A study has determined that 19 percent of younger siblings who were diagnosed with autism had an older sibling that was also autistic.
While it has been long known that autism runs in families, the recent study has significantly increased the statistics of previous research, which stated that 3 to 14 percent of the younger siblings of autistic children will also be autistic.
The study, conducted by Sally Ozonoff of the University of California-Davis, was the largest of its kind and followed 664 infants aged six to eight months until three years that had at least one sibling with an autism-spectrum disorder.
Research found that as 80 percent of autistic children are boys, younger brothers are more likely to develop autism than younger sisters. It was also found that siblings of families with more than one autistic child had the greatest risk of developing autism themselves.
While one in 110 children in the general population will develop some form of autism, one in nine younger sisters and one in five younger brothers of children with autism will also develop autism. If there is more than one child in a family with autism, the likelihood jumps to one in three.
"It's the first thing families ask: How likely is this to happen again?" Ozonoff told NPR. "Parents are already concerned, already watching their child's development carefully. We are able to supply some answers."
The study was funded by the National Institutes of Health and the advocacy group Autism Speaks.
Alycia Halladay, a member of Autism Speaks says that the new findings should help parents and doctors of siblings be more proactive about looking for the signs of autism in younger children earlier, which will help with earlier diagnoses and more prompt treatments.
Judith Ursitti, mother to two autistic children, 11-year-old daughter, Amy with Asperger's and 7-year old son, Jack with severe autism, agrees. NPR has revealed the story of Ursitti and her family.
Despite having impeccable pediatric care, Jack was not diagnosed until the age of two, NPR has reported. Since then, the family has been extremely diligent with Jack's care, enrolling him in a special school for autistic children and continuing his behavioral therapy at home has allowed him to develop communication skills far beyond what was expected for his severity of autism.
Ursitti also notes that she was amazed at how often families are affected by autism more than once.
"I remember when Amy started getting intervention for Asperger's, and she was with a group of girls for therapy. And they all had brothers who had autism. And I thought, 'Wow, this is incredible!'," she told NPR.
Despite the statistics, Ozonoff says that parents should not lose hope, as the chance of a child not developing autism is always greater than the chance that they will.