Are meals at full-service and fast-food restaurants "normal" or "inferior"?

Whereas some studies show statistically significant linear associations between consumption at full-service restaurants and consumer incomes, studies of fast-food restaurants fail to find statistically significant linear associations. In this study, nationally representative data were drawn from the 1994-1996 Continuing Survey of Food Intakes by Individuals and the accompanying Diet and Health Knowledge Survey. The sample contained 4972 individuals who were 21 years of age or older. Dependent variables measured number of restaurant visits on 2 nonconsecutive days. Income was total annual household income. Control variables reflected sociodemographic, economic, lifestyle, and attitudinal variables. To capture possible curvilinear relationships between income and food consumption, we analyzed frequency distributions, regressions on full samples including income squared, and we divided samples into above-and below-average income groups. Zero-inflated negative binomial regressions accounted for excessive zeros within dependent variables. We found that fast-food restaurants were "normal goods" for below-average income, but "inferior goods" for above-average income, whereas full-service restaurants were "normal" for virtually all income levels. Earlier studies were flawed because they only tested for linear associations. Our results have implications for the poverty and obesity debate.