Importance Autism Spectrum Disorders (ASD) aggregates in families, but the individual risk and to what extent this is caused by genetic factors, or shared or non-shared environment remains unresolved. Objective To provide estimates of familial aggregation of ASD. Design, Setting and Participants A population based cohort of all Swedish children born 1982–2007. We identified all twins, full siblings, maternal and paternal half siblings and cousin pairs and all diagnosis of ASD to 31-December-2009. Main Outcome Measure(s) The relative recurrence risk (RR) measure familial aggregation of disease. The RR is the relative risk of autism in an participant given a sibling or cousin has the diagnosis, compared with the risk in a participant with no diseased family member. We calculated RR for both ASD and Autistic Disorder (AD). We estimated how much of the probability of developing ASD can be related to genetic (additive and dominance) and environmental (shared and non-shared) factors. Results In the sample of 2,049,899 children, 14,516 obtained an ASD diagnosis of which 5,689 were AD. The ASD RR was estimated to 153.0 (95%CI 56.7–412.8; 27 vs 6,273 per 100,000 person-years) for monozygotic twins, 8.2 (95%CI 3.7–18.1; 55 vs 805 per 100,000 person-years) for dizygotic twins, 10.3 (95%CI 9.4–11.2; 49 vs 829 per 100,000 person-years) for full-siblings, 3.3 (95%CI 2.6–4.2; 94 vs 492 per 100,000 person-years) for maternal half siblings, 2.9 (95%CI: 2.2–3.7; 85 vs 371 per 100,000 person-years) for paternal half siblings, and 2.0 (95%CI: 1.8–2.2; 49 vs 155 per 100,000 person-years) for cousins. The RR pattern was similar for AD but of slightly higher magnitude. We found support for a disease etiology including only additive genetic and non-shared environmental effects. The ASD heritability was estimated to 0.50 (95%CI 0.44–0.55) and the AD heritability was estimated to 0.54 (95%CI 0.44–0.64). Conclusion and Relevance Among children born in Sweden, heritability of ASD and AD were estimated to be approximately 50%. For an individual, the risk of autism is increased 10 fold if a full sibling has the diagnosis and about 2 fold if a cousin has the diagnosis. These findings may inform counseling families with affected children.

Author Contributions: Drs Sandin and Kuja-Halkola had full access to all of the data in the study and take responsibility for the integrity of the data and the accuracy of the data analysis.

Twins, full siblings, and half-siblings are increasingly used as comparison groups in matched cohort and matched case-control studies. The "within-pair" estimates acquired through these comparisons are free from confounding from all factors that are shared by the siblings. This has made sibling comparisons popular in studying associations thought likely to suffer confounding from socioeconomic or genetic factors. Despite the wide application of these designs in epidemiology, they have received little scrutiny from a statistical or methodological standpoint. In this paper we show, analytically and through a series of simulations, that the standard interpretation of the models is subject to several limitations that are rarely acknowledged.Although within-pair estimates will not be confounded by factors shared by the siblings, such estimates are more severely biased by non-shared confounders than the unpaired estimate. If siblings are less similar with regard to confounders than to the exposure under study, the within-pair estimate will always be more biased than the ordinary unpaired estimate. Attenuation of associations due to random measurement error in exposure will also be higher in the within-pair estimate, leading within-pair associations to be weaker than corresponding unpaired associations, even in the absence of confounding. Implications for the interpretation of sibling comparison results are discussed.

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Autism spectrum disorders (ASD) and attention-deficit/hyperactivity disorder (ADHD) frequently co-occur. The presence of a genetic link between ASD and ADHD symptoms is supported by twin studies, but the genetic overlap between clinically ascertained ASD and ADHD remains largely unclear. We therefore investigated how ASD and ADHD co-aggregate in individuals and in families to test for the presence of a shared genetic liability and examined potential differences between low- and high-functioning ASD in the link with ADHD. We studied 1 899 654 individuals born in Sweden between 1987 and 2006. Logistic regression was used to estimate the association between clinically ascertained ASD and ADHD in individuals and in families. Stratified estimates were obtained for ASD with (low-functioning) and without (high-functioning) intellectual disability. Individuals with ASD were at higher risk of having ADHD compared with individuals who did not have ASD (odds ratio (OR)=22.33, 95% confidence interval (CI): 21.77–22.92). The association was stronger for high-functioning than for low-functioning ASD. Relatives of individuals with ASD were at higher risk of ADHD compared with relatives of individuals without ASD. The association was stronger in monozygotic twins (OR=17.77, 95% CI: 9.80–32.22) than in dizygotic twins (OR=4.33, 95% CI: 3.21–5.85) and full siblings (OR=4.59, 95% CI: 4.39–4.80). Individuals with ASD and their relatives are at increased risk of ADHD. The pattern of association across different types of relatives supports the existence of genetic overlap between clinically ascertained ASD and ADHD, suggesting that genomic studies might have underestimated this overlap.