In the age-old debate between nature and nurture, scientists studying breast-feeding and IQ have declared a tie.

The international team of psychologists, psychiatrists and geneticists wanted to know whether genes are primarily responsible for determining a person’s intellect or if environment plays a dominant role. They decided to examine breast-feeding because studies have found that nutrients in breast milk boost neurodevelopment in babies and that children who were breast-fed tend to have higher IQs.

So they searched for a gene that was likely to help babies get an intelligence boost from breast milk. The team focused on FADS2 because it plays a crucial role in metabolizing the fatty acids DHA and ARA that help neurons build connections in the developing brain. DHA and ARA are abundant in breast milk but only recently have been added to infant formula.

The study hinged on the fact that babies are born with one of three versions of FADS2. For children in two of those categories, those who were breast-fed scored an average of 6.8 points higher on IQ tests compared with kids who were raised on formula, the researchers found. Breast-feeding didn’t make a difference for youngsters in the third group.

The disparity was found in two separate groups of children — about 1,000 born in New Zealand in the early 1970s and 2,200 born in Britain in the mid-1990s.

The scientists were able to rule out other possible explanations for the difference.

, including maternal IQ and family social class, two factors that are linked to higher rates of breast-feeding.

The results showed that both nature and nurture — the right genes and a parental decision to breast-feed — are necessary to gain the IQ advantage.

“Genes are not a blueprint,” said Avshalom Caspi, a professor of personality development at King’s College London and lead author of the study published online this week in Proceedings of the National Academy of Sciences. “We hope that this research will help put a nail in the nature-versus-nurture coffin.”

Figuring out precisely how certain versions of the gene allow babies to benefit from breast milk will require further research, Caspi said. Besides, he added, an array of other genes almost certainly play a role in helping babies get the most out of the myriad nutrients found in breast milk.

AP-NY-11-09-07 1636EST