Early Warning Signs of Autism: A Systematic Review of Nonverbal Behavioral Markers and Early Developmental Red Flags in Children Under 36 Months.

Early identification of autism spectrum disorder (ASD) remains a priority because clinical diagnosis often occurs after the earliest stages of development, and early behavioral signs can be subtle and variable across children. This systematic review summarized evidence on early nonverbal behavioral markers measured before 36 months that are associated with later-ASD diagnosis or classification. A PRISMA-aligned search was conducted. Eligible studies assessed nonverbal behavioral markers before age three and reported ASD outcomes at a later follow-up. Data were extracted on study design, population, marker domains, assessment methods, and predictive performance when reported. Risk of bias was assessed using tools matched to the study type. Findings were synthesized narratively because studies differed in cohorts, tasks, and outcome definitions. Across the included studies, early ASD-related signals most often appeared as differences in developmental change over time rather than as fixed abnormalities present from birth. Evidence was most consistent for differences in social visual engagement and attention flexibility during the first year of life, with more apparent differences from late infancy into the second year in attention disengagement, repetitive and sensory-linked behaviors, and motor and postural development. Predictive performance for single markers was generally limited. At the same time, combinations across domains and multimodal approaches appeared more useful, although they still require external validation and more transparent reporting before use in practice. Overall, the evidence supports longitudinal surveillance that combines multiple early behavioral domains to improve early risk identification.