During counselling, parents and their defiant children, aged two to seven, use a playroom while a counsellor observes them through a one-way mirror and advises the parents through earphones.

Social worker Kitty Heung Yin-kwan, of TWGH's Lui Wing Cheung Children Centre, said counsellors mainly asked parents to give more praise, avoid criticism and describe what their children were doing and saying.

'When a parent says what his child is doing, he will think his parent is paying attention to him. It will help enhance linguistic ability as well,' Ms Heung said. 'If a parent repeats what his child has just said, it will show that the parent is listening attentively.'

Participants have to attend a one hour-long session a week for 14 to 20 weeks.

About 120 families have attended courses so far.

For the study, Sandra Tsang Kit-man, head of the University of Hong Kong's department of social work, and Cynthia Leung Man, the deputy head of the Hong Kong Institute of Education's department of educational psychology, interviewed 62 parents who had not attended the course and 48 who had.

The 34 course attendees who completed three assessments - before, immediately after and six months after the counselling - on average praised their children 17 times and criticised them only once in five minutes after the counselling. The result remained the same six months after the treatment.

Before counselling, they praised on average only once and criticised them 26 times in five minutes.

The findings also showed that children had fewer behavioural problems and parents were less stressed after they had attended the course.