The More the Heavier? Family Size and Childhood Obesity

A growing body of research suggests that home environment plays a critical role in influencing children’s diet and activity behaviors and body weight. Studies have also shown that children consume more “empty” calories at home rather than outside home. In this paper, we examine the contribution of one specific aspect of the home environment, namely family size, on children’s obesity and diet and activity behaviors.

Research surrounding family size has focused around the quality-quantity trade-off issue, which says that larger families might lead to “resource dilution” that would result in fewer resources (both time and money) for all children in the household, and consequently worse child outcomes. Indeed, a significant body of literature has examined the relationship between family size and individuals’ educational and labor market outcomes and has generally found a negative relationship. However, larger families might also mean lower likelihood of mothers working, which has been shown to have a protective effect on children’s BMI. Larger families might also imply economies of scale in home meal preparation. Given that foods prepared at home have been shown to have lower fat density and higher nutrient density relative to foods prepared away from home, larger families might imply better nutrition and lower obesity. Therefore, ultimately this is an open empirical question.

Very few studies have specifically examined how family size might influence children’s body weight. This limited literature suggests that last-born and only-kids are more likely to be obese. However, these studies have not been able to address the potential endogeneity of family size, which might bias their estimates. Moreover, very little is known about the mechanisms linking family size with childhood obesity, largely due to lack of information about mechanisms in the datasets used in these studies. We add to this limited literature in two important ways. First, we address the potential endogeneity of family size by instrumenting family size using the gender-composition of the oldest two children and whether there was a multiple birth in the family. These have been used in prior studies to instrument for family size when examining its effects on other child outcomes. Second, we conduct a detailed examination of the mechanisms that link family size with children’s BMI and obesity, including diet quality, physical activity levels, eating out vs. at home, maternal work, adult supervision, time spent in watching TV or playing video games.

We pool several waves of data (1999-2013) from the National Health and Nutrition Examination Study (NHANES) to examine the effect of family size, defined as number of siblings, on children’s BMI and obesity and the potential mechanisms at work. Our preliminary findings suggest that children in larger families are more likely to be obese. The robustness of these findings and their underlying mechanisms are explored in this paper.