Advocates of so-called attachment parenting take an all-encompassing approach: assiduous attention to a child’s every need, including prolonged breast-feeding. But those who claim that these practices benefit children’s intelligence miss an important point.

A child’s biggest advantage arises from having the family resources that make such care giving possible in the first place. In this sense, attachment parenting is a trend among families whose children need it least -- the affluent. Fortunately, there is a way for parents of lesser means to get similar benefits.

Attachment parenting has not yet been subjected to rigorous scientific study. It is not known, for example, if it makes kids more independent or secure. But one claim has been thoroughly investigated: that breast-feeding increases a child’s IQ. Even many mothers who return to work believe their breast milk to be essential to their babies’ future intellect. Research shows that this is a myth.

Although it is true that children who were breast-fed as babies have higher intelligence than bottle-fed children, the reason for the correlation is in the mother’s brain, not her breast. A U.S. mother whose IQ is 15 points higher than her neighbor’s is more than twice as likely to breast-feed. Women who breast-feed are also more educated and less likely to smoke. Intelligent parents pass along their genes and also create a more stimulating environment, two advantages for the baby’s development. In short, smart mothers have smart babies.

Mom Differences

In one analysis of multiple studies, combining data from more than 5,000 children, the IQ differences associated with breast-feeding were eliminated when the mothers’ characteristics were taken into account. Among 332 pairs of siblings in which one was breast-fed and the other bottle-fed, researchers also found no difference in IQ.

So mothers who are unable to breast-feed need not worry that they are harming their baby’s intellectual development. Indeed, adopted children, many of whom are not breast-fed, have higher IQs, on average, than their siblings who remain in the birth family, presumably because their adoptive families provide an environment better suited to cognitive development.

A key factor in development is the family’s socioeconomic status -- a combined measure of income, occupation and education. The possible effect of environment can be measured by comparing differences within the same household in outcomes between identical twins, who share all their DNA, with those between fraternal twins, who share half their DNA. In middle-class households, identical twins have more similar outcomes than fraternal twins have, suggesting that environmental influences have a smaller impact than genetics on the growth of children’s intelligence. It seems that most middle-class environments are good enough to allow most children to approach their full genetic potential.

For children growing up in households of low socioeconomic status, on the other hand, good parenting is crucial. In one recent study, parents’ IQ influenced the child’s IQ at 2 years of age -- but not if the household was low-status. In 7-year-olds, only 10 percent of IQ variability in low-status children was attributable to genes. In other words, a poor environment can wipe out genetic advantages.

Low social status often means economic insecurity, unsafe neighborhoods and chaotic households. And it means increased secretion of stress hormones. At age 10, low-status children have twice as much stress hormone in their blood as high-status children do. Chronic stress reduces the volume of the hippocampus, a brain region involved in learning, and leads to memory problems. It also interferes with development of the frontal cortex, which is important for planning and organizing behavior and for self-control.

Social Advantages

The influence of social environment can even outweigh physical hazards: High-status children recover from lead poisoning better than low-status children do.

For these reasons, efforts to encourage young mothers to breast-feed should take into account their economic circumstances. Working mothers, already strapped by the expenses of new parenthood, cannot necessarily afford to shell out hundreds of dollars for a breast pump and accessories.

The next barrier comes at work, where pumping requires a private room with an electrical outlet. One woman we know, a phlebotomist, couldn’t find a regular place or time to pump until her baby was more than 3 months old, too late for the mother to begin lactating. In contrast, a physician who gave birth around the same time regularly caught up on work at her computer while using an electric breast pump in her private office.

Of course, breast-feeding is a healthy thing to do. It enhances the baby’s immune system, and builds a bond with mom. But social pressure on mothers may itself be harmful to children. Stress hormones can be passed from mother to baby during pregnancy and nursing.

Even in families of modest means, effective strategies are available to help a baby’s brain grow. Babies learn best from communicating with other people, especially with those who respond to them promptly and appropriately. Support programs that train mothers to be more responsive to their babies lead to improved cognitive development. An analysis of multiple studies involving 900 mother-infant pairs showed that short-term behavioral training for mothers to enhance sensitivity also made children more likely to form a secure attachment to the mother, a major goal of attachment parenting. Such interventions are especially effective for mothers of fussy infants.

There is no One True Way to be a mother, as humans have been raising children successfully under a variety of circumstances for thousands of years. Putting pressure on women to adhere to a particular set of practices that may not fit their lives is likely to be counterproductive. Societal support, on the other hand, including support for breast-feeding where possible, can help all parents to help their children grow up well.

(Sam Wang, an associate professor of molecular biology and neuroscience at Princeton University, and Sandra Aamodt, a former editor in chief of Nature Neuroscience, are the authors of “Welcome to Your Child’s Brain: How the Mind Grows from Conception to College.” The opinions expressed are their own.)

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To contact the writer of this article: Sam Wang at sswang@princeton.edu Sandra Aamodt at sandra.aamodt@gmail.com

To contact the editor responsible for this article: Mary Duenwald at mduenwald@bloomberg.net