The WHO research also reveals a strong correlation between ADHD and other disorders. For instance, adults with ADHD are four times more likely to suffer from a mood disorder or an addiction.

A sub-study of the WHO research found that employees with ADHD work an average of 22 days less yearly than colleagues without ADHD. This equates with an annual productivity loss of approximately one month. The loss of productivity is caused not only by days absent, but also by reduced productivity during the working day, resulting from inefficient or unfocussed behaviour.

A second sub-study examined how many children ‘carry’ ADHD into adulthood. Of all the respondents who had ADHD during childhood, an average of 50% still had the disorder as adults. Here too, however, there are striking differences between countries, meriting further study. Mexico, for instance, had a low score of 33%, compared with Italy which scored 84%.

Whether the disorder continues into adulthood seems to depend mainly on the type of ADHD. The ADHD variant which combines attention deficit with impulsive and hyperactive behaviour appears more likely to exist in adulthood. Other factors associated with the continuation of the disorder include poor functioning in childhood, depression before the age of 16, a paternal mental disorder, or an anti-social personality disorder in one of the parents.