The findings, published in the journal Archives of Disease in Childhood, add to the evidence base on the benefits of breastfeeding.

'Our results provide even more evidence for the benefits of breastfeeding,' says Maria Quigley of the National Perinatal Epidemiology Unit at Oxford University, who led the work with Katriina Heikkilä.

'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.'

There are a number of well-known health benefits of breastfeeding: for example, breastfed babies have lower rates of infections and mothers have a reduced risk of breast cancer. A range of other health and child development benefits have also been suggested -- such as higher IQ, fewer behavioural problems and lower levels of obesity -- but the evidence for these can be inconsistent across different studies.

The researchers from the University of Oxford, along with colleagues from the University of Essex, University College London and the University of York, set out to investigate associations between the duration of any breastfeeding and child behaviour at age 5.

'We found that children who were breastfed for at least four months were less likely to have behavioural problems at age 5,' says Maria Quigley.

'However, that observation might not have been the direct result of breastfeeding -- it could have been down to a number of factors,' she explains. 'As a group, mothers who breastfed for four months were very different socially to those who formula fed. They were more likely to be older, better educated and in a higher socio-economic position, on average.

'Having controlled for these and other differences between the groups, we found there was still a 30% lower risk of behaviour problems associated with prolonged breastfeeding.'

The team used a nationwide survey of infants born in a 12-month period in 2000-2001 called the Millennium Cohort Study. This cohort study, for which the data are openly available, involved carrying out home interviews with parents when their children were 9 months old, with further follow-up interviews roughly every two years.

The Oxford researchers included data for over 9,500 mothers and babies born at full term to families of white ethnic background. They used answers from the initial interview when the children were 9 months old to determine whether mothers had breastfed and how long for.

They combined these data with the results of a standard questionnaire used for identifying children with possible behavioural problems. This was filled out by a parent (normally the mother) when their child was 5 years old. Children scoring in the top 10% are classified as having an abnormal score.

Abnormal scores in the questionnaire can result from a range of emotional (eg clinginess, anxiety), conduct (eg lying, stealing) or hyperactivity (restlessness) problems.

'We're not necessarily talking about tearaway, unmanageable 5-year-old kids,' says Maria Quigley. 'It might be unusual anxiousness, restlessness, inability to socialise with other children or play fully in groups.'

The raw figures showed that 16.1% of formula-fed babies (530 out of 3,292 formula-fed babies) had abnormal scores at age 5. Of the babies breastfed for at least four months, 6.5% had abnormal scores (179 out of 2,741 babies).

However, these two groups of mothers and children are very different across a number of measures, such as mother's age, education and socio-economic position. It could be that breastfeeding is serving as a proxy for something else causing the difference in rates of behavioural problems among the children.

So the researchers then adjusted their analysis to account for all these potential factors. Children who were breastfed for at least four months were still about 30% less likely to have behavioural problems at age five.

It is possible to suggest possible causes for the relationship between breastfeeding and reduced likelihood of problem behaviour. It may be that there is something in the breast milk that leads to improved neurological development and behavioural learning in children. Or the close physical contact during breastfeeding may lead to more mother-baby interaction and better communication. Or the reduced illness experienced by babies who are breastfed.

Maria Quigley says, '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 mum during breastfeeding, or whether it is a knock-on effect of the reduced illness in breastfed babies. But it does begin to look like we can add fewer behavioural problems as another potential benefit of breastfeeding.'

Mothers and children from non-white and mixed-ethnic groups were not included in the analysis. A significant proportion of mothers in these groups did not complete the questionnaire on child behaviour, making it difficult to know how representative the data would be for these groups.

Note: If no author is given, the source is cited instead.

• Breastfeeding
• Infant's Health
• Children's Health

• Child Development
• Child Psychology
• ADD and ADHD

• Maternal bond
• Colostrum
• Nutrition and pregnancy
• Baby colic
• Breastfeeding
• Infant