Approximately one in five siblings of children with autism who don’t meet criteria for the disorder display mild or “subclinical” autism traits. . . . These traits consist of language delays, the use of odd or repeated phrases and other unusual speech qualities and difficulties interacting with others. Most such children come from families with at least two other youngsters who have an autism spectrum disorder.
Four times as many boys as girls meet psychiatric criteria for autism. But the inclusion of mild autism traits narrows that ratio to three boys for every two girls. . . .
[The study] assessed signs of autism in 2,920 children from 1,235 families participating in a national online research registry. Each family in the registry includes at least one child with an autism spectrum disorder and at least one biological sibling. Data came from questionnaires completed by parents.
Their responses indicated that 134 families, or 11 percent, had more than one child diagnosed with autism. About one in four families, including nearly all of those with multiple autism cases, also contained siblings with mild symptoms.
Among mildly affected boys and girls, 20 percent had received a diagnosis of language delay or speech problems early in life, double the prevalence in the general population.
The study is J.N. Constantino et al. Sibling recurrence and the genetic epidemiology of autism. American Journal of Psychiatry, in press, 2010. doi:10.1176/appi.ajp.2010.09101470.