Skip to main content

Smaller Families Produce Bigger Children

New research finds kids with more siblings are less likely to be obese.
  • Author:
  • Updated:
(Photo: Cameron Spencer/Getty Images)

(Photo: Cameron Spencer/Getty Images)

Once upon a time, American families were larger, and American children were smaller. Coincidence? Perhaps not.

When it comes to childhood corpulence, “family size matters,” writes economist Ashlesha Datar of the University of Southern California. In a new study, she presents evidence that “having more siblings is associated with significantly lower body-mass index, and lower likelihood of obesity.”

Writing in the journal Social Science and Medicine, Datar notes that, between the late 1960s and the late 2000s, the number of children in an average American family decreased from 2.44 to 1.86. During that same period, the rate of childhood obesity ballooned from 5 percent to 17 percent, according to the Centers for Disease Control and Prevention.

To determine if the two trends are related, she turned to the Early Childhood Longitudinal Study. It featured a nationally representative cohort of American youngsters who were first surveyed as kindergartners, during the 1998–99 school year.

Data on the children — including height and weight, family size, and the amount of vigorous physical activity they typically engage in — was collected in the first, third, fifth, and eighth grades. Parents provided information on their dietary habits beginning in fifth grade. They also reported how often the family ate meals together, and how frequently the child engaged in sedentary activities such as watching television.

Crunching the numbers, Datar discovered that “every additional sibling is associated with a 2.6 percentage point decline in the likelihood of obesity in early adolescence.”

“A back-of-the-envelope calculation based on these estimates suggests the decline in average family size in U.S. households between the late 1960s and 2008 may account for up to 13 percent of the increase in childhood obesity during the same period,” she writes.

Additional analysis revealed some likely explanations for this finding. For one thing, the study found children in bigger families watched less television, on average — perhaps because there was more competition for the set.

In addition, “Children with siblings are more likely to eat meals together as a family, and are significantly less likely to eat away from home,” Datar writes. “A growing literature suggests that family mealtimes are associated with better nutritional intake.”

Also, she writes, “children with siblings are less likely to spend time without an adult,” in part because their mothers are less likely to work outside the house. Unsupervised kids, of course, have more opportunities to pig out on unhealthy foods.

One implication of these findings, Datar notes, is that only children may be at particularly high risk for weight gain, and should be targeted for obesity-prevention programs. Meanwhile, if you’re a svelte adult, you might want to call that younger brother or sister you used to torture and thank them.

Better yet, offer to buy them a healthy lunch.