An estimated 15% of children and adolescents will be diagnosed with a mental disorder before age 18, according to a new study published in JAMA Psychiatry last month.

The researchers, led by Soren Dalsgaard, M.D., Ph.D., with the National Centre for Register-Based Research, Aarhus University in Denmark, looked at data from more than 1.3 million individuals born in Denmark between 1996 and 2016. The study looked at incidence of mental disorders (new cases per unit of time) and cumulative incidence, which provides an estimate of the risk of developing a mental disorder by a specific age.

Overall, 15% of children and youth were diagnosed with a mental disorder before age 18, including 14.6% of girls and 15.5% of boys. The overall risk of being diagnosed with a mental disorder before age 6 was 2%.

The overall risk of mental illness diagnosis during childhood identified by Dalsgaard and colleagues is consistent with surveys done in the U.S.

The study identified several differences in rates of specific mental illnesses in boys and girls. Anxiety disorder was the most common mental health diagnosis in girls at 8% and ADHD was the most common diagnosis in boys at 6%. Girls had a higher risk than boys of depression, anxiety, schizophrenia, obsessive-compulsive disorder (OCD) and eating disorders. Boys were at higher risk than girls of intellectual disability, autism, ADHD, oppositional defiant disorder/conduct disorder, attachment disorders and tic disorders.

There were also significant differences between boys and girls in the timing of illnesses. For example, incidence of ADHD peaked at an earlier age in boys than girls – at 8 years of age for boys versus age 17 for girls. Incidence of intellectual disability peaked at 5 years for boys compared to 14 years for girls; and incidence of other development disorders peaked at age 5 for boys compared to age 16 for girls.

The trends in diagnosis of autism were particularly distinct between girls and boys. For boys, diagnosis of autism spectrum disorder increased from birth to a peak at age of 4 years and then remained almost constant until age 15. For girls, incidence of autism was low until age 10 then increased and peaked in early adolescence.

The pattern of later diagnosis of several disorders among girls, the authors note, suggests possible delays in detecting conditions that typically begin at an early age. Understanding the differences with respect to age and gender and the age-specific risks is important for public health decisions, prioritizing resources and research on risk factors and outcomes, the authors conclude.