You can bet that the increase in the percent of newborns who are breastfed, from 68 percent in 1999 to 74 percent in 2004, didn’t happen because more mothers cared about strengthening their child’s immune system (one of many reported benefits of breast over bottle, according to the American Academy of Pediatrics). Instead, this period coincides with more reports that breastfeeding spurs a baby’s brain development, conferring an extra half-dozen or so IQ points by the time he or she enters school. Talk about feeding into the neuroses of middle-class parents.

But not all breastfed babies are little Einsteins, and some parents may well wonder why all the months of milk-stained blouses and balky breast pumps didn’t seem to boost Junior's cognitive development. A remarkable study unveiled Monday evening offers a clue. Researchers are reporting in the Proceedings of the National Academy of Sciences that only babies who carry a particular form of a gene derive an IQ benefit from being breastfed. Without this form of the gene, breastfeeding has no effect on later IQ.

The study, of more than 3,000 children in Britain and New Zealand, cuts through the stultifying debate about whether intelligence reflects nature or nurture. Of course it reflects both, which is not exactly a stop-the-presses statement. More interesting is the finding that intelligence reflects a specific interaction of genes and environment: in children with a particular version of a gene called FADS2, breastfeeding raises intelligence an average of nearly 7 IQ points, find scientists led by Terrie Moffitt and Avshalom Caspi of King's College London and Duke University.