The popular press has triumphantly announced that the cause of the obesity epidemic is “junk food.” After a moment’s reflection, however, it seems likely that the true causal structure of the obesity epidemic can be neither single-equation nor univariate. Therefore, while the hypothesis that “junk food” is the cause of obesity has little a priori plausibility, these articles in the popular press present a testable hypothesis that, in spite of some measurement impossibilities, is tested here. While one can always argue about p values etc., it is safe to say that the results show no evidence to indicate support for a causal link. The second section of the paper explains this result and suggests a rudimentary structural model of obesity that begins to address the issues of specification error, simultaneity, etc., that plague much of the obesity research. This model shows that because of the dynamic nature of weight status, there is no necessary reason to expect to find a statistical relation between a person’s observed weight and the amount he or she is currently eating or exercising. Therefore, studies which regress weight, obesity, or the probability of obesity on eating and exercise patterns have serious specification error. Further development of structural econometric models of obesity may lead to consistent estimates of the partial effects of exogenous variables on obesity levels. We conclude with a discussion of the implications for policy development and industry.
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