The argument over whether intelligence is innate or environmentally influenced has raged for more than a century. One of the most recent issues in the nature versus nurture debate is the effect of breast-feeding on IQ.

Research shows that the fatty acids in human milk may influence brain development. Using that data as a springboard, a group of scientists, led by a team at the King's College London Institute of Psychiatry, set out to determine how the makeup of infants interacts with their mothers' milk to affect intelligence.

Their findings, published in Proceedings of the National Academy of Science USA: breastfeeding can boost a baby's intelligence quotient if the newborn has a certain version of a gene, called FADS2 (fatty acid desaturase 2), which affects how fatty acids are processed.

"We were searching for an empirical example that would allow us to show scientists that it is possible to use the environment as a tool, to uncover novel genes that are important for human outcomes—including diseases," says study co-author Terrie Moffitt, a psychiatry professor of at King's College. "Our chain of logic from environment to genetic marker allowed us to discover for the first time the link between the FADS2 gene and the IQ, an important child health outcome."

The genetic marker that Moffitt refers to is located in the FADS2 gene, which has two primary variations. The new study, based on 1,000 New Zealander children (a portion of whom were breast-fed) in the early 1970s as well as on more than 2,000 breast-fed kids who lived in the U.K. in the mid 1990s, showed that 90 percent of the subjects had at least one copy of the more common version of FADS2 and 50 percent of them had two copies.

The researchers found that breast-fed infants with at least one or more of the common variation had IQ scores that were, on average, six to seven points higher than those of nonnursed kids with similar genetics. But breast-feeding did not appear to affect those children (10 percent of the population) with only the less common variant. The scientists ruled out other factors, including birth weight and the mother's social class and IQ, finding that they had no impact.

"Those who were breast-fed scored on average three points above the population mean of 100 on the IQ test, whereas those who were not breast-fed scored about three points below the population mean," Moffitt says. In other words, breast-feeding led to a gain of a few IQ points, whereas those using baby formula in lieu of mom's milk led to a slight dip.

As for the study's implications on the nature / nurture debate, Linda Gottfredson, a professor of education at the University of Delaware, says that a person's DNA is not really a blueprint, as it is commonly portrayed. "[Genes] are more like playbooks," she says. "It's not nature or nurture, but your genes operate frequently by making you more susceptible or less susceptible to certain environmental conditions." Hence, the withdrawal of breast milk from the diets of babies with a certain genetic predisposition resulted in a negative effect on intelligence.

The exact mechanism by which the enzyme coded by FADS2 might influence IQ is not known, but Moffitt suggests two possible roles: The gene variants may affect the conversion of dietary precursors to long-chain polyunsaturated fatty acids, which aggregate in the brain in the early months after birth. Alternatively, the presence of long-chain polyunsaturated fatty acids may act on the gene itself, causing it to turn on or off, thereby affecting the metabolic pathway the acids use.

The authors note that since the time that study subjects were breast-fed, many baby formula manufacturers have begun adding fatty acid supplements to their products, potentially giving them an IQ boosting effect.

"What's critical about this paper is that we haven't known entirely what are the mechanisms by which breast-feeding supports higher IQ," says Joseph Hibbeln, lead clinical investigator at the Unit of Nutrition in Psychiatry at the National Institute on Alcohol Abuse and Alcoholism. "This really helps to dissect one of those mechanisms: that is…, if your body can't make [fatty acids] efficiently, you better get it through the breast milk to support optimal IQ."