A cross-syndrome approach to the social phenotype of neurodevelopmental disorders: Focusing on social vulnerability and social interaction style

•Heightened social vulnerability is evident across multiple neurodevelopmental disorders.•The limitation of IQ to explain social vulnerability is shown by a cross-syndrome approach.•Atypical social interaction styles vary within and across neurodevelopmental groups.•Social interaction styles make a unique contribution to heightened social vulnerability.•Social phenotypes are best understood as distributed across diagnostic boundaries. Following Annette Karmiloff-Smith’s approach to cognitive research, this study applied a cross-syndrome approach to the social phenotype, focusing on social vulnerability (SV) and the factors that contribute to it. To (i) identify syndrome-specific differences in SV across four neurodevelopmental disorder (NDD) groups, (ii) determine the contribution of intellectual disability (ID), age or gender to SV, and (iii) explore its relationship with social interaction style (SIS). 262 parents of children: Autism (n = 29), Williams syndrome (n = 29), Attention deficit hyperactivity disorder (n = 36), Fragile X syndrome (n = 18), and Neurotypical (n = 150) reported on their child’s SV, quality of SIS and other factors (ID, age, gender). Heightened SV was not syndrome-specific. Instead it was found equally across NDD groups (and not in the neurotypical group), and independently of ID, age and gender. Different atypical SISs were also distributed across NDD groups and each were significantly related to SV, independent of the factors above and beyond neurodevelopmental diagnosis. The findings emphasise that social phenotypes are best understood as distributed across diagnostic boundaries and offer opportunities to further test the role of varied atypical SISs in the development of heightened SV.