Breastfeeding may have long-term positive effects on cognitive and intellectual development

Dr Erik Lykke Mortensen

However, the study published in the Journal of the American Journal of Medicine, also found that the IQ benefits started to taper off for those who had been breast-fed beyond nine months.

Dr Erik Lykke Mortensen and colleagues from the Institute of Preventive Medicine in Copenhagen analysed data on 9,125 Danish babies born between 1959 and 1961, recording how they were fed as babies and their IQs as adults.

IQ scores

They found that those babies breast-fed for less than a month had an average IQ of 99.4 as adults, close to the 100 average for the population as a whole.

Those breast-fed for two to three months had IQs averaging 101.7, while those breast-fed for seven to nine months scored highest with 106.

They found breast-feeding beyond nine months had no further benefit for IQ, with the score dropping to 104.

The authors said their findings showed a strong link between breast-feeding and intelligence.

They rejected the conclusion of similar studies that the IQ scores may have more to do with the education and social class of mothers.

They stated: "We observed a positive significant association between duration of breastfeeding and intelligence in young adults."

Other factors

They added that a number of factors might be responsible for the link.

"These results indicate that breastfeeding may have long-term positive effects on cognitive and intellectual development.

"The nutrients in breast milk, behavioural factors and factors associated with choice of feeding method may all contribute to the positive association."

Rosie Dodds, policy research officer at the National Childbirth Trust, welcomed the findings.

But speaking to BBC News Online, she added: "It is not surprising given that there have been other studies which have found a link between breastfeeding and in particular duration of breastfeeding and intelligence."