The study revealed another provocative finding: Adopted children born in another country most in this study were from South Korea were slightly less likely to have ADHD than adopted children born in the United States.
Kim said the prevalence estimates in the study, which happened to take place in South Korea, are higher than previous estimates elsewhere, including but not exclusively the United States.
Hence the value of a methodical study that gives a glimpse of conditions in North Korea and was published recently in the Lancet, a British medical journal.
The maths study examined the availability of resources at home - such as books - with pupils in South Korea, Norway, Sweden and the US being the best equipped.