By Emma Young

For every girl with ADHD, there are three boys with the same diagnosis. But among adults, the gender ratio is more like 1:1. That’s a big discrepancy. So what’s going on?

In 2017, Aja Louise Murray and colleagues investigated possible predictors of childhood vs. later (adolescent/adult-onset) ADHD, and they found hints that girls tend to develop ADHD at a later age than boys. Now a team that includes the same researchers has investigated this explicitly and in their paper in Developmental Science, they’ve confirmed it seems to be the case, which could partially explain the discrepancy in the ADHD gender ratio between children and adults.

The researchers analysed data on 1,571 children living in Zurich, Switzerland, whose teachers used a standard scale to assess symptoms of inattention and also of hyperactivity/impulsivity every year from age 7 (when the children started school) through to age 15. The two domains of ADHD were assessed separately, as previous work has found that they can develop at different rates.

When it came to inattention, most of the children (about 60 per cent of boys and girls) had low levels of symptoms between the ages 7 to 15. However, whereas the remaining 40 per cent of the boys had persistently high inattention levels during this period, among the girls there was more variation, and generally lower symptom levels: almost a third had moderate symptoms that declined with increasing age, while the others started out with relatively serious symptoms at age 7, which then declined, but still remained above average at age 15.

For hyperactivity/impulsivity, there were also some contrasting developmental profiles between the sexes. Again, the majority of children (81 per cent of the girls and 61 per cent of the boys) started out with low levels of symptoms, which decreased even further into late adolescence. There was also a group (13 per cent of boys and 10 percent of girls), who had mildly elevated symptoms in childhood, followed by a dip around age 11 to 13, but then a rapid increase. “This group could speculatively be characterised as ‘adolescence-triggered’,” the researchers write. Finally, there was also a group of children (24 per cent of boys, versus only 9 per cent of girls) who had high levels of symptoms all the way through the study period.

In other words, according to this data, more boys than girls show consistently high levels of hyperactivity/impulsivity – and inattention – symptoms from a young age, whereas proportionately more girls than boys develop high levels of these symptoms (which may lead to a diagnosis of ADHD) only after early adolescence.

Whatever the reasons for this difference, it could be relevant to the sex difference in diagnosis rates because current ADHD diagnostic criteria for children require symptoms to have begun before the age of 12. This could mean that a greater proportion of girls with ADHD, than boys, are being missed by clinicians. (Exactly why adolescence might be associated with a sudden rise in these symptoms in some children, and especially girls, is not known – it could be to do with hormones, and/or increased social and academic pressures, the researchers note.)

“More attention should be paid to early adolescence as a period of risk for hyperactivity/impulsivity symptom onset or worsening,” Murray and her colleagues write. And, they add: “It should be investigated whether removing the ‘onset before 12’ stipulation in diagnostic tests would help to identify more girls who would benefit from intervention.”

—Sex differences in ADHD trajectories across childhood and adolescence

Emma Young (@EmmaELYoung) is Staff Writer at BPS Research Digest