Out of every 4 children who suffer from ADHD, 3 are boys. But in adulthood, the ratio gets very close to 1:1.

Image in public domain.

Part of the explanation emerged in 2017 when a study found that girls tend to develop ADHD at a later age than boys, but that doesn’t tell the whole story. A new study analyzed data from 1,571 children living in Zurich, Switzerland. The kids were involved in a program where teachers assessed two aspects associated with ADHD: inattention and hyperactivity/impulsivity.

Every year from the age of 7 to the age of 15, these behavioral elements were measured with a standard scale. They were assessed separately as previous studies have shown that they can develop at different rates and with different intensities.

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Firstly, when it comes to inattention, most children had low levels of symptoms. However, almost 40% of boys seemed to have persistently high inattention levels during this period — a level that stayed consistent with age. Among girls, however, there was much more variation. Around a third of them had moderate symptoms, but these symptoms declined consistently with age.

When it came to hyperactivity/impulsivity, there were also strong differences between boys and girls. Again, the majority of all children had fairly low levels of symptoms (81% for girls and 61% for girls). Symptoms tended to decrease with age, but there were two groups who stood out: one group (13% of boys, 10% of girls) had somewhat elevated symptoms in childhood. Their symptoms first decreased, but as they entered their teens, the symptoms re-emerged quickly — researchers speculate that the symptoms could be “adolescence-triggered”. There was also a group of children (24% of boys, 9% of girls) who had high levels of symptoms which did not seem to decrease over the years.

This confirms that boys tend to develop ADHD-type symptoms earlier than girls. It also shows that for both males and females, early adolescence represented a period of vulnerability characterized by relatively sudden symptom increases.

However, it also suggests that many girls may be slipping diagnosis altogether, since most of the diagnosis emphasis is placed on younger children — as most diagnostic criteria require symptoms to begin before the age of 12.

“Females affected by hyperactivity/impulsivity may be more likely to be excluded from diagnosis due to current age of onset criteria. More attention should be paid to early adolescence as a period of risk for hyperactivity/impulsivity symptom onset or worsening,” researchers write.

The study “Sex differences in ADHD trajectories across childhood and adolescence” has been published in Developmental Science.