Researchers at Ohio State University compared 1,773 sibling pairs, one of whom had been breast-fed and one bottle-fed, on 11 measures of health and intellectual competency. The children ranged in age from 4 to 14 years.
The researchers recorded various health and behavioral outcomes in the sibling pairs, including body mass index, obesity, asthma, hyperactivity, reading comprehension, math ability and memory-based intelligence. The study, published online in Social Science & Medicine, found no statistically significant differences between the breast-fed and bottle-fed siblings on any of these measures.
By studying "discordant" siblings — one of whom had been breast-fed and the other not — the authors sought to minimize the possibility that racial, socioeconomic, educational or other differences between families could affect the results. Many earlier studies on breast-feeding failed to control for such factors, they say.
Campaigns to increase the rate of breast-feeding have been highly successful in recent years. According to the Centers for Disease Control and Prevention, about three-quarters of American mothers now breast-feed, compared with less than two-thirds in 2000, and about 49 percent are still breast-feeding at six months, compared with 34 percent in 2000.
Yet despite this increase, researchers have consistently found large socioeconomic and racial disparities in breast-feeding rates. A C.D.C. survey in 2008 found that 75 percent of white infants and 59 percent of black infants were ever breast-fed, and in 2013, the agency reported that 47 percent of white babies but only 30 percent of black babies were still being breast-fed at 6 months. Compared with bottle-fed infants, breast-fed babies are more likely to be born into families with higher incomes, have parents with higher educational attainments, and live in safer neighborhoods with easier access to health care services.
Still, sibling studies such as this latest one do not solve all the problems of bias. "We were not able to control for everything that could affect what would make a mom breast-feed one child and not the other," said the lead author, Cynthia G. Colen, an assistant professor of sociology at Ohio State. "But we did control for premature birth, birth order, the age of the mother, and whether she was working when she had one infant and not when she had the other."
Geoff Der, a statistician at the University of Glasgow who has worked with the same data in previous studies, said that the findings in the present study were robust and the authors' method for eliminating selection bias was powerful. He had reassuring words for women who do not or cannot breast-feed.
"In a society with a clean water supply and modern formulas," he said, "a woman who isn't able to breast-feed shouldn't be feeling guilty, and the likelihood that there's any harm to the baby is pretty slim."
Image: Baby having a bottle, via Shutterstock