Autism is identified by how a person communicates, relates to others and behaves, not by a single lab test. Many of the clearest markers are things a parent can notice at home, often before a child turns two.1PubMed. Infants at Risk for Autism Spectrum Disorder: Frequency, Quality, and Variety of Joint Attention Behaviors Scientists have found differences in brains, blood, gut bacteria and eye movements, but these remain research tools, not tests you can order to confirm a diagnosis.
The Two Core Signs Clinicians Look For
The international diagnostic manual first requires lasting difficulty starting and keeping up back-and-forth social interaction and communication.2PubMed Central. Innovations of the ICD-11 in the Field of Autism Spectrum Disorder: A Psychological Approach Day to day, that can mean struggling to begin or sustain a natural conversation, using or reading gestures, facial expressions and eye contact differently, or finding friendships hard to form or keep at a typical age. In young children it often shows up as less response to their name and less sharing of interest with others.
The manual also requires restricted, repetitive or inflexible behaviours, interests or activities that are clearly unusual for the person’s age and culture.3PubMed Central. Innovations of the ICD-11 in the Field of Autism Spectrum Disorder: A Psychological Approach Think of repeated movements such as hand-flapping, rocking or lining up objects, strong distress at small changes in routine, or an intense focus on narrow topics well beyond what peers show. Unusual reactions to sounds, textures, lights or smells now count as part of this second area.4PubMed Central. Innovations of the ICD-11 in the Field of Autism Spectrum Disorder: A Psychological Approach
The signs must have started in early development and must cause significant difficulty at home, school, work or socially.5PubMed Central. Innovations of the ICD-11 in the Field of Autism Spectrum Disorder: A Psychological Approach That requirement matters because one or two traits, like loving routine, are common and are not autism.
These differences usually show up across settings, though they can look milder in some places than others.6PubMed Central. Autism spectrum disorder in ICD-11—a critical reflection of its possible impact on clinical practice and research No single social behaviour is mandatory, so two autistic people can look quite different.7PubMed Central. Autism spectrum disorder in ICD-11—a critical reflection of its possible impact on clinical practice and research A child who talks fluently and earns top grades can still meet the social communication area, since the difficulty lies in the give-and-take of interaction rather than in vocabulary. Traits are judged against what is expected for the person’s age, so lining up toys is read differently in a two-year-old than in a ten-year-old.
Early Signs in Babies and Toddlers
One early sign involves sharing attention: a baby looking between you and an object to share interest, or pointing and showing things to you. From 12 months, toddlers later diagnosed with autism start fewer of these moments, especially pointing and showing.8PubMed. Infants at Risk for Autism Spectrum Disorder: Frequency, Quality, and Variety of Joint Attention Behaviors At 12 months, though, children with language delay show the same pattern, so it is not specific to autism yet.9PubMed. Infants at Risk for Autism Spectrum Disorder: Frequency, Quality, and Variety of Joint Attention Behaviors By 18 months, fewer attempts at sharing attention set autistic toddlers apart from those with language delay.10PubMed. Infants at Risk for Autism Spectrum Disorder: Frequency, Quality, and Variety of Joint Attention Behaviors How a 12-month-old responds when you point at something also helps predict autism signs at 18 months.11PubMed Central. Joint Attention and Its Relationship with Autism Risk Markers at 18 Months of Age
At home, a few questions are useful. Does your toddler bring toys just to show you, not only for help? Does your child point to share something interesting, like a dog? Does your child respond to their name most of the time?
The kind of pointing matters. A toddler who points to get something, such as a cup out of reach, is making a request, which is different from pointing just to share interest in an aeroplane or a dog. Seeing requests but few shared moments is worth mentioning to a doctor, since that gap is the pattern clinicians pay attention to.
Movement and sensory differences can appear too. Motor differences can show as late milestones or as more frequent, varied repetitive movements, with or without objects.12PubMed Central. Early Motor Signs in Autism Spectrum Disorder Toddlers with autism can walk with a measurably different gait.13PubMed. Analysis of unsupported gait in toddlers with autism In one group of autistic children, about 27% toe-walked compared with about 5% of other children.14PubMed Central. The Prevalence and Clinical Significance of Toe Walking in Autism Spectrum Disorder: A Cross-Sectional Study in an Italian Pediatric Sample But in that group toe walking appeared only in children with more severe autism, so it is a weak sign on its own.15PubMed Central. The Prevalence and Clinical Significance of Toe Walking in Autism Spectrum Disorder: A Cross-Sectional Study in an Italian Pediatric Sample Sensory patterns include under-reacting, over-reacting, and seeking out sensations,16PubMed. Sensory abnormalities in children with autism spectrum disorder and these differences can begin in infancy.17PubMed Central. Cortical markers of excitation/inhibition balance are associated with sensory responsivity from infancy in longitudinal cohorts enriched for autism and ADHD
A single sign is not a diagnosis. Many typical toddlers toe-walk or are late to point for a while. Clinicians look for several signs together, persisting over months.
If your toddler seems to be missing these moments, watch over a week or two how often the behaviours happen in different settings, such as play, meals and outings, rather than judging from one quiet afternoon. Note the age at which you first noticed each behaviour and whether it seems to be missing or was once present and faded, because clinicians ask about this. Then bring it up with your child’s doctor at the next visit instead of waiting to see whether it resolves. A hearing check and developmental screening are straightforward first steps.
Sensory Differences Across Ages
Sensory markers come in four patterns. Over-reacting means sounds, tags or lights feel overwhelming.18PubMed. Sensory abnormalities in children with autism spectrum disorder Under-reacting means not noticing pain, cold or your voice as expected.19PubMed. Sensory abnormalities in children with autism spectrum disorder Sensation seeking means craving spinning, touching or chewing.20PubMed. Sensory abnormalities in children with autism spectrum disorder And some autistic people notice fine details others miss.21PubMed. Sensory abnormalities in children with autism spectrum disorder These can sit side by side, so one child can over-react to noise yet seek deep pressure.
In daily life, this might be covering ears at hand dryers or vacuum cleaners, refusing many foods because of texture, or having meltdowns in busy shops that ease in quiet places. Constant movement or crashing into furniture can be a way of getting input. Ordinary pickiness looks different: what stands out is the intensity, how much it disrupts daily routines, and whether it persists across settings and time.
Biological Markers, Brain, Blood, Gut and Eyes
A “biomarker” is a measurable biological sign, such as a brain feature or a substance in blood, that can point to a condition. For autism, these show group differences but are not reliable for one person.
Brain differences are real but inconsistent. Some young autistic children have larger-than-usual brains, especially before age 6,22PubMed. A systematic review of structural MRI biomarkers in autism spectrum disorder: A machine learning perspective but this early overgrowth appears in only a subset of children.23Nature. Neuroimaging and Brain Morphology in Autism Spectrum Disorders Findings on brain thickness and surface area differ between studies.24PubMed Central. Evidence against the normalization prediction of the early brain overgrowth hypothesis of autism A brain network active during rest and social thinking often works differently in autism, with links to social difficulties.25PubMed Central. The Default Mode Network in Autism Yet that network can be more connected in some studies and less in others.26PubMed Central. A Review of the Default Mode Network in Autism Spectrum Disorders and Attention Deficit Hyperactivity Disorder Results are mixed partly because studies scan children of different ages, brain features change quickly with age, and the autistic groups studied mix children with different underlying causes.
That is also why scans can’t diagnose a child. Studies compare averages across large groups, and a difference in the average does not tell you which category any single child belongs to.
Genes, blood and immune markers come next. Genetics are the biggest known contributor, through rare large-effect changes and thousands of common small ones.27PubMed Central. A phenotypic spectrum of autism is attributable to the combined effects of rare variants, polygenic risk and sex Genetic testing can find a specific cause, such as a deletion on chromosome 16, in some children. Doctors often offer it after diagnosis, not to make one. It can explain a cause and inform family planning, though most results do not find a single cause.
Chemical tags on DNA in blood differ in autistic children and are being explored as markers,28PubMed Central. DNA Methylation Biomarkers for Young Children with Idiopathic Autism Spectrum Disorder: A Systematic Review but blood-based tag tests so far have only moderate accuracy in small studies.29PubMed Central. Blood-based DNA methylation markers for autism spectrum disorder identification using machine learning Signs of inflammation have been found in the brain and spinal fluid of some autistic people.30PubMed. Neuroglial activation and neuroinflammation in the brain of patients with autism
Autistic children often have a different mix of gut bacteria,31PubMed Central. Altered gut microbiota and short chain fatty acids in Chinese children with autism spectrum disorder although which bacteria differ varies between studies.32PubMed Central. Altered Gut Microbiota and Short-chain Fatty Acids in Chinese Children with Constipated Autism Spectrum Disorder Autistic toddlers tend to look less at eyes and mouths and more at objects,33PubMed. Social Visual Engagement in Preterm and Term Children With Autism: Consistent Eye-tracking Patterns Irrespective of Gestational Age so eye-tracking is being tested as an aid to early diagnosis.34PubMed Central. Eye-Tracking-Based Measurement of Social Visual Engagement Compared With Expert Clinical Diagnosis of Autism
Commercial tests sold to diagnose autism deserve caution. No blood, stool, hair or brain scan test is approved to diagnose autism, so a company claiming otherwise is going beyond the evidence.
Why Girls and Adults Are Often Missed
Camouflaging, or masking, means hiding autistic traits and copying others to fit in.35Review Journal of Autism and Developmental Disorders. The Female Autism Phenotype and Camouflaging: a Narrative Review Girls tend to mask more, so observers can miss them,36PubMed Central. Sex/Gender Differences in Camouflaging in Children and Adolescents with Autism and girls are often diagnosed later than boys.37PubMed Central. Sex/Gender Differences in Camouflaging in Children and Adolescents with Autism
In practice, masking can mean rehearsing conversations beforehand, forcing eye contact that feels uncomfortable, and copying others’ expressions or phrases. Deep exhaustion after social days is common.
A girl who copies classmates, follows rules closely and is quiet in class can look fine at school, with difficulties coming out at home. Her interests may look typical, with the difference lying in how intense they are. Because the usual picture of autism was built mostly from studies of boys, girls who don’t match it can be told it is anxiety or shyness instead. More masking is linked to worse mental health,38PubMed. Camouflaging in autism: A systematic review which is partly why some people first seek help for anxiety or depression and are only later found to be autistic.
Autistic adults often struggle most with flexibility, planning, starting tasks and tracking work.39PubMed Central. Real-World Executive Functions in Adults with Autism Spectrum Disorder: Profiles of Impairment and Associations with Adaptive Functioning and Co-morbid Anxiety and Depression Trouble shifting plans is linked to anxiety, and planning difficulty to depression.40PubMed Central. Real-World Executive Functions in Adults with Autism Spectrum Disorder: Profiles of Impairment and Associations with Adaptive Functioning and Co-morbid Anxiety and Depression Some late-diagnosed adults, though, perform above average on speed tasks.41PubMed Central. Exploratory Study of Executive Function Abilities Across the Adult Lifespan in Individuals Receiving an ASD Diagnosis in Adulthood
If you are seeking an adult assessment, note lifelong patterns, including childhood examples, and ask relatives about your early behaviour. Mention any anxiety or depression, which often coexist.
Culture Changes How Signs Look
The broad areas of autism appear worldwide, but how they show up varies.42PubMed Central. A conceptual framework for understanding the cultural and contextual factors on autism across the globe In some cultures children avoid eye contact with adults out of respect, so less eye contact is not a red flag there.43PubMed Central. A conceptual framework for understanding the cultural and contextual factors on autism across the globe Reported symptoms also differ between countries in communication and insistence on sameness.44Research in Autism Spectrum Disorders. A multinational study examining the cross cultural differences in reported symptoms of autism spectrum disorders: Israel, South Korea, the United Kingdom, and the United States of America
Eye contact is best judged alongside other signs, such as sharing attention. What matters is whether a child differs from other children in the same community, not from a single standard of what looks “normal”.
It helps to tell the clinician your family’s norms: explain what counts as polite in your culture, and mention that bilingual homes can affect speech timing. You can also ask whether the assessor knows your cultural background.
What to Do If You Notice Signs
A tablet app watching toddlers’ behaviour correctly flagged about 88% of autistic children, but only about 4 in 10 children it flagged were autistic.45PubMed Central. Early detection of autism using digital behavioral phenotyping A screening tool is built to catch most autistic children, so it casts a wide net and flags many who aren’t. A positive result is a reason to look closer, not a cause for alarm. A negative result isn’t full reassurance either, since some autistic children are missed, so persistent worries still deserve a conversation with a doctor.
Raise concerns with your child’s doctor rather than waiting, because early support helps. Say plainly that you are worried about your child’s development and would like a developmental assessment, not just “is this normal?” If you’re told to wait and see, ask what specific signs would change that advice and when your child should be rechecked, and ask for a referral if worries persist. Early support doesn’t depend on a final diagnosis, so asking about therapies for speech or other needs while you wait is reasonable.
To prepare, write down examples with ages and situations, take short videos of behaviours that worry you, and bring a milestone list. Ask about referral to a developmental specialist.
The assessment itself usually includes a parent interview about development, play-based observation by specialists, and a hearing check to rule out other causes. Results usually take several visits.