Behavioural markers for autism in infancy: Scores on the Autism Observational Scale for Infants in a prospective study of at-risk siblings

•Behavioural atypicalities in emergent ASD in infancy include both social and non-social behaviours.•Some of these atypicalities are found in HR siblings who do not go on to have ASD, supporting the notion of an early broader autism phenotype.•Understanding the interplay between different neurodevelopmental domains across the first years of life is important to understand developmental mechanisms and to develop early interventions.