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By Maggie Fox

An early warning sign of depression in teenagers may not be sadness, but rather anxious and even irritable behavior, British researchers reported Wednesday.

They found that the children of parents with major depression often gave clues not only in the form of anxiety, but also with angry, resentful behavior.

Irritability may be a first sign of depression in teensGetty Images/Image Source

Such clues are important for flagging parents, teachers and doctors to the risk of major depression, which can disable adolescents just as they’re starting life. And it’s the leading cause of suicide, which is itself a major cause of death for children and teenagers.

“Both irritability and fear/anxiety predicted new adolescent-onset major depressive disorder."

“Major depressive disorder is a leading global cause of lifelong disability. The incidence markedly rises during mid-adolescence,” Frances Rice, of Cardiff University in Britain, and colleagues wrote.

Related: Depression Worsening in Teens, Especially Girls

“Even when the onset of depression is in adult life, many of its contributing risk factors begin during childhood, highlighting the importance of understanding the etiology of early-onset major depressive disorder,” they added in their report, published in the Journal of the American Medical Association’s JAMA Psychiatry.

“The most common major risk factor for early-onset major depressive disorder is depression in a parent.”

Related: Teens at Risk Can Hide Mental Illness

The Cardiff team followed 337 families in which one parent, usually the mother, had major depression. Depression can be genetic, so the children were considered to have some risk.

It can be hard to study depression in families. The disorder itself can make people hard to reach or unable to answer surveys. And conditions that can fuel depression — unemployment, uncertainty and instability — make it hard to follow families consistently.

Related: Suicide Kills More Middle-Schoolers Than Car Crashes

The Cardiff team was able to follow the families for four years, regularly interviewing parents and children. Over those four years, 20 children developed major depression, on average at age 14. The team used standard definitions of depression, which requires the patients to have at least five symptoms, including persistent low mood, irritability and loss of interest in normal activities.

“While anxiety and depression cross-predict each other over time, anxiety typically emerges earlier."

“Irritability and fear and/or anxiety were significant independent clinical antecedents of new adolescent-onset major depressive disorder, but disruptive behavior and low mood were not,” the researchers wrote.

“Both irritability and fear/anxiety predicted new adolescent-onset major depressive disorder."

Related: Antidepressants May Not Always Work in Kids

Irritability was assessed by asking if the children were touchy or easily annoyed, angry or resentful, or threw temper tantrums; if they were frequently disobedient, broke rules, annoyed or blamed others, and were spiteful or vindictive.

“While anxiety and depression cross-predict each other over time, anxiety typically emerges earlier,” the researchers wrote.

Social stresses — such as being poor or having other struggles in life — also contributed, the team found.

Related: Young kids can have depression

So doctors will need to be on the lookout for these symptoms, they said.

“Family-based programs may be indicated in children at high familial risk of depression because parental depression is associated with social adversity (poverty and stress exposure),” they added.

Major depression affects about 3 percent of children aged 6 to 12 years and about 6 percent of teenagers aged 13 to 18. Suicide is now the second leading cause of death for adolescents aged 15 to 19.

"In 2015, an estimated 3 million adolescents aged 12 to 17 in the United States had at least one major depressive episode in the past year," the National Institute on Mental Health said. "This number represented 12.5 percent of the U.S. population aged 12 to 17."