Attention-deficit/hyperactivity disorder is often thought of a boy thing. In explaining the jump in cases in recent years, numerous researchers, educators and parents have theorized that perhaps boys are hardwired to be more impulsive, wiggly and less able to stay on task in the early years than their female counterparts.

That may be a myth.

A study published in The Journal of Clinical Psychiatry on Tuesday shows a surprising 55 percent increase in prevalence of diagnoses among girls — from 4.7 percent to 7.3 percent from 2003 to 2011.

The rise in cases in girls mirrors a similar but less-sharp rise in cases in boys from a prevalence of 11.8 to 16.5 percent. During the same period, the researchers found an increase in cases across all races and ethnicities but especially in Hispanic children. In all children, the prevalence increased from 8.4 percent to 12 percent.

The analysis, conducted by George Washington University biostatistician Sean D. Cleary and his co-author Kevin P. Collins of Mathematica Policy Research, was based on data from the National Survey of Children's Health in which parents were asked whether they had been told by a doctor or other health-care provider that their child has ADHD.

ADHD is generally thought to be more difficult to diagnose in girls than boys because the condition manifests itself differently. A boy with ADHD may show more symptoms that are "external," clinicians suggest. So a boy may yell or shout in certain situations whereas a girl may resort more to teasing or name-calling. A boy may be very vocal about not being able to finish a task whereas a girl might internalize her frustration.

"It's hard to know what's causing the increase, but it's possible it is greater awareness. People are recognizing that there are different types of symptoms in females so they may be more likely to be brought in and diagnosed than in the past," Cleary, an associate professor of epidemiology and biostatistics, said in an interview.

The study is also believed to be among the first to look at the change in ADHD diagnoses across racial and ethnic lines. Whereas previous studies have noted that it is mostly boys who are white being diagnosed, Cleary's work shows that the diagnoses among children who are black, Hispanic and of other races are also on the rise.

The prevalence of ADHD in white children rose from 9.6 to 14 percent from 2003 to 2011 and from 8.1 to 12.8 in black children. The most dramatic increase was in Hispanic children who saw a rise from 4.2 to 7.7 percent — an 83 percent increase.

While the research wasn't designed to look at the underlying reasons for the change, Cleary theorized that it's possible the fact that the survey that he drew data from began to be administered in Spanish starting in 2003.

Cleary and Collins also found several other revelations in the data: that there was significantly higher ADHD prevalence in blended families (with a stepparent) or living with guardians with non-traditional makeups such as with a single father, a legal guardian or a grandparent; and that in 2011, children ages 15-17 reported the highest ADHD prevalence, contradicting the idea that ADHD is usually diagnosed mostly in the elementary years.

Furthermore, the researchers wrote that "although results from this study did not support the hypothesis that sociodemographic factors explained all racial/ethnic differences in ADHD, results indicated that particular factors were highly influential, including socioeconomic, family, and insurance status, as well as neighborhood safety and primary language."

Cleary concluded that his main concern from the numbers is the children who remain undiagnosed.

"We know already that this may lead to difficulties in school but not being treated could mean sustained difficulties into adulthood and could impact their getting and keeping a job," he said. "My main recommendation is if a teacher, school counselor or whoever is in a position to advise a parent about ADHD symptoms that that parent go see a physician and get assessed."

This post has been updated.

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