Breastfeeding your baby seems like a good economic move: it doesn't cost a penny, and you don't have to invest in formula or the baby bottles to put it in. But a study published in the April issue of the American Sociological Review suggests that breastfeeding actually can exact a high cost — not in terms of immediate monetary outlay but in its long-term impact on a woman's earnings.

A decal reading "Breastfeeding Welcomed Here" is displayed on the door to a store on Thursday, Aug. 11, 2011, in Nashville, Tenn. (Mark Humphrey/AP)

Sociologists Mary Noonan of the University of Iowa and Phyllis Rippeyoung of Acadia University in Nova Scotia, Canada looked at data from 1980 to 1993 for 1,313 women who had their first baby in their 20s or 30s and had been employed for the year preceding their child's birth. That data came from the National Longitudinal Survey of Youth.

The researchers divided the women into three groups — those who exclusively fed their babies formula, those who breastfed for the first six months of their baby's life, and those who breastfed for longer than six months. They projected the women's earning trajectories for the years after giving birth.

In short, they found that while all the new mothers experienced income losses after they gave birth, women who breastfed for six months or longer saw greater reductions in their long-term earnings. As it happens, those women who breastfed the longest also were more likely to be married to college-educated men, to be older than other mothers and to be white.

The authors argue that the public-health community's insistence that "breast is best" unduly pressures women — but not men — to alter their approaches to employment to accommodate their children's feeding needs. (The American Academy of Pediatrics, for instance, urges exclusive breastfeeding for the first six months of life and continued breastfeeding even after solid foods are introduced.)

They note that while breastfeeding was once imperative to the survival of the species, it has always placed women in a subordinate position to men as it limited their mobility and thus lowered their ability to hone other skills that might have helped them advance in society.

To remedy all this, the authors suggest that federal legislation should be enacted that would make it easier for women to maintain their positions in the workforce while breastfeeding their babies.

That would require more paid leave time and/or requiring workplaces to have child-care facilities that would give women easy access to their hungry babies during the work day. That would go far beyond current laws that simply make it easier for women to express milk (to be stored and fed later to their babies) during the work day, “so that breastfeeding does not have to beome a disembodied practice involving machines and bottles,” the study says.

The authors conclude, "For the time being, despite catchy slogans, it remains to be seen if breast is always best."