The study took a bite out of the nature versus nurture debate by showing that intellectual development is influenced by both environmental and genetic factors.

'There has been some criticism of earlier studies about breastfeeding and IQ that they didn't control for socioeconomic status, or the mother's IQ or other factors,' said study co-author Terrie Moffitt, a professor of psychological and brain sciences at Duke University and King's College in London.

'Our findings take an end-run around those arguments by showing the physiological mechanism that accounts for the difference.'

Researchers examined more than 3,000 breast-fed infants in Britain and New Zealand and found that the child's IQ was an average of 6.8 points higher if the child had a particular version of a gene called FADS2.

This difference remained after researchers were able to rule out the influence of socioeconomic status, the IQ scores of the mother, birth weight and gestational age as factors.

'The argument about intelligence has been about nature versus nurture for at least a century,' Prof Moffitt said. 'We're finding that nature and nurture work together.'

Ninety per cent of the children had at least one copy of version of the gene which yielded higher IQ if they were breast-fed.

The IQ scores of the other 10 per cent were not influenced by breastfeeding, according to the study published in the Proceedings of the National Academy of Sciences.

The gene was studied because it produces an enzyme found in breast milk which has been associated with higher IQ. The enzyme helps convert dietary fatty acids into the polyunsaturated fatty acids that have been shown to accumulate in the human brain during the first months after birth.

This enzyme and the fatty acids has been added to many infant formulas since the first findings about breastfeeding and IQ appeared a decade ago, but tests have not been able to show whether it has an impact.

The authors suggest that may be because those studies did not account for whether or not the gene was present.

Lab studies on rodents and primates fed supplemental fatty acids have shown enhanced abilities in tests of learning, memory and problem-solving. -- AFP