Scientists at Rush University Medical Center in Chicago said the three key factors they have identified could also help to develop better screening tests and treatments for children with social and emotional learning difficulties.

"The number of children who cannot negotiate all these steps, and who are at risk of social rejection, is startling," said Clark McKown, the research director of the Rush Neurobehavioral Center who headed the research team.

About 4 million school children, nearly 13 per cent, in the United States have social-emotional learning difficulties, according to the researchers.

"They simply don't notice the way someone's shoulders slump with disappointment, or hear the change in someone's voice when they are excited, or take in whether a person's face shows anger or sadness," McKown explained in a statement.

In two studies reported in the Journal of Clinical Child and Adolescent Psychology, McKown and his team found that recognising and responding to the non-verbal cues was essential in developing skills for making and keeping friends and avoiding problems later in life.

Some children do not recognise the cues, while other youngsters do not understand the meaning of them or have the ability to reason about social problems.

The scientists studied 158 children in schools in Chicago, Illinois, a state which requires the school district to assess the social-emotional needs of pupils, and 126 children who had been referred to the clinic.

They found that children who picked up the cues and responded appropriately were most likely to have successful friendships.

"Now it will be possible to pinpoint which abilities a child needs to develop and offer help," McKown added.

Reuters