Obesity is generally blamed on both genetics and behavior. Economists, though, like to look at incentives, and a new National Bureau of Economic Research study suggests a strong link between obesity, which we tend to think of as a bad thing, and health insurance, which is usually thought of as something desirable.

The study asks the question, “Does health insurance make you fat?”, and it answers in the affirmative. It finds “weak evidence” that body mass index goes up as one’s health plan becomes more generous, and “stronger evidence” that people with insurance are fatter than those without. The study controls for income and other demographic variables. Here’s part of the conclusion:

Our estimates suggest that, by insulating people from the costs of obesity-related medical care expenditures, insurance coverage creates moral hazard in behaviors related to body weight. These effects are larger in public insurance programs where premiums are not risk adjusted and smaller in private insurance markets where obese might pay for incremental medical care costs in the form of lower wages.

The authors, led by Jay Bhattacharya of Stanford University, aren’t arguing that health insurance is a bad thing. Rather, they say, the lesson is that anyone designing a health-care plan must pay attention to incentives. That’s a message that needs to be heard in Washington right now.