Autism spectrum disorders and gender identity issues often occur together in children, according to new research that supports previous findings.

Children with an autism spectrum disorders (ASDs) were seven times more likely than other children to report gender variance on a questionnaire, researchers at one U.S. medical center found.

"Like many research ideas, this is one that was born from clinical experience," said lead researcher Dr. Aron Janssen, director of NYU Langone Medical Center's Gender and Sexuality Service in New York City.

People who work with transgender and gender variant youth have been noticing the overlap with ASDs for the last several years, he told Reuters Health.

Gender variance describes variability between the sex a person is assigned at birth and their experienced or expressed gender, Janssen and colleagues write in the journal Transgender Health.

ASDs can affect behavior as well as social and communication skills. Studies in Europe and North America have found higher rates of ASDs among gender variant youths, and vice versa.

For the new study, the researchers examined the Child Behavior Checklist of 492 children, ages six to 18, diagnosed with ASDs between 2011 and 2015 - including 409 who were assigned male at birth, and 83 who were assigned female.

The children's answer to one question on the checklist about gender identity was then compared to the answers of 1,605 other children.

Overall, 5.1 percent of children with ASDs said they wished to be the opposite sex, compared to 0.7 percent of the other children.

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In a 2014 study that used a similar design, 5.4 percent of children with ASDs said they wished to be the opposite sex on the same questionnaire.

"It's quite striking that through a very similar procedure he had almost the same numbers," said John Strang, the lead researcher of the 2014 paper from the Center for Autism Spectrum Disorders at Children's National Medical Center in Washington, D.C.

In both studies, children with ASDs were about seven times more likely to report gender variance, compared to other children. Likewise, a 2010 Dutch study found children at a gender clinic were about seven times more likely to meet the criteria to be diagnosed with ASDs.

No study answers why gender identity issues and ASDs often occur together, but there are several possible explanations.

Strang told Reuters Health people with ASDs may be less aware of social expectations to conform to their sex assigned at birth. As a result they may be less likely to hide their gender variance.

It could also be that people who are gender variant are more socially isolated and demonstrate traits similar to those of ASDs, Janssen said.

"When you have two conditions that are both on a spectrum and you see there is an overlap, I think it gets even more complex and more interesting," he said.

Janssen said identifying an overlap between ASDs and gender variance is just a first step.

"At the end of the day it's about how we help people, whether or not they're on the spectrum, lead happy fulfilling lives," he said.

Strang said people with ASDs and gender variance need a lot of support.

"We just concluded a two-year project bringing together specialists who either published in this are or working with this co-occurrence to produce a set of guidelines," he said. "Hopefully those will be out soon."