Context: The presence of autistic-like traits in relatives of individuals with autism spectrum disorder (ASD) is well recognized, but, to our knowledge, the emergence of these traits early in development has not been studied.
Objective: To prospectively investigate the emergence of autistic-like traits in unaffected (no ASD diagnosis) infant siblings of probands diagnosed as having ASD.
Design: Two groups of children unaffected with ASD were assessed prospectively-siblings of probands diagnosed as having ASD (high risk [HR]) and control subjects with no family history of ASD (low risk [LR]). Scores on a measure of autistic-like traits at 12 months of age were used in a cluster analysis of the entire sample.
Setting: A prospective study of infant siblings of probands with ASD from 3 diagnostic centers in Canada.
Participants: The study included 170 HR and 90 LR children, none of whom was diagnosed as having ASD at age 3 years.
Main outcome measures: The Autism Observation Scale for Infants was used to measure autistic-like traits and derive clusters at 12 months of age. Clusters were compared on ASD symptoms, cognitive abilities, and social-emotional difficulties at age 3 years.
Results: Two clusters were identified. Cluster 1 (n = 37; 14.2% of total sample) had significantly higher levels of autistic-like traits compared with cluster 2. Within cluster 1, 33 children came from the siblings (19.4% of HR group) and only 4 came from the control subjects (4.5% of LR group). At age 3 years, children from cluster 1 had more social-communication impairment (effect size > 0.70; P < .001), lower cognitive abilities (effect size = -0.59; P < .005), and more internalizing problems (effect size = 0.55; P = .01). Compared with control subjects, HR siblings had a relative risk of 4.3 (95% CI,1.6-11.9) for membership in cluster 1.
Conclusions: Study findings suggest the emergence of autistic-like traits resembling a broader autism phenotype by 12 months of age in approximately 19% of HR siblings who did not meet ASD diagnostic criteria at age 3 years.