This study, conducted by researchers from Sweden and the United States, aimed to estimate the familial aggregation of Autism Spectrum Disorders (ASD) and Autistic Disorder (AD) in a large, population-based cohort of Swedish children born between 1982 and 2007. The study identified all twins, full siblings, half-siblings, and cousin pairs, and analyzed the recurrence risk (RR) of ASD and AD within these families. The RR was calculated for monozygotic (MZ) and dizygotic (DZ) twins, full siblings, maternal and paternal half-siblings, and cousins. The results showed that the RR for ASD was significantly higher in full siblings (10.3) compared to cousins (2.0), and for AD (116.8) compared to cousins (2.3). The heritability of ASD was estimated to be 50%, and for AD, it was 54%. These findings suggest that genetic factors play a significant role in the development of ASD and AD, but environmental factors also contribute. The study provides valuable insights for family counseling and genetic research.This study, conducted by researchers from Sweden and the United States, aimed to estimate the familial aggregation of Autism Spectrum Disorders (ASD) and Autistic Disorder (AD) in a large, population-based cohort of Swedish children born between 1982 and 2007. The study identified all twins, full siblings, half-siblings, and cousin pairs, and analyzed the recurrence risk (RR) of ASD and AD within these families. The RR was calculated for monozygotic (MZ) and dizygotic (DZ) twins, full siblings, maternal and paternal half-siblings, and cousins. The results showed that the RR for ASD was significantly higher in full siblings (10.3) compared to cousins (2.0), and for AD (116.8) compared to cousins (2.3). The heritability of ASD was estimated to be 50%, and for AD, it was 54%. These findings suggest that genetic factors play a significant role in the development of ASD and AD, but environmental factors also contribute. The study provides valuable insights for family counseling and genetic research.