Nearly half of serious cases of attention deficit hyperactivity disorder in children are closely tied to social factors such as single parenting and poor maternal education, reports a new study.

Genes can play a key role in the development of so-called ADHD, whose signature symptoms include poor concentration, hyperactivity and impulsive behaviour.

Studies of identical twins separated at or near birth show that if one sibling is affected, there is a better-than-average chance the other will be as well.

And research published last month found that youngsters exposed to very high levels of organophosphate pesticide, used on many commercially grown US fruits and vegetables, were also at greater risk.

But to date few large-scale studies have tried to isolate the potential impact of social and family influences on ADHD.