Babies who are breastfed are less likely to have behavioural problems by the age of five than those given formula milk, according to new research. The question of whether breastfeeding has a long-term impact on behaviour has been investigated before, but the studies have usually been small-scale and inconsistent in their findings. But a large research project carried out by the universities of Oxford, Essex and York, together with University College London, has come to a more robust conclusion. They used data from the ongoing Millennium Cohort Study, a survey of babies born in the UK during a 12-month period between 2000 and 2001. More than 10,000 mother and baby pairs of white ethnic background took part. They were interviewed when the baby was nine months old and again at two-yearly intervals. The researchers asked parents to fill in questionnaires to assess their children's potential behavioural difficulties. These included emotional issues such as clinginess and anxiety, hyperactivity such as restlessness, and conduct problems such as lying and stealing. Fewer than a third of the babies born at full-term (29%) and a fifth (21%) of those born prematurely were breastfed for at least four months. But only 4% of the breastfed babies showed a tendency to behavioural problems compared with 16% of those fed formula milk. The difference in the full-term babies was still significant even when other influences were taken into account, such as socioeconomic status and the mother's education, age and smoking habits. However, the association was not clear among the 512 children who were born prematurely. The researchers, writing in the journal Archives of Disease in Childhood, say they are not concerned with ordinary childhood misbehaviour. "Children learn appropriate behaviour from people around them and, during the learning process, all children sometimes behave inappropriately – for example, have temper tantrums or are aggressive," they write. "Behavioural problems, however, are inappropriate behaviours that occur repeatedly over a period of time, have a negative impact on the child's development and interfere with the child's or their family's everyday life." They offer two possible explanations for their findings. One is that breast milk contains large amounts of essential fatty acids, which are known to have an important role in the development and function of the brain and central nervous system. But in the past decade, they note, formula manufacturers have been supplementing their products with essential fatty acids and it is likely that the children in the study were given supplemented formula milk. The other possible answer, they write, is that "breastfeeding leads to more interaction between the mother and the child, better learning of acceptable behaviours and fewer behavioural problems". Peter Kinderman, professor of clinical psychology at Liverpool University, called it "a very good piece of research published in an important journal". He said he suspected the mother-child bonding that takes place during breastfeeding might be the most important factor. "Positive bonding between parent and child is known to be fantastically helpful for development," he said, noting that the authors specifically took into account factors such as childcare arrangements because they are so well-established as important influences in development. "This is more evidence of the importance of breastfeeding and mother-baby attachment, not just for physical health but also for the psychological development of the child," he said. The authors said that more work needed to be done to see if their findings would contrast with other ethnic groups.