Individuals with Sex Chromosome Trisomies (SCT; XXX, XXY, XYY) have an increased vulnerability for developing challenges in social adaptive functioning. The present study investigates social interaction behavior in the context of varying social load, and Autism Spectrum Disorder (ASD) symptomatology in young children aged 1–7.5 years old, with SCT (N= 105) and control children (N= 101). Children with SCT show less interaction behaviors and more social withdrawal, as compared to their control peers, which were most evident in the high social load condition. Second, social impairments related to ASD are more prevalent, as compared to controls (27.1% at clinical level). These findings stress the importance of early monitoring and (preventive) support of early social development in young children with SCT.