One in three children who have an older sibling with an autism related disorder show higher levels of autism-related behaviors or lower levels of developmental progress.

The autism spectrum of disorders is a group of developmental conditions characterized by problems with social interaction and communication. About 1 in 100 children today are diagnosed with an autism spectrum disorder (also known as an ASD). Previously, researchers found that almost one in five of younger siblings of children with an autism spectrum disorder themselves developed one.

"It is clear that the younger siblings of a child with an ASD may face challenges even if they are not themselves identified with an ASD," study researcher Daniel Messinger, of the University of Miami, said in a statement. "This new work identifies classes of outcomes in these children."

The study reveals that difficulties faced by the younger siblings of children autism involve both lower levels of verbal and nonverbal functioning and higher levels of autism-related problems. Examples of a child's autism-related problems — which are not as severe as those of children with an ASD — include lower levels of back-and-forth play with others and lower levels of pointing to express interest in what is going on around them.

"We found that the majority of these high risk siblings appear to be developing normally," Messinger said. "However, a higher than expected proportion of the children face challenges related to higher levels of autism-related behaviors or lower levels of verbal and non-verbal developmental functioning."

Overall, the research says, the majority of high-risk siblings are developing typically at three years of age, but the development of a substantial minority is affected by subtler forms of ASD-related problems or lower levels of developmental functioning. Lower levels of developmental functioning and higher levels of autism-related problems in the at-risk siblings define what researchers refer to as the broad autism phenotype.

The research will be presented at the International Meeting for Autism Research in Toronto from May 17 to 19, 2012.