New research shows that children who were breastfed for more than four months as babies are less likely to be anxious, restless and antisocial by the time they turn five.

"Our results provide even more evidence for the benefits of breastfeeding,' said Maria Quigley of Oxford University, co-author of the study, in a university press release.

"Mothers who want to breastfeed should be given all the support they need. Many women struggle to breastfeed for as long as they might otherwise like, and many don't receive the support that might make a difference."

Breastfeeding is already associated with higher IQs, lower levels of infection and lower rates of obesity.

The study, published in the Archives of Disease in Childhood, looked at 9,500 mothers and babies, all of whom were born full-term within a 12-month period to families with a white ethnic background.

Those moms filled out a questionnaire when their babies were nine months old detailing whether they breastfed and for how long.

When the kids were five years old, the researchers sent the mothers a second questionnaire to determine possible behaviour problems, such as anxiety, restlessness or an inability to socialize with other kids.

While 16.1% of the formula-fed babies had behavioural problems, only 6.5% of breastfed babies showed the same.

But the study's authors warn other factors might be in play. For instance, moms who breastfeed tend to be older, richer and better educated -- all factors that could contribute to their children's behaviour.

Still, adjusting for age, education and socio-economic background, the researchers found breastfed babies were 30% less likely to have behaviour problems.

"We just don't know whether it is because of the constituents in breast milk which are lacking in formula, or the close interaction with the mom during breastfeeding, or whether it is a knock-on effect of the reduced illness in breastfed babies," said Quigly.

"But it does begin to look like we can add fewer behavioural problems as another potential benefit of breastfeeding."