Within-family variation in obesity

We use data from the Children of the National Longitudinal Survey of Youth 1979 to document the degree to which childhood obesity varies among siblings. We find considerable differences in body weight between siblings with over half of the siblings differing by more than 20 age-specific percentiles in terms of the body mass index. Even among identical twins, there is an average BMI difference of 12 percentiles. This variation is important for the use of econometric approaches that involve sibling comparisons.

► We document the degree to which BMI varies between siblings. ► Half of sibling pairs differ by more than 20 percentiles in BMI. ► Even among identical twins the average difference in BMI is 11 percentiles. ► Among fraternal twins, same-gender siblings are more similar in BMI.