Standard utility functions in economics are assumed to be quasiconcave to ensure convex indifference sets and diminishing marginal rates of substitution, which is a foundational definition in microeconomic theory.
The claim states a foundational definition and standard mathematical property of utility functions taught in every microeconomics text to ensure well-behaved consumer optimization (convexity of preferences). None of the retrieved applied papers directly dispute or test this axiomatic foundational assumption, making the claim a standard definitional property of economic theory.