More than a third of teenage girls experience depression, new study says

But what if by that time it's already too late?

A large new study out this week contains some alarming data about the state of children's mental health in the United States, finding that depression in many children appears to start as early as age 11. By the time they hit age 17, the analysis found, 13.6 percent of boys and a staggering 36.1 percent of girls have been or are depressed.

These numbers are significantly higher than previous estimates. Understanding the risk of depression is critically important because of the close link between depressive episodes and serious issues with school, relationships and suicide.

While researchers have long known about the gender gap in depression, with more adult women than men suffering from the condition, the new numbers show that whatever divergent paths boys and girls take happens even earlier than expected.

Published in the journal Translational Psychiatry, the study was based on data compiled from in-person interviews with more than 100,000 children who participated in the National Survey of Drug Use and Health from 2009 to 2014. The NSDUH is an annual survey on a representative sample of the U.S. population.

Among the standard questions asked are ones about insomnia, irritability, and feelings of guilt or worthlessness that researchers used to "diagnose" survey participants with depression using diagnostic criteria from the Diagnostic and Statistical Manual of Mental Disorders. Through the survey, they were able to capture a broader group of children than those who have a formal diagnosis and who may be in treatment.

Joshua Breslau, a senior researcher in the division of health for RAND and lead author of the study, explained in an interview that one of the main working theories about why there is such a big gender difference in depression had to do with the teen years.

"The idea was that it was something in particular, socially or biologically, that was happening about mid-adolescence in girls that led to this increase," Breslau said. "What we found partially contradicts that."

He says "partially" because the analysis did find that the gap between boys with depression and girls grows between ages 12 and 17, which is consistent with the old way of thinking. The surprise was that the gap is already quite large at age 12, Breslau explained, "which indicates that the differences have origins that starts earlier in childhood than was previously thought."

The idea that children can be depressed is something that has only been recently accepted by psychologists. As recently as the 1980s, adolescents were considered too developmentally immature to be able to experience such a grown-up affliction. Today most scientists recognize that children as young as 4 or 5 years of age can be depressed.

There are numerous theories about why boys develop differently than girls from a mental health perspective. While depression may be more common among girls, other conditions are more common among boys, such as conduct problems, aggression and substance abuse. One way of explaining this pattern involves a possible single underlying phenomenon, with different people branching off to develop different disorders because of social influences. There's also the possibility it may be connected to biological differences, perhaps involving changes in hormones or other ways that are distinct to how girls are socialized.

Breslau said the researchers initially hoped the data would be able to provide insights into how to differentiate between high-risk and low-risk cases of depression, but they were not able to answer that question. The findings showed that children who have had recent depression and those who had depression a while ago are very similar to each other in terms of school function and likelihood of suicide. But clearly not all will go on to develop serious problems.

Study author Elizabeth Miller, director of the division of adolescent medicine at Children's Hospital of Pittsburgh, said that until researchers can get a better understanding of why some people can come out of depression without intervention while others need help, teachers and others who work with children should learn to recognize the signs of childhood depression. They include extended periods - usually two weeks or longer - of low mood, feeling unable to enjoy normally pleasurable activities, insomnia, irritability, weight gain or loss, and feelings of guilt or worthlessness.