What is Autism Spectrum Disorder?

Autism Spectrum Disorder is a neurodevelopmental disorder that affects the growth, development and functioning of the brain. Children with ASD typically find social interaction challenging and will often have restricted, repetitive patterns of behaviour. Let’s explore ASD in more detail to learn more about brain development and the behaviours exhibited by children with ASD.

A child with autism spectrum disorder poses for a photograph

Brain Development in Children with Autism Spectrum Disorder

The DSM-5 classifies Autism Spectrum Disorder (ASD) as a type of neurodevelopmental disorder, which means there is atypical growth and development of the brain or central nervous system, resulting in their abnormal functioning.


This diagram below highlights a few of brain regions that are structurally distinct in individuals with ASD. Please note this is not a comprehensive list of all the structural differences.

  1. Cerebral Cortex
  2. Amygdala
  3. Hippocampus
  4. Cerebellum
  5. Temporal Lobe

Regions of the brain that when underdeveloped cause autism spectrum disorder


1. Cerebral Cortex - structural variations cause difficulty with attention span, processing sensory information, engaging in complex thought and abstract reasoning as well as producing and understanding language.

2. Amygdala - structural variations in the size of the Amygdala cause more difficulty with regulating their emotions and stress.

3. Hippocampus - structural variations in the size of the Hippocampus causes problems with sleep, learning and memory.

4. Cereberllum - structural varations in the amounts of brain tissue causes difficulties with posture, balance, coordination and speech production.   

5. Temporal Lobe - structural variations cause difficulties with:

  • sensory processing,
  • speech production,
  • memory storage,
  • object recognition (such as faces),
  • processing emotions,
  • controlling unconscious reactions such as appetite, thirst and hunger.

Each individual with ASD has a unique brain. No two brains of individuals with ASD are alike. This means each individual will have their own unique profile of needs, abilities and preferences.  

To truly understand what ASD is, we need to look next at the symptoms and the ways in which the changes in the brain actually manifest as recognisable behaviours we can observe and relate to.  

Symptoms of Autism Spectrum Disorder

The symptoms of ASD can be categorised into two domains:

  1. Difficulties with social communication and social interaction; or
  2. Restricted, repetitive patterns of behaviour, interests or activities. 

It’s worth remembering that Autism is a spectrum disorder which means that the symptoms can vary from person to person. To learn more about how these symptoms can present, let’s explore the categories in more detail.

Difficulties with Social Communication and Social Interaction

For someone to actually be diagnosed with ASD they will demonstrate persistent difficulties in social communication and social interaction as evidenced by the presence of all of the following symptoms, across multiple contexts, currently or historically:

Difficulty with social-emotional reciprocity.

This can range from abnormal initiation of social interactions and difficulty with normal back-and-forth conversation; to reduced sharing of interests, emotions, or thoughts; to a complete lack of initiating or responding to social interactions.

Difficulties with nonverbal communication. 

This can range from poorly integrating nonverbal communication like body language with verbal communication; to abnormalities in eye contact and body language or difficulty understanding and using gestures; to a total lack of facial expressions and nonverbal communication.

Deficits in developing, maintaining, and understanding relationships. 

This can range from difficulties adjusting behaviour to suit various social contexts; to difficulties making friends; to a complete lack of interest in peers.
To put the definitions into context, you may notice how children with ASD can often appear withdrawn from social groups, perhaps in the playground for example, or wound up in a confrontation with other children because they are struggling to process the social interaction and are, as a result, having a stress response.

Restricted, repetitive patterns of behaviour, interests, or activities.

You may have noticed children with Autism will often obsess over a particular toy or have an intense interest, at the exclusion of other interests, in a particular computer game, hobby or other activity.  

For someone to be diagnosed with Autism, there will be at least two of the following symptoms present either currently or historically:

Repetitive Movements

Stereotyped or repetitive movements, use of objects, or speech, such as lining up toys or flipping objects, repeating the words of others back to them, or repeating idiosyncratic phrases.

Routine and inflexibility

Insistence on sameness, inflexible adherence to routines, or ritualised patterns of verbal and nonverbal behaviour, such as extreme distress at small changes, difficulties with transitioning between places or activities, rigid thinking patterns, or greeting rituals.

Abnormally intense interests

Highly restricted, fixated interests that are abnormal in intensity or focus, such as strong attachment to or preoccupation with unusual objects.

Sensory sensitivity and interest

Hyperreactivity or hyporeactivity to sensory input or unusual interest in sensory aspects of the environment, such as apparent indifference to pain or temperature, adverse response to specific sounds or textures, excessive smelling or touching of objects, or visual fascination with lights or movement.

For an individual to be diagnosed with ASD, symptoms must be present at an early age, but may not fully manifest until social demands exceed the ability of the individual to deal with them. 

The symptoms should cause significant impairment on social, occupational or other important areas and not be better explained by intellectual disability or global development delay. 

Diagnosing Autism

If an individual is diagnosed with ASD, the medical professional will also specify if it is:

  1. With or without accompanying intellectual impairment, 
  2. With or without accompanying language impairment,
  3. Associated with a known medical or genetic condition or environmental factor,
  4. Associated with another neurodevelopmental, mental, or behavioural disorder, 
  5. With catatonia.

Individuals with Autism Spectrum Disorder may have one or more of the following conditions at any time during their development:


Severity Levels of ASD

An ASD diagnosis includes a severity level, which is used to show how much support the individual needs.
The diagram below shows how Autism is a ‘spectrum disorder’ because there is a wide variation in the type and severity of symptoms people experience.

Autism Spectrum Disorder severity spectrum graphic

When diagnosing Autism, there are three levels of severity.  This table describes the characteristics of the condition at each level:


Severity Level Social Communication Restricted, Repetitive Behaviour
Level 3 – Requiring very substantial support

The individual has severe deficits in verbal and nonverbal social communication skills that cause severe impairments in functioning; very limited initiation of social interactions; and limited response to social interactions from others.

For example, a person with few words of intelligible speech who rarely initiates interaction and, when he or she does, makes unusual approaches and does so only to meet their needs, and responds to only very direct social approaches.

The individual demonstrates inflexibility of behaviour, extreme difficulty coping with change, or other restricted/ repetitive behaviours that substantially interfere with all areas of functioning. The individual has great distress or difficulty changing focus or activity.

Level 2 – Requiring substantial support

The individual has significant deficits in verbal and nonverbal social communication skills; social impairments are apparent even with supports in place; limited initiation of social interactions; and reduced or abnormal responses to social interactions from others. 

For example, a person who speaks in simple sentences, whose interaction is limited to narrow special interests, and who has markedly odd nonverbal communication.

The individual experiences inflexibility of behaviour, difficulty coping with change, or other restricted/ repetitive behaviours that appear frequently enough to be obvious to the casual observer and interfere with functioning in a variety of contexts. The individual experiences distress or difficulty changing focus or activity.
Level 1 – Requiring support

Without supports in place, deficits in social communication cause noticeable impairments. The individual has difficulty initiating social interactions, and there are clear examples of atypical or unsuccessful responses to social interactions of others. They may appear to have decreased interest in social interactions. 

For example, a person who is able to speak in full sentences and engages in communication but whose to-and-fro conversation with others fails, and whose attempts to make friends are odd and typically unsuccessful.

The individual’s inflexibility of behaviour causes significant interference with functioning in one or more contexts. They have difficulty switching between activities. Difficulty with organisation and planning hampers their independence.

To conclude, ASD is a type of neurodevelopmental disorder characterised by deficits in social communication and social interaction and the presence of restricted, repetitive behaviours. With the right supports and services we can empower individuals with ASD to lead a fulfilling and valued life.

Further reading and support

We have a more technical factsheet about Autism available on this website to read and we also offer online courses on supporting individuals with ASD together with the book by Dolly Bhargava: Positive Behaviour Support Strategies for Students with Autism Spectrum Disorder.



American Psychiatric Association (2013). Diagnostic and Statistical Manual of Mental Disorders -5th ed. Arlington, VA: American Psychiatric Association.

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