Economic theories of choice suggest that more options are better, and people should prefer choosing from among more options to find their most valued alternative. But in an intriguing counter-example, Iyengar and Lepper (2000) observed that while people were attracted to more options while shopping, the larger set size increased the likelihood that they would leave the store empty-handed. Surprisingly, this too-much-choice effect has not been consistently observed in situations where it would be expected (e.g., Chernev, 2003; Scheibehenne, 2008). This paper describes boundary conditions for the too-much-choice effect that were determined by evaluating three different psychological explanations within a unified theoretical framework, decision field theory (Busemeyer & Townsend, 1993). The effect of environmental structure on choice was also tested by varying the distribution of quality in the option sets between low variance (roughly uniform) and high variance (exponential distribution). Based on these simulations, two explanations were identified that differentially predicted the too-much-choice effect: avoiding choice when the most preferred option changes too often, or when time runs out. Moreover, the magnitude of the too-much-choice effect depended on the distribution of option quality. These mechanism environment structure combinations can help explain why the too-much-choice effect is observed some—but not all—of the time. © 2009 Wiley Periodicals, Inc.