Is It Rational To Eat Dessert On A Diet? The Economic Perspective

Economists know that there is an optimal level of a bad thing, and that the optimal level is not necessarily zero. This phenomenon is most often mentioned in the context of negative production externalities, for example, where there is a rational level of pollution that is greater than zero. Last week, however, we examined this economic understanding as it applies to voting and political information, an area in which there are not obvious negative production externalities. We discovered that individuals are rationally ignorant about many political issues. The rational level of ignorance for different subjects is found by graphing their marginal cost and benefit curves and calculating where the curves intersect, or where the costs of ignorance equal the benefits.

The economic inference of the rationality of a non-zero level of a “bad thing” applies in a myriad of everyday situations. One such arena is dieting. Eating sweets on a diet is generally considered to be irrational -- why behave in a way that works against your goal of losing weight? According to our economic principle, however, this behavior, of completely cutting out “bad foods” is likely a mistake.

Looking at the rough graph above, we can see that the marginal benefit of eating a small amount of dessert per week while on a diet is relatively large and the costs, or overall harm to the weight-loss process, are relatively low. Therefore, there is an optimal amount of dessert to eat per week while on a diet. While dietitians would likely have to calculate the exact number of calories where the costs and benefits of dessert meet, the economic theory suggests that it is rational to eat some amount of dessert while on a diet. Luckily, diet science agrees with the economic perspective, so go ahead, enjoy that piece of cake!