The current study investigated how item formats and their inherent affordances influence test-takers' cognition under uncertainty. Adult participants solved content-equivalent math items in multiple-selection multiple-choice and four alternative grid formats. The results indicated that participants' affirmative response tendency (i.e., judge the given information as True) was affected by the presence of a grid, type of grid options, and their visual layouts. The item formats further affected the test scores obtained from the alternatives keyed True and the alternatives keyed False, and their psychometric properties. The current results suggest that the affordances rendered by item design can lead to markedly different test-taker behaviors and can potentially influence test outcomes. They emphasize that a better understanding of the cognitive implications of item formats could potentially facilitate item design decisions for large-scale educational assessments.