Intellectual disability in Autism Spectrum Disorder. Investigation of prevalence in an Italian sample of children and adolescents

1. Introduction Autism Spectrum Disorder (ASD) is a neurodevelopmental disorder characterized from early developmental period by the presence of persistent deficits in social communication and social interaction, and restricted and repetitive patterns of behaviour, interests or activities that cause clinically significant impairment in several areas of functioning (American Psychiatric Association, 2013). The clinical expression of this disorder varies greatly, depending on the severity of autistic symptoms and on the developmental level.