Over the past 35 years, the percentage of U.S. mothers who hold down a job while raising kids has soared, from less than 50 percent to more than 70 percent. The childhood obesity rate -- which is now close to 17 percent -- has more than tripled during the same time frame.
These overlapping trends may not be a coincidence. The longer a mother is employed, the more likely her children are to be overweight or obese, a new study of grade-schoolers published in the journal Child Development suggests.
For each additional five-month period his or her mother is employed, a child of average height can be expected to gain one extra pound over and above normal growth, the study estimates. In addition, sixth graders with working mothers were found to be six times more likely than those with stay-at-home moms to be overweight.
Mothers who have jobs don't directly cause weight problems in their children, but busy families may accelerate weight gain by relying too much on fast food and frozen dinners rather than preparing fresh, healthy meals, the researchers say.
"It is not the mother's employment, but the environment," says the lead author of the study, Taryn Morrissey, Ph.D., an assistant professor of public administration and policy at American University in Washington, D.C. "There needs to be improved access to healthy foods."
Focusing on kids in grades 3, 5, and 6, Morrissey and her colleagues analyzed data from a government-funded study that began in 1991 and followed more than 1,000 children nationwide from infancy through age 15. As part of that study, researchers interviewed families about their everyday lives and measured the children's body mass index (BMI), a simple ratio of height to weight that estimates total body fat.
Roughly three-quarters of the mothers in the study were employed, and they were working an average of 27 hours per week when their children were in third grade. More than 80 percent of mothers were married or living with a partner, and 90 percent of those husbands or partners worked full time.
The study didn't collect data on family eating habits, so Morrissey and her colleagues weren't able to confirm their hunch that diet is largely responsible for their findings. But they were able to effectively rule out several alternative explanations.
None of the factors the researchers looked at -- including average time spent in front of the TV, daily physical activity, and parental supervision -- helped explain the link between a mother's employment and her child's BMI. Nor did it seem to matter whether the mother worked a standard nine-to-five schedule (as opposed to night shifts, for instance).
Michele Mietus-Snyder, M.D., co-director of the Obesity Institute at Children's National Medical Center in Washington, D.C., emphasizes that the study only shows association, not causation. Still, she adds, the findings are "provocative" and the food hypothesis is compelling.
"Foods that are the healthiest are often the most expensive and the least accessible, and the healthiest foods are often the most time-consuming to prepare," says Dr. Mietus-Snyder, who was not involved in the new study. "There are nutritional guidelines -- and then there are all the barriers that the world puts up that make them hard to follow."
The study had some key shortcomings. Although they controlled for family income, the number of children in a home, and other household characteristics, Morrissey and her colleagues weren't able to account for many other potentially important factors, including disruptions to family life (such as divorce) or the father's work history.
"We didn't have information on both parents' work schedules, so we couldn't look at whether they overlapped or complemented each other," Morrissey says.
The data also provides a narrow view of American households. Too few low-income, minority, and single-parent households were included for the study to be nationally representative. (78 percent of the children were white and only one-quarter of the families were low income.)
Despite its limitations, the study adds to a growing body of scientific literature linking childhood weight gain to the amount of time a mother works. More research will be needed to fully explain this apparently strong yet enigmatic relationship, the researchers say.
"Over the past few decades, there's been a tripling of the obesity rate," Morrissey says. "The total time a mother works is one factor at play, but there's no single smoking gun."