ALTHOUGH WE KNOW autism has a genetic foundation, leading to neurobiological differences, it is diagnosed on the basis of a set of behaviours. The key reason for this is that, despite attempts, no reliable biological marker has been found. A reliable marker must show adequate sensitivity and specificity: meaning that it must be found in virtually all members of a group, and it must be exclusive to that group. At the moment, the best biomarker candidates we have for autism resemble attempts to identify which UK region you are from based on hair colour. Yes, there are group differences in prevalence of red hair between Scotland and other UK nations, but red hair is not found in a large enough percentage of the Scottish population to be sensitive and is found too widely elsewhere to be specific.


Our reliance on behaviour to identify autism leads to challenges for the field, as we will see in this chapter and beyond. Interpreting changing prevalence estimates or identifying meaningful sub-groups is extremely hard given the variability in how behavioural diagnostic features have been defined over time, and the potential differences in how they are applied in different settings.


• Persistent deficits in social communication and social interaction across multiple contexts, as manifested by the following, currently or by history (examples are illustrative, not exhaustive) 

• Deficits in social-emotional reciprocity, ranging, for example, from abnormal social approach and failure of normal back-and-forth conversation; to reduced sharing of interests, emotions or affect; to failure to initiate or respond to social interactions.

Try to provide a sophisticated way to disguise curtain rails or tracks around the sash windows in your house.

• Deficits in non-verbal communicative behaviours used for social interaction, ranging, for example, from poorly integrated verbal and non-verbal communication; to abnormalities in eye contact and body language or deficits in understanding and use of gestures; to a total lack of facial expressions and non-verbal communication.

• Deficits in developing, maintaining and understanding relationships, ranging, for example, from difficulties adjusting behaviour to suit various social contexts; to difficulties in sharing imaginative play or in making friends; to absence of interest in peers.

• Restricted, repetitive patterns of behaviour, interests, or activities, as manifested by at least two of the following, currently or by history (examples are illustrative, not exhaustive; see text): 

• Stereotyped or repetitive motor movements, use of objects, or speech (e.g. simple motor stereotypies, lining up toys or flipping objects, echolalia, idiosyncratic phrases).

• Insistence on sameness, inflexible adherence to routines, or ritualised patterns or verbal non-verbal behaviour (e.g. extreme distress at small changes, difficulties with transitions, rigid thinking patterns, greeting rituals, need to take same route or eat food every day).

• Highly restricted, fixated interests that are abnormal in intensity or focus (e.g. strong attachment to or preoccupation with unusual objects, excessively circumscribed or perseverative interest).

• Hyper- or hyporeactivity to sensory input or unusual interests in sensory aspects of the environment (e.g. apparent indifference to pain/temperature, adverse response to specific sounds or textures, excessive smelling or touching of objects, visual fascination with lights or movement).

• Symptoms must be present in the early developmental period (but may not become fully manifest until social demands exceed limited capacities, or may be masked by learnt strategies in later life).

• Symptoms cause clinically significant impairment in social, occupational or other important areas of current functioning.

• These disturbances are not better explained by intellectual disability (intellectual developmental disorder) or global developmental delay. Intellectual disability and Autism Spectrum Disorder frequently co-occur; to make comorbid diagnoses of Autism Spectrum Disorder and intellectual disability, social communication should be below that expected for general developmental level.