Changes in dietary and physical activity patterns are driven by changes in the environment and by the incentives that people face. Many factors have been suggested as causes of the "obesity epidemic" - automobiles, television, fast food, computer use, vending machines, suburban housing developments, portion sizes, and countless others. Putting a multitude of isolated data points into a coherent picture is a challenging but necessary task to assess whether proposed solutions are promising or likely to lead us down a blind alley. Conventional wisdom is an unreliable guide and, as I will show in this presentation, some widely held beliefs are simply false. While understanding the economic forces behind the changes that actually occurred is not difficult, there is a big gulf in how health professionals and economists perceive efforts to change trends. Economists see only a valid rationale for interventions if there are market failures, primarily externalities, under-provision of public goods and services, or information problems. In contrast, a clinical or public health view focuses on health: intervene if health could be improved, regardless of other consequences. Expert opinion evaluates decisions based on how they affect health; consumer sovereignty or individual preferences plays no major role. Unfortunately, ignoring the economic component is inconsistent with much of the U.S. institutional framework and reduces the political feasibility or sustainability of proposals. For example, federal agencies must demonstrate evidence that proposed regulations address market failures before implementing them. Public health and economics are not necessarily at loggerheads, however, and I argue that an economic research agenda that measures market failures in the physical activity area will be a major advance towards developing successful interventions.
