Dr Elizabeth Hoy and colleagues from Queen's University, Belfast, compared 26 eighteen-month-old toddlers who had been born with very low birthweights with infants of the same age, gender and social background who had normal birthweights.

The children were videotaped as they completed psychological tests that involved playing with toys.

The researchers analysed the videotapes and examined the social behaviour of the children.

The toddlers with very low birthweights showed less positive feelings and were less sociable overall.

There were many occasions when they showed no positive feelings or social behaviour at all.

Compared to the children with normal birthweights, they also had a shorter attention span when playing with toys and a higher level of activity and changes of atttention.

Previous research has found that children born prematurely show poorer academic achievement compared to children with normal birthweights.

Dr Hoy suggests that these children's problems with social behaviour may in turn damage their ability to learn because they make it hard for caregivers and teachers to create the right environment.

On average, the premature babies were born after 28 weeks of pregnancy, and weighed 972 grams. The babies who went to full term weighed more than 2,500 grams.

Dr Hoy believes that when premature babies are subjected to the bright noisy environment of an intensive care baby unit they try to shut it out.

Shell shocked

When the babies get home, shell-shocked from their experience, their parents often make the problem worse by over stimulating their unusually quiet infant, she said.

The result is a child that has difficulty expressing emotions and engaging socially.

Dr Hoy said the flaw could prove a life-long handicap, affecting a child's progress at school and job prospects.

Previous research, in which she had also taken part, showed that at the age of seven children who had been born prematurely were seen as withdrawn, sad and unhappy by their classmates.

"After being in the womb you're suddenly thrust into this world very early before you able to cope," she said.

"It's a highly stimulating world of pre-natal intensive care, with all kinds of lights and procedures, and this can be highly stressful.

"Then because these children don't respond like they should, instead of listening parents often dominate their children and make the situation even more stimulating.

"The child is unable to handle the stimulation and tends to become withdrawn."

Her advice to parents of prematurely born children was to dispense with mobiles and other stimulating toys and not to be too overbearing.

Ill health

Ann Furedi, director of communications for the British Pregnancy Advisory Service, said the findings were not surprising, but that the problems were probably a result of the trauma associated with a premature birth.

"We are talking about children who are severely premature with very low birth weight," she said.

"These children are often very ill when they are born and can have very difficult first months - or even a first year - of life.

"It is not really surprising that their social development is affected as well."