Groundbreaking study links autism to siblings’ gender

If a family has a girl with autism, they are more likely to have another child with autism – especially if that next child is a boy.

That is the groundbreaking finding on autism’s gender connection from Harvard Medical School, published today in the Journal of the American Medical Association. 

We have long known that having a child with autism is a risk factor for having another one with the same disorder.

But this study is the first to show that the older sibling’s gender could also influence the likelihood.  

Sibling connection: We have long known that having a child with autism is a risk factor for having another with the same disorder. Now we know the sibling’s gender matters (file image)

‘Our results give us a fair degree of confidence to gauge the risk of autism recurrence in families affected by it based on a child’s gender,’ said study first author Nathan Palmer, instructor in biomedical informatics at Harvard Medical School. 

‘It is important to be able to provide worried parents who have one child with the condition some sense of what they can expect with their next child. That information is critical given how much better we’ve become at screening for the disease earlier and earlier in life.’ 

The new results stem from the largest study of its kind.

Researchers analyzed health insurance records of more than 1.5 million American families with two children between the ages of four and 18.

They then tracked patterns of recurrence among siblings over a year or longer.

There were more than 3.1 million children in the study. Of those, 39,000 – about 1.2 percent overall, two percent of boys and 0.5 percent of girls – received a diagnosis of autism.

The results confirm previous research showing that, overall, boys have a higher risk of autism and related disorders than girls.

The last few years have seen a rapid acceleration in research to understand how to detect autism. 

Now, physicians are far more equipped than they were 10 years ago at detecting the disorder’s manifestations early in a child’s life, in order to start treating them early.

The new study backed up past research that shows having one child with autism increases the risk for subsequent children, and that the disorder is still rare, accounting for just 1.2 percent of children in the study.

They also confirmed that the disorder is more common in boys than in girls. 

And they say their new finding about gender can further equip doctors to make diagnoses or intelligent estimates to help families prepare.  

‘This study is a powerful example of how big data can illuminate patterns and give us insights that allow us to empower parents and pediatricians to implement anticipatory and far more precise medicine,’ said study senior author Isaac Kohane, head of the Department of Biomedical Informatics at Harvard Medical School. 

The results, however, also reveal a curious pattern of recurrence based on gender. 

Siblings born after a female child with autism or a related disorder had a higher risk than siblings born after a male child with autism. 

Male children were, overall, more susceptible to autism than females. 

That means boys with older female siblings with autism had the highest risk for autism themselves, while female siblings with older brothers with autism had the lowest risk.

For every 100 boys with an older female sibling with autism, 17 received a diagnosis of autism or a related disorder. 

Male children with older male siblings with ASD had a 13 percent risk of an ASD diagnosis, followed by younger female siblings with older male siblings with ASD (7.6 percent). 

The lowest risk (four percent) was observed among younger female siblings who had an older brother with autism or an ASD.

The investigators caution that families should keep the risk in perspective because autism and related disorders remain relatively rare, affecting roughly one percent of the general population.

‘Even for the group at highest risk – males with an older female sibling with autism – the odds are still about five to one that the child will be unaffected,’ Palmer said. 

‘What we have provided here is context for families who already have children with autism or another similar disorder and need a clearer perspective on recurrence risk.’