Research suggests that the rate of autism is rising, but the causes are not so well understood. Previously, scientists have found mutations and common genetic variations in people with autism, implying that this developmental condition has a genetic component. A new study might have provided the best evidence yet to support this assertion.

Increasing diagnoses

Researchers have found that siblings of autistic children face a 20% chance of being autistic themselves. This rate is about seven times higher than the general population, according to findings reported by a team of researchers led by Sally Ozonoff of the University of California Davis MIND Institute.

The study builds on Ozonoff’s earlier work, reaffirming findings from a 2011 paper that reported a similar recurrence rate. The new study involved a larger and more diverse group of families. The participants spanned the United States, Canada, and the United Kingdom.

“The rate of autism diagnosis in the general community has been steadily increasing since our previous paper was published,” said Ozonoff. The latest estimates from the Centers for Disease Control and Prevention show that about 1 in 36 children has autism. This is up from 1 in 68 in 2011.

Ozonoff also noted changes in autism diagnostic criteria and increased awareness of autism in girls as factors prompting the new study.

“So, it was important to understand if these had any impact on the likelihood of autism recurrence within a family,” she said.

The recurrence rate of autism among siblings found in the new paper (20.2%) was slightly higher than in the 2011 paper (18.7%). But this increase was not statistically significant.

A family component

The study included data from 1,605 infants who had older autistic siblings. Researchers followed these children from as early as 6 months old, conducting up to seven visits. Assessments were made using the Autism Diagnostic Observation Schedule (ADOS-2), a well-validated tool for diagnosing autism spectrum disorder.

Several factors influence the likelihood of autism recurrence within families. Researchers found that if the first autistic child in a family was a girl, the likelihood of having another autistic child increased by 50% compared to families whose first autistic child was a boy. Moreover, families with multiple autistic siblings had a higher recurrence rate (37%) than those with just one autistic sibling (21%).

“This points to genetic differences that increase recurrence likelihood in families who have an autistic daughter,” Ozonoff said.

Boys were almost twice as likely to be diagnosed with autism as girls who were born after a sibling was diagnosed with autism. The familial recurrence rate was 25% for boys and 13% for girls. In the general population, boys are nearly 4 times more likely to be diagnosed with autism than girls.

Race and maternal education level also factored in the recurrence of autism. In non-white families, the recurrence rate was 25%, compared to 18% in white families. Recurrence rates varied by the mother’s education level, from 32% in those with a high school education or less, to 16.9% in those with a graduate degree.

A critical area for review

“These findings are new — and critical to replicate,” Ozonoff explained. “They do mirror the recent CDC findings that autism is more prevalent in children of historically underrepresented groups.” She noted that this reversed a longtime trend of lower prevalence in those groups.

The study also addressed concerns that families who stayed in the study might have biased the results. The findings showed no significant difference in outcomes between families who stayed in the study and those who dropped out.

“So, now we have two large, independent studies that report familial recurrence in the same range,” Ozonoff said, who noted the importance of closely monitoring the siblings of autistic children for developmental delays, particularly in families with reduced access to care.

The new study appeared in the journal Pediatrics.