Breast-fed babies grow into better behaved and more emotionally stable children than those that are bottle fed, a study has shown.
Five-year-olds who had been breast-fed were almost a third less likely to suffer behavioural problems severe enough to disrupt family life.
Those reared on bottled milk formula tended to display more troublesome traits such as neediness, anxiety, hyperactivity or lying and stealing.
The link between breastfeeding and better behaviour could be explained by fatty acids in mother's milk or bonding between child and parent, say scientists.
Researchers analysed data from a survey of around 10,000 infants born in the UK over a 12-month period between 2000 and 2001.
In total, 29% of children born after a full-term pregnancy, and 21% of those born prematurely, were breast fed for at least four months.
Parents were asked to complete Strength and Difficulties Questionnaires (SDQs) designed to assess the behaviour of their children via a scoring system.
The results showed that 16% of formula-fed children and 6% of breast-fed children were given abnormal scores indicating behavioural problems.
For full-term babies, the pattern persisted after taking account of influences such as social and economic background and parental factors.
The evidence for a link between breastfeeding and fewer behavioural problems in premature children was unclear.
Writing in the journal Archives of Disease in Childhood, Dr Maria Quigley from Oxford University and colleagues concluded: "Our findings suggest that longer duration of breastfeeding (at all or exclusively) is associated with having fewer parent-rated behavioural problems in term children."
Two likely explanations were given for the results.
One was that breast milk contains long chain polyunsaturated fatty acids and other chemicals that aid brain development. However, it was probable that the children in the study were fed baby formula supplemented by essential fatty acids, said the researchers.
The other theory was that breastfeeding leads to more interaction and bonding between mother and child which may affect learned behaviour.
Commenting on the findings, clinical psychologist Professor Peter Kinderman, from the University of Liverpool, said: "I suspect it may be a combination of both mechanisms, but I also suspect that maternal attachment may be more important.
"Research into maternal bonding - including research here at the University of Liverpool - shows that mother rats who interact well with their offspring, by licking and grooming them, moderate the baby rats' stress response (even affecting mechanisms involving the stress hormone, cortisol, which has a role in development and behaviour).
"The human version of grooming may include behaviours such as breastfeeding and this may be an important feature of bonding between mother and child. Positive bonding between parent and child is known to be fantastically helpful for development."
Janet Fyle, from the Royal College of Midwives, said: "The weight of evidence about the benefits of breastfeeding is well established and this is a good study which adds to the evidence.
"However, it is vital that women get the help, advice and support from their midwives, particularly once they have gone home, because this is when breastfeeding rates fall.
"We also know that many of the mothers who are motivated to breastfeed tend to be better educated and spend more time socialising or engaged in interactions with their children. We should therefore not underestimate the role of parenting in the emotional and social development of children as these approaches are also important.
"Nevertheless, we need to be careful to keep a balance when interpreting the results, so that we do not send a negative message to mothers that they have failed or make them feel guilty because they bottle-fed their babies."