It can take surprisingly little to improve the mental health of a teenager. In fact, as a new study has found, it may take only two group therapy sessions to help teens take control of many of the mental health problems they face.

Mental illness is the largest cause of misery in the world. About one in every four 8- to 15-year-olds in the U.S. has experienced a mental health disorder in the past year. But because it was assumed for so long that teenagers didn't really have mental health issues the way adults do, little information has been collected on the types of interventions that truly help adolescents solve emotional and behavioral problems.

Depression, anxiety and conduct disorders dropped by 21-36%, with the conduct disorders of the impulsive teens showing the largest drop — 36% — compared to similar students who did not attend the sessions.

Canadian researchers studied roughly a thousand teens. Half of the students, those who tested high for impulsiveness, thrill-seeking, hopelessness or anxiety, were offered two 90-minute therapy sessions.

The researchers focused on these tendencies because they are known to raise the risk that a teen will have behavioral problems. The idea was to give teens the tools to manage them better. For example, being rated as highly impulsive makes it more than five times likelier that a teen will develop serious conduct problems in school.

Students who had previously exhibited difficulty in any of these four areas attended two cognitive behavioral therapy sessions where they explored the thoughts, emotions and behaviors that were causing trouble, particularly the real-life triggers that commonly set them off with a therapist. Then together they developed concrete strategies that would help students feel less hopeless, less anxious, act less impulsively and lower their need for stimulation and excitement.

These strategies often worked: Based on questionnaires filled out by the teens every six months for the next two years, depression, anxiety and conduct disorders dropped by 21-36%, with the conduct disorders of the impulsive teens showing the largest drop, 36%, compared to similar students who did not attend the sessions. Similarly, teens high in anxiety sensitivity were 33% less likely to experience severe anxiety problems over the next two years.

“Our study shows that teacher delivered interventions that target specific risk factors for mental health problems can be immensely effective at reducing the incidence of depression, anxiety and conduct disorders in the long term,” study leader, Patricia Conrod of the University of Montreal said in a statement.

A similar study is now underway in 32 high schools in Montreal to further test the effectiveness of the brief CBT sessions.

The study appears in the Journal of the American Academy of Child & Adolescent Psychiatry.