 in 100. The WHO's modelled estimate is about 1 in 127 people.

Why the numbers have risen

The ABS attributes the increase to "growing awareness about autism and changes to diagnostic criteria", and to under-diagnosis of females being addressed. The research agrees:

  • Diagnostic practice changed, not children. In Denmark, a 1994 change in criteria and the 1995 inclusion of outpatient diagnoses accounted for 60% of the rise in autism prevalence among 677,915 children. In Sweden, the autism symptom phenotype measured in 19,993 twins was stable over ten years while registered diagnoses in more than a million children rose steadily.
  • Women and adults are catching up. UK primary-care records show a 787% rise in recorded diagnoses from 1998 to 2018, biggest among females and adults, which the authors put down to increased recognition.
  • Girls were missed. A meta-analysis of 54 studies found a male-to-female ratio of about 4:1 among people already diagnosed but 3:1 when whole populations were screened: girls who meet the criteria "are at disproportionate risk of not receiving a clinical diagnosis". See autism in women and girls.
  • Older adults remain undiagnosed. An English study estimated 59% to 72% of autistic people were undiagnosed in 2018; 2.94% of 10 to 14 year olds had a diagnosis against 0.02% of over-70s. See signs of autism in adults.

What causes autism

Mostly genes. A Swedish study of children born 1982 to 2006, including 37,570 twin pairs and more than 2.6 million sibling pairs, estimated autism's heritability at 83%, with shared environment contributing minimally. There is no single autism gene: Raising Children Network explains that "several genes combine and act together" and play a stronger role than environmental factors. Our is autism genetic guide goes deeper.

Two things do not cause autism. Parenting is one: Raising Children Network states that "autism isn't caused by poor parenting, vaccines or diet", and Healthdirect adds paracetamol in pregnancy to the list of things that do not. Vaccines are the other. A 2014 meta-analysis of five cohort studies (1,256,407 children) and five case-control studies (9,920 children) found no association between vaccination and autism, including the MMR vaccine and thiomersal. The WHO's 2025 fact sheet says the same.

Conditions that often come with autism

Autism rarely travels alone. A 2019 meta-analysis of 96 studies in The Lancet Psychiatry estimated how often other diagnoses occur in autistic people:

ConditionEstimated share of autistic people
ADHD28%
Anxiety disorders20%
Sleep-wake disorders13%
Disruptive, impulse-control and conduct disorders12%
Depressive disorders11%
Obsessive-compulsive disorder9%

All were more common than in the general population; estimates vary between studies.

Intellectual disability. Intellectual ability varies widely. In US surveillance of eight-year-olds in 2022, 39.6% of autistic children with IQ data were classified as having an intellectual disability; the rest were not.

Epilepsy. A 2022 meta-analysis of 66 studies put epilepsy at 10% of autistic people, higher in adults (19%) than children (7%) and more common with intellectual disability.

If both autism and ADHD fit, see AuDHD.

How autism is diagnosed in Australia

Australia has a national guideline: Autism CRC's National Guideline for the Assessment and Diagnosis of Autism in Australia, with a second edition approved by the NHMRC on 8 December 2023. It supports diagnosis against the DSM-5-TR or the ICD-11.

Who can diagnose autism

The guideline says a diagnostic evaluation should involve practitioners with specified medical or allied health qualifications "combined with advanced training and/or experience relevant to the differential diagnosis of autism". It allows a lead practitioner, with input from at least one other qualified practitioner, or a consensus team. The clinicians it lists as commonly involved include paediatricians, psychiatrists, neurologists, psychologists, speech pathologists and occupational therapists.

In practice, Healthdirect says a GP who suspects autism in a child may refer to a paediatrician, who can rule out physical causes, and to a psychiatrist or psychologist for diagnosis, and that adults are diagnosed by a psychiatrist or psychologist. Young children are usually diagnosed by multidisciplinary teams, sometimes by one professional. There is no blood test or scan; the diagnosis rests on developmental history, observation and information from more than one source. The guideline also asks for an Assessment of Functioning, so the report says what support a person needs. For a child, see signs of autism in children.

Medicare

For people under 25, Medicare rebates a paediatrician's (item 135) or psychiatrist's (item 289) assessment and treatment plan for a complex neurodevelopmental disorder such as autism, once per lifetime ($272.50 benefit). On that referral, item 82000 rebates up to eight allied health assessment services in a lifetime, including a psychologist's, at $101.55 each. There is no equivalent item for adults assessed by a psychologist. See free and low-cost assessments.

Autism and the NDIS

A diagnosis does not by itself decide NDIS access. The NDIA treats autism at level 2 or 3, diagnosed by a specialist multidisciplinary team, paediatrician, psychiatrist or clinical psychologist experienced in assessing autism, as likely to meet its disability requirements; autism not meeting that description, usually level 1, needs evidence of everyday impact. NDIS access rules are unchanged until 1 January 2028, and from October 2026 the Thriving Kids program supports children aged 8 and under with developmental delay and/or autism with low to moderate support needs, no diagnosis required. See how the NDIS works and Thriving Kids.

Autistic person or person with autism?

Identity-first language ("autistic person") treats autism as part of who someone is; person-first ("person with autism") separates the two. When 198 autistic Australian adults rated and ranked terms, "autistic", "person on the autism spectrum" and "autistic person" were rated most preferred and least offensive, and "person on the autism spectrum" came first when ranked. Participants split on whether autism "defines who I am" or is "something I have", and the researchers concluded there is no one right term. This site mostly says "autistic", and the person's own choice wins.

Questions people ask

What is autism in simple terms?

Autism is a lifelong difference in how a person's brain develops. It affects how they communicate and relate to other people, how they deal with routine, change and interests, and how they experience sound, light, touch and other sensations. It is there from early childhood, though sometimes not recognised until adulthood.

What is ASD?

ASD stands for autism spectrum disorder, the formal name for autism in both manuals used in Australia, the DSM-5-TR and the ICD-11 (code 6A02). It covers what were once separate diagnoses, including Asperger's disorder and PDD-NOS. "Autism" and "ASD" mean the same thing; many autistic Australians prefer "autistic".

Is autism a disability?

Under Australian law, yes. The Disability Discrimination Act 1992 defines disability to include "a disorder or malfunction that results in the person learning differently" and a disorder that "affects a person's thought processes, perception of reality, emotions or judgment", so autistic people are protected from discrimination. In the 2022 ABS survey, 91.1% of autistic Australians reported a disability. How much it disables any one person varies widely.

Is autism a mental illness?

No. Autism is a neurodevelopmental condition: Healthdirect calls it "a lifelong neurodevelopmental disorder", and the ICD-11 places it among the neurodevelopmental disorders. It is a difference in how the brain developed, not an illness that comes and goes. Mental health conditions are more common in autistic people (anxiety disorders in about 20%, depressive disorders in about 11% in a 2019 meta-analysis) and are treated in their own right.

What are the 3 types of autism?

There are no longer types of autism; since 2013 there has been one diagnosis, autism spectrum disorder. What people usually mean are the DSM-5-TR support levels: level 1 "requiring support", level 2 "requiring substantial support" and level 3 "requiring very substantial support". They describe support needs at the time of assessment and, the manual says, should not decide eligibility for services.

Can you be a little bit autistic?

Not as a diagnosis. Autism is diagnosed when the full pattern is present: differences in all three areas of social communication, at least two kinds of restricted or repetitive behaviour, there since early childhood and causing real difficulty in daily life. Many people have some autistic traits without meeting that bar. If traits are affecting daily life, an assessment can work out whether the pattern is there, and a careful one will say so if it isn't.

Wondering whether an assessment makes sense? The free screener takes about three minutes, for your child, your adult son or daughter, or yourself. It suggests which assessment fits, and it will tell you if one does not look like the right next step. Start the free screener.

Related

Autism levels · Signs of autism in children, from babies to teens · Is autism genetic? · Adult autism assessment · Free autism and ADHD screener · All guides

Where this information comes from

We would rather you checked us than took our word for it. Every link below is the original source, not a summary of one.

Links to external sites are provided for reference. We do not control them and they may change. Last checked 5 October 2026.

This guide is general information, not medical advice. If you are worried about yourself or your child, talk to your GP.

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