Symptoms of autism in Williams syndrome: a transdiagnostic approach.

Williams syndrome (WS) is associated with atypical social communication and cognition reminiscent of the behaviours observed in autism. Nonetheless, WS also differs significantly from autism, such as regarding social motivation, which is typically enhanced in WS and reduced in autism. This study sought to examine the conditions' transdiagnostic similarities and differences for autistic symptoms and social functioning, and their developmental trajectories, by comparing individuals with WS (n = 24) and those diagnosed with idiopathic autism (n = 24) and attention deficit hyperactivity disorder (ADHD; n = 24), aged 9 to 53 years, on measures of autism, social functioning, IQ and cooccurring psychiatric conditions. Although only 12.5% in the WS group met the criteria for an autism diagnosis, a majority exhibited distinct difficulties within social communication, social cognition, repetitive behaviours, and atypical sensory reactivity resembling autism. Conversely, elevated social motivation and a high number of social initiatives accompany these characteristics. No group differences in the developmental trajectories of autism symptoms were found. Our results demonstrate that autistic behaviours are more frequent in individuals with WS, than in individuals with idiopathic ADHD, and emphasize the need for clinical management of these behaviours.