A study published in *Arch Gen Psychiatry* (2011) analyzed the genetic and environmental factors influencing autism spectrum disorder (ASD) using twin pairs from California. The study identified 192 twin pairs with at least one twin diagnosed with ASD, born between 1987 and 2004. Structured diagnostic assessments, including the Autism Diagnostic Interview–Revised (ADI-R) and Autism Diagnostic Observation Schedule (ADOS), were used to determine diagnoses. Concordance rates were calculated for both strict autism and ASD, revealing higher concordance in monozygotic (identical) twins compared to dizygotic (fraternal) twins. For strict autism, male monozygotic twin concordance was 58% (95% CI, 42–74), while female monozygotic concordance was 60% (95% CI, 28–90). For ASD, male monozygotic concordance was 77% (95% CI, 65–86), and female monozygotic concordance was 50% (95% CI, 16–84). Dizygotic concordance rates were lower, with male dizygotic concordance at 21% (95% CI, 9–43) and female dizygotic concordance at 27% (95% CI, 9–69). The study found that shared environmental factors explained a large portion of the variance in liability for ASD (55% for autism, 58% for ASD), in addition to moderate genetic heritability (37% for autism, 38% for ASD). These findings suggest that both genetic and environmental factors contribute significantly to ASD susceptibility. The study also noted that the heritability estimates for ASD were similar to those for strict autism, indicating a similar liability spectrum. The results highlight the importance of shared environmental factors in ASD development, which may include factors such as parental age, birth weight, and maternal infections during pregnancy. The study's findings challenge previous assumptions about the primary role of genetic factors in ASD and emphasize the need for further research into environmental influences. The study was conducted using rigorous methods, including twin zygosity determination and statistical analysis, and was reviewed by ethics committees. The results provide valuable insights into the complex interplay between genetic and environmental factors in ASD.A study published in *Arch Gen Psychiatry* (2011) analyzed the genetic and environmental factors influencing autism spectrum disorder (ASD) using twin pairs from California. The study identified 192 twin pairs with at least one twin diagnosed with ASD, born between 1987 and 2004. Structured diagnostic assessments, including the Autism Diagnostic Interview–Revised (ADI-R) and Autism Diagnostic Observation Schedule (ADOS), were used to determine diagnoses. Concordance rates were calculated for both strict autism and ASD, revealing higher concordance in monozygotic (identical) twins compared to dizygotic (fraternal) twins. For strict autism, male monozygotic twin concordance was 58% (95% CI, 42–74), while female monozygotic concordance was 60% (95% CI, 28–90). For ASD, male monozygotic concordance was 77% (95% CI, 65–86), and female monozygotic concordance was 50% (95% CI, 16–84). Dizygotic concordance rates were lower, with male dizygotic concordance at 21% (95% CI, 9–43) and female dizygotic concordance at 27% (95% CI, 9–69). The study found that shared environmental factors explained a large portion of the variance in liability for ASD (55% for autism, 58% for ASD), in addition to moderate genetic heritability (37% for autism, 38% for ASD). These findings suggest that both genetic and environmental factors contribute significantly to ASD susceptibility. The study also noted that the heritability estimates for ASD were similar to those for strict autism, indicating a similar liability spectrum. The results highlight the importance of shared environmental factors in ASD development, which may include factors such as parental age, birth weight, and maternal infections during pregnancy. The study's findings challenge previous assumptions about the primary role of genetic factors in ASD and emphasize the need for further research into environmental influences. The study was conducted using rigorous methods, including twin zygosity determination and statistical analysis, and was reviewed by ethics committees. The results provide valuable insights into the complex interplay between genetic and environmental factors in ASD.