The study also examined the effects of substance use and abuse on high school graduation and found that among students who engage in substance use, including alcohol and other drugs, teens who smoke cigarettes are at greatest risk of dropping out.
There are three types of ADHD: the hyperactive type, the inattentive type and the combined type. Symptoms include not being able to pay attention, daydreaming, being easily distracted and being in constant motion or unable to remain seated.
"Most people think that the student who is acting out, who is lying and stealing, is most likely to drop out of school. But we found that students with the combined type of ADHD — the most common type — have a higher likelihood of dropping out than students with disciplinary problems," said Julie Schweitzer, an expert on ADHD at the UC Davis MIND Institute, an associate professor of psychiatry and behavioral sciences and the study's senior author. "This study shows that ADHD is a serious disorder that affects a child's ability to be successful in school and subsequently in a way that can limit success in life."
Published online in July in the Journal of Psychiatric Research, the study "Childhood and Adolescent-onset Psychiatric Disorders, Substance Use, and Failure to Graduate High School on Time" found that 32.3 percent of students with the combined type of ADHD — which incorporates hyperactive and inattentive symptoms — drop out of high school. Fifteen percent of teens with no psychiatric disorder drop out.
"Understanding the factors that contribute to dropping out of high school has major public-health implications, given that a third of youth in this country do not complete high school on time. Supporting mental-health interventions for students may have a significant impact on reducing high school dropout," said study author Elizabeth Miller, an assistant professor of pediatrics and an adolescent medicine specialist at UC Davis Children's Hospital.
In 2006 an estimated 4.5 million children in the United States between 5 and 17 years of age were diagnosed with attention-deficit/hyperactivity disorder, according to the Centers for Disease Control and Prevention. An estimated 9.5 percent of boys and 5.9 percent of girls are diagnosed with the condition.
The next most at-risk teens are students with conduct disorder, whose symptoms include aggression, lying, stealing, truancy, vandalism and a general pattern of rule-breaking. Thirty-one percent of students with conduct disorder drop out, said Joshua Breslau, associate professor of internal medicine and the study's lead author. Breslau said the research shows there are different pathways to poor high school performance.
"This study identifies multiple ways in which mental-health problems can affect education at the high school level. Attention-deficit/hyperactivity disorder impacts achievement because it affects how well students are able to perform basic classroom tasks from paying attention to turning in their homework," said Breslau. "Students with conduct disorder are able to do just as well as everyone else academically but disciplinary issues and dealing with the routines of school life may cause them to drop out."
For the study, the researchers examined the joint, predictive effects of childhood- and adolescent-onset psychiatric and substance-use disorders on failure to graduate high school on time, using data collected during 2001 and 2002 from the National Epidemiological Survey of Alcohol and Related Conditions. A total of approximately 43,000 racially diverse male and female participants over 18 from throughout the United States were interviewed by U.S. Census Bureau representatives about the age of onset of psychiatric diagnoses, substance use and high school graduation. Respondents were excluded if they had less than eight years of education or arrived in the U.S. after age 13. A total of 29,662 of the respondents were included in the UC Davis study.
Among childhood and adolescent psychiatric disorders, diagnosis with either the combined type of ADHD or the inattentive type — at 28.6 percent — resulted in the highest dropout rates. Students with mania, a mood disorder, and panic disorder dropped out at 26.6 and 24.9 percent respectively. Students with other mental-health disorders had dropout rates in the high teen- to low 20-percent range. The disorders included specific phobias (like fear of water), social phobia (fear of people), post-traumatic stress disorder, generalized anxiety disorder and depression.
But more predictive of dropping out than all other mental-health disorders except ADHD and conduct disorder was tobacco use. The study found that 29 percent of students who used tobacco failed to complete high school on time. Only 20 percent of teens who used alcohol and 24.6 percent of teens who used drugs dropped out. However, when the three substances were examined together, the effect of drinking and using drugs was no longer significant, Breslau said.
"Kids who smoke had a much higher risk of dropping out than kids who drink alcohol or use other drugs. When we looked at smoking in combination with other substances, drinking and using drugs did not increase one's risk of not completing high school on time. There's no additional increment of risk of dropping out once you account for smoking," Breslau said.
The reasons why this is the case merits further investigation, he said. However, existing literature suggests that poor educational performance contributes to smoking. If this is true, then breaking the connection between smoking and education may be essential to further reduction in the prevalence of smoking, Breslau said.
The implication of the findings, according to Breslau, is that the impact of mental health on education is likely to arise from a small set of conditions.
"This study suggests that focusing on a relatively narrow and hopefully more manageable range of mental-health conditions may have a consequential impact on improving school performance in secondary education."
Schweitzer said that devising effective interventions to help students with ADHD graduate high school would have important long-term societal consequences.
"If you don't have your high school degree, you're going to have less income. You can't buy houses and cars. People who drop out of high school are more likely to be reliant on public assistance. This is a disorder that has serious long-term impacts on your ability to be successful and contribute to society, not just in school, but for the rest of your life," she said.
The UC Davis School of Medicine is among the nation's leading medical schools, recognized for its research and primary-care programs. The school offers fully accredited master's degree programs in public health and in informatics, and its combined M.D.-Ph.D. program is training the next generation of physician-scientists to conduct high-impact research and translate discoveries into better clinical care. Along with being a recognized leader in medical research, the school is committed to serving underserved communities and advancing rural health.