With symptoms of inattentiveness, hyperactivity and impulsiveness, attention deficit hyperactivity disorder (ADHD) is challenging for families. Forgetfulness and disorganization cause problems at home as well as in school, and characteristics such as low self-esteem, aggressiveness and emotional immaturity affect the daily lives of all family members.

Symptoms usually start before 12 years of age, sometimes as early as age 3. According to parent reports, 6.4 million children (11 percent of this age group 4 to 17 years) have ever been diagnosed with ADHD, and rates of ever-diagnosed ADHD increased an average of approximately 5 percent per year from 2003 to 2011.

More than half of children (60 percent) with ADHD had another mental disorder — most often conduct disorder (CD) or oppositional defiant disorder (ODD). About 1 in 30 adults suffers with ADHD, and 60 percent of children with ADHD in the United States become adults with ADHD about 4 percent of the adult population or 8 million adults.

Although the exact cause of ADHD remains unknown, it is thought that the disorder runs in families, and there is ongoing research focusing on genes as a causative factor.

Twin studies have shown that monozygotic twin pairs have much higher concordance rates for ADHD than dizygotic twin pairs. Factors that may play a role in the development of ADHD include genetics, environment (lead exposure) and development (problems in nervous system) as well as maternal alcohol use, drug use, cigarette smoking and premature birth.

Rather than a problem of motivation or parenting, a new study with the largest data set to date and conducted by an ADHD working group called ENIGMA (Enhancing Neuro Imaging Genetics through Meta Analysis) points to a problem of delayed brain maturation.

The study included 1,713 children, adolescents and adults (ages 4 to 63 years, median age, 14 years) and 1,529 controls from 23 sites. The individual sites analyzed T1-weighted magnetic resonance imaging (MRI) of individuals with ADHD compared with those who did not have this diagnosis. The primary outcome was to assess differences in subcortical structures and intracranial volume through pooling of all individual data.

Study participants with ADHD showed structural differences and evidence of delayed development compared with those without ADHD. The researchers discovered patterns in the brains of children with ADHD that revealed delayed brain development in five brain regions linked to impulse control, attention and processing emotions.

The volumes of the accumbens, amygdale, caudate, hippocampus, putamen and intracranial volume were smaller in those with ADHD compared with controls in the mega-analysis. There was no difference in volume size in the pallidum and thalamus between ADHD sufferers and controls.

Because the greatest differences were seen in children, the researchers concluded that people can eventually outgrow the disorder. There was no difference found between those who were taking or had taken ADHD drugs and those who had never taken such medications suggesting the brain changes were not caused by psychostimulants.

Previous studies that associated changes in brain volume with ADHD had been too small to be conclusive. This study confirms that people with ADHD have differences in their brain structure, suggesting ADHD is a disorder of the brain.

Lead author, Martine Hoogman of Radboud University Medical Centre in the Netherlands, hopes the study will help reduce stigma that ADHD is a label for difficult children or that ADHD is caused by poor parenting.