ADHD, a major public health issue, affects 9% of all children with a sex ratio heretofore of male to female of 2:1. Diagnosis of ADHD has increased in recent years with the diagnosis in female increasing more rapidly than in males. Recent large epidemiological studies by Froehlich et.al. found 8.7% met criteria for ADHD of which 51% were boys and 48% were girls. Adult ADHD reveals a higher incidence in females than males suggesting a delayed onset which in turn suggests differential brain maturation rates in the two sexes. Girls show a more rapid neurobiological, cognitive, motor, and social development. During the onset of puberty increased estrogen and dopamine receptors may lead to increased symptoms in adolescence.
Barkley, RA et.al have reported that those with ADHD are less likely to finish school, have few or no friends, exhibit more autistic social behavior, drive faster, have more accidents, have a higher rate of STDs, and are at increased risk for other psychiatric conditions in adulthood. Mick, E et.at. found that 16% of females with ADHD showed adequate social adjustment compared to 86% of females without ADHD. Females who met criteria for ADHD at 16 years of age showed a persistence of the condition into adulthood of 71% with deficits in executive function especially prominent. Earlier studies showed males as more impaired but they are likely in error because of failure to correct for ADHD subtypes. Current studies point to equal impairment in the two sexes.
The diagnosis of ADHD in girls is more complex and difficult than in boys because of the age of onset, more subtle clinical findings and problems problems associated with the criteria as set out in the DSM-IV as well as a variety of ratings which are tilted to the male presentation. Also mental health professionals tend to have a bias toward more diagnosis in boys. In addition, boys have a delayed developmental maturity especially in self-control processes which likely contributes to over-diagnosis.
Males have brains that are 8-10% larger than females; however, female brains mature earlier and follow a different developmental course. Females are about three weeks ahead of males in physical maturation at birth and are often about one year more mature by the start of school at 5 years old. This is important because different maturations are associated with different patterns of cognitive skills. Total cortical volume is reached in females by 10.5 years and in boys by 14.5 years. This is also true for brain areas considered of importance in ADHD pathology. These areas include especially the frontal lobes and basal ganglia.
To summarize, sexual dimorphism in human neurobiological development extends to patterns of behavior along the developmental course of ADHD. Current research supports the equal prevalence in males and females. Previous cross sectional research is flawed because comparable ages in males and females did not take into account the differential developmental trajectory of the two sexes. Future studies will undoubtedly nail down the differences more clearly.