Isolating the Effect of Major Depression on Obesity: Role of Selection Bias

There is suggestive evidence that rates of major depression have risen markedly in the U.S. concurrent with the rise in obesity. The economic burden of depression, about $100 billion annually, is under-estimated if depression has a positive causal impact on obesity. If depression plays a causal role in increasing the prevalence of obesity, then policy interventions aimed at promoting mental health may also have the indirect benefits of promoting a healthy bodyweight. However, virtually the entire existing literature on the connection between the two conditions has examined merely whether they are significantly correlated, sometimes holding constant a limited set of demographic factors. This study utilizes multiple large-scale nationally-representative datasets to assess whether, and the extent to which, the positive association reflects a causal link from major depression to higher BMI and obesity. While contemporaneous effects are considered, the study primarily focuses on the effects of past and lifetime depression to bypass reverse causality and further assess the role of non-random selection on unobservable factors. There are expectedly no significant or substantial effects of current depression on BMI or overweight/obesity, given that BMI is a stock measure that changes relatively slowly over time. Results are also not supportive of a causal interpretation among males. However, among females, estimates indicate that past or lifetime diagnosis of major depression raises the probability of being overweight or obese by about seven percentage points. Results also suggest that this effect appears to plausibly operate through shifts in food consumption and physical activity. We estimate that this higher risk of overweight and obesity among females could potentially add about 10% (or $9.7 billion) to the estimated economic burden of depression.