To estimate the degree of correlation in BMI between generations, I use data from the National Longitudinal Survey of Youth 1979 (NLSY79) and the Children and Young Adults of the NLSY79 (YA NLSY79). The NLSY79 began in 1979 with interviews of a nationally-representative sample of people born between 1957 and 1964 (so that all respondents were between ages 14 and 22 at the beginning of the survey). This sample was interviewed annually until 1986 and biennially since 1988. Height and weight data for women in the NLSY79 were first collected in the 1981 survey. Beginning in 1986, data on the children of the women in the NLSY79 sample were collected. In 1994, children aged 14 or older of women in the NLSY79 were surveyed in the YA NLSY79.
The structure of these surveys provides the highly desirable feature of being able to measure BMI at similar stages of life for mothers and their children. Previous studies of intergenerational mobility in economics have identified two critical shortcomings of data used to measure such relationships: potential measurement error from using only a single observation of the outcome of interest (assuming there exists temporal variance in the measure of mobility studied) and observing the outcome at different stages of the lifecycle (such as in cross sectional observations of earnings for two generations at a single point in time).14
For example, an observation of income for a father at age 50 might be paired with an observation on his son’s earnings in the same year at age 25. While measurements of the age of each generation are often incorporated to attempt to adjust for such lifecycle differences, it is evident that knowledge of the son’s earnings when he is age 50 would provide much more accurate estimates of the intergenerational correlation of income. Using the NLSY79 and YA NLSY79 data, I calculate the weight status of mothers and their children when both generations are between the ages of 16 and 24. This generates an estimate of the intergenerational persistence of BMI across generations at a crucial stage for determining the likelihood of developing weight problems later on in life.
A second critique of studies of intergenerational mobility focuses on whether the observed data are representative of the true level of the variable of interest. The transitory nature of weight and height during late adolescence could result in significant deviations from a more permanent measurement when data are only collected for a given year. Obviously, weight is a variable that is likely to fluctuate over time and multiple observations are desirable for generating an accurate measure of weight status. The longitudinal structure of the NLSY data allows for multiple observations of BMI for both generations to generate an estimate of its intergenerational dependence.15
Since the NLSY79 cohort contains individuals who were first interviewed at age 14 to 22 in 1979 and height and weight data are only available beginning with the 1981 survey, the age range for averaging BMI observations extends up to age 24. Although the period from age 16 to 24 is likely to feature significant changes in BMI (potentially as a result of individuals making food choices without supervision from parents for the first time), averaging BMI values over this range should accurately reflect weight status during this phase of development and reduce potential bias from only a single observation of BMI.16
For the determination of whether individuals are obese, overweight or underweight, the cutoffs for adults of BMI over 30 indicating obesity, BMI above 25 and below 30 as indicating overweight and BMI below 18.5 indicating underweight are used.17
Among the 12,686 people initially surveyed in the NLSY79, 6,283 were female. Of these women, approximately two-thirds have had children who have been included in at least one of the YA surveys. The selection criterion for inclusion in this analysis requires that the child have reached at least age 16 by the 2004 YA NLSY79 survey. This leaves 4,748 children born to 2,560 women in the NLSY79 sample for analysis. Given this criterion, the included sample contains an oversampling of younger mothers and the demographics displayed in make apparent that the racial composition of the sample is not nationally representative. Thus, results for several of the specifications will be presented separately by racial and ethnic categories.
Summary Statistics for NLSY Sample
Due to the structuring of the data for this analysis, there is no evident method to incorporate the available sample weights from the NLSY79 and YA NLSY79 surveys since height and weight measurements are collected for both generations in multiple survey years. Sample weights are provided for each NLSY survey year to generate a representative cross-section for a given survey year. However, since this study combines observations on mothers interviewed in the beginning of the NLSY79 panel (who were assigned sample weights for the first survey in 1979) and averages BMI over multiple survey years (each of which has unique sampling weights) with observations on their children who are also assigned sample weights in each survey year, this structure of data from multiple NLSY surveys does not lend itself to using the cross-section NLSY sample weights. The results reported in this paper are unweighted, but an appendix includes results generated using custom longitudinal sampling weights available for the YA NLSY data that provide representative weights for multiple survey years of the survey. The limitation of these custom weights for this application is that height and weight observations for the offspring are not all taken from the same set of survey years. Thus, the weighted results in the appendix use the custom sample weights for the YA NLSY survey from 1986 to 2004. Results ( and ) are not sensitive to the oversampling of black and Hispanic individuals in the unweighted data, but most results are presented separately by ethnicity given this potential concern.
Weighted Intergenerational BMI Elasticities and Correlations by Ethnicity and Gender, 1981 – 2004
Weighted Distributions of Intergenerational Weight Status Transitions
A limitation of the health data available in the NLSY79 is the lack of measures of actual body fatness (adiposity) which limits the analysis to using height and weight in the calculation of BMI which may be a poor proxy for adiposity. While some studies include measures of skinfold thickness and waist circumference, only self-reported height and weight are available in the NLSY data.18