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Autism spectrum disorder matters because it names a lifelong pattern of social-communication difficulty plus restricted, repetitive behaviour that is present from early childhood, not something that appears suddenly later in life. It is diagnosed as persistent deficits in social communication and social interaction across multiple contexts, together with restricted, repetitive patterns of behaviour, interests, or activities. For example, imagine a three-year-old who has never pointed to share interest with a parent, lines up toy cars in a fixed order every day, and becomes distressed by loud noises. In practice, that combination of social difficulty and repetitive behaviour, present since early childhood, is what defines the diagnosis.

Core features include reduced eye contact and joint attention, difficulty developing and maintaining relationships, restricted interests pursued with unusual intensity, insistence on sameness or routines, and sensory sensitivities, such as distress at certain sounds or textures. Symptoms must be present in early development, though they may not fully manifest until social demands exceed the person's capacityA person's ability to understand, weigh up, and use relevant information to make a specific decision.Read more →, such as starting school. For example, imagine a bright child who copes well at home but is only noticed to struggle once she starts secondary school, where unstructured social demands increase sharply. In practice, this means a normal-looking early childhood does not rule out autism spectrum disorder if demands later expose the difficulty.

• What is the role of medication in autism spectrum disorder?
• Why is autism spectrum disorder often recognised later in girls than in boys?
• A parent asks whether the MMR vaccine causes autism. What is the accurate reply?

• Schizoid personality disorder, social withdrawal without repetitive behaviours or childhood onset, unlike autism spectrum disorder's early, repetitive pattern.
• Social anxiety disorder, desire for social contact limited by fear of judgement, rather than autism spectrum disorder's more fundamental social-communication difficulty.
• Attention deficit hyperactivity disorder (ADHDAttention-deficit hyperactivity disorder, a condition of inattention, impulsivity and sometimes hyperactivity.Read more →): inattention and hyperactivity, unlike autism spectrum disorder's restricted, repetitive behaviours and sensory sensitivities, though the two commonly co-occur.
• Autism spectrum disorder, symptoms trace back to early childhood, unlike a sudden change in social functioning, which points toward an acquired condition instead.

• Psychosocial and educational interventions are first-line rather than medication.
• Diagnosis rests on developmental history and observation, not on a drug response.
• No drug reverses core autistic features.
• Onset is in early development in both sexes, whatever the age at recognition.
• Sensory differences vary between individuals rather than by sex.

• Extract the defining phenomenology, duration, severity and functional-impact clues.
• Match the pattern to the most specific diagnostic or assessment construct.
• Actively test the closest look-alike against the missing or contradictory clue.
• Choose the precise term rather than a broader category that is merely compatible.