Is assessment biased against girls?
The diagnosis of ADHD in girls is more complicated than in boys because of the later age of onset, more subtle clinical manifestations, and limitations associated with the DSM-IV diagnostic schema and nomenclature.8,19 One concern has been that the wording of DSM-IV diagnostic criteria for ADHD and the majority of rating scales are geared toward the manifestation of ADHD in boys. Some have argued that there is a subset of girls who present with symptoms of ADHD and are rated by parents and teachers as having elevated scores when sex-specific norms are used, yet they do not meet DSM-IV criteria for the disorder.20 Parents tend to rate behaviors in boys (ie, inattention, hyperactivity, aggression) as more problematic than behaviors in girls (ie, relational aggression and other “feminine” behaviors).21
A Dutch study found that while mothers reported similar levels of disruptive behavior in boys and girls, teachers reported higher levels of concern with attention and aggression among the boys in the same sample.22 Mental health professionals may also display this bias. In a recent study, therapists were presented with 4 diagnostic vignettes: 3 vignettes illustrated some symptoms of ADHD, but not enough to diagnose the disorder according to DSM-IV criteria. Male and female versions of each vignette were created. In the male version of the vignettes, therapists diagnosed ADHD twice as often as they did with the female vignettes. This suggests that therapists do not adhere strictly to DSM-IV criteria, with consequent overdiagnosis of ADHD in boys and underdiagnosis (and undertreatment) in girls.23
Developmental maturity also appears to play a part in the diagnosis of ADHD, and boys may be at increased risk for diagnosis because of their later development of self-control skills. A recent US study found that the rate of diagnosis of ADHD varied according to the month of the child’s birthday, relative to the cutoff date for entrance into kindergarten. For example, in approximately 8.4% of children born in the month before the cutoff, ADHD was diagnosed, compared with 5.1% of children born in the month after the cutoff.24 Similar findings were also reported in a Canadian study that showed higher rates of diagnosis (relative to sex-matched peers) for both boys and girls born in the month before kindergarten cutoff.25
Sexual dimorphism
There are sex differences in incidence or nature of many CNS-related disorders. Human brains are approximately 8% to 10% larger in males than in females26; however, girls’ brains mature earlier and are on a different trajectory. These differences are observed at birth and may be driven by sex chromosome–specific pathways that underlie sexual dimorphism or even by differences in the presence of fetal testosterone.27,28 It has been estimated that at birth, girls are 3 weeks ahead of boys in physical maturation and by the time they enter school, girls are approximately 1 year ahead.10These findings are highly salient, since different maturation rates appear to be associated with different patterns of cognitive skills (eg, children identified as “late-maturing” performed better than early-maturing children of the same sex on spatial measures).29
A recent meta-analysis identified sex differences in reward sensitivity—women were more sensitive to punishment than men but not more sensitive to reward; men showed higher rates of sensation-seeking than women.30 It has been argued that these patterns of behavior are related and may also have sexually dimorphic phenotypic expression of genes linked to ADHD.31
