Transgender adolescents are far more likely to attempt suicide than teens whose identity matches their sex assigned at birth, and trans male youth are especially at risk, a U.S. study suggests.

Roughly half of transgender teens who identify as male but were assigned a female gender at birth have attempted suicide at least once, the study found. And 42 percent of adolescents who don’t identify exclusively as male or female have at least one prior suicide attempt.

About 30 percent of trans female teens — who identify as female but have birth certificates that label them as male — have tried suicide at least once, as have 28 percent of adolescents who are questioning their gender identity, the study also found.

In contrast to all of these groups of transgender teens, just 18 percent of females and 10 percent of males who are cisgender — meaning their gender identity matches what it says on their birth certificate — have attempted suicide.

“Our findings are startling,” said study leader Russell Toomey of the University of Arizona in Tucson.

“Previous studies had already demonstrated that transgender teens reported higher levels of suicidal behavior compared to cisgender adolescents, but our study is the first to go beyond this type of crude comparison to examine whether there are critical differences in suicidal behavior within transgender youth populations,” Toomey said by email.

“While all four transgender subgroups reported higher levels compared to cisgender female and male youth, it is important for targeted prevention and intervention efforts to know that transmasculine and nonbinary trans youth are at higher risk,” Toomey added.

To assess the connection between gender identity and suicide risk, researchers examined survey data collected between 2012 and 2015 from more than 120,000 youth nationwide ranging in age from 11 to 19.

Participants were 15 years old on average, and less than 1 percent of them identified as transgender.

Among other things, the survey asked teens if they had tried to kill themselves one or more times.

Overall, nearly 14 percent of the participants said they had, researchers report in Pediatrics.

Parents’ education levels and family socioeconomic status didn’t appear to influence whether teens would attempt suicide, the study also found.

Suicide is the second leading cause of death among teens and young adults in the U.S., researchers note.

The study wasn’t a controlled experiment designed to prove whether or how teens’ gender identity might influence suicidal behavior.

It’s possible trans teens might have a higher suicide risk as a result of being marginalized or experiencing discrimination, victimization or harassment, Toomey said.

“For transgender youth, we know, for example, that peer, school, community, and family based rejection, discrimination, and victimization are associated with greater risk for suicidal behaviors,” Toomey noted. “Transgender youth might respond to these experiences by internalizing this rejection (e.g., shame), feeling like a burden to others, or perceiving that they do not belong.”

Another limitation of the study is that, even though it was conducted nationwide, it wasn’t nationally representative; it included a lot of rural and suburban teens and relatively few urban adolescents.

And it’s possible that teens might not accurately report any history of suicide attempts, the authors acknowledge.

“The fact is we do not know why transgender teens have these incredibly high suicide rates,” said John Ayers, a researcher at the University of California, San Diego, who wasn’t involved in the study.

“It is critically important that we begin investing in asking why, instead of simply counting how many, especially for designing effective prevention campaigns,” Ayers said by email.

FOLLOW NBC OUT ON TWITTER, FACEBOOK AND INSTAGRAM