Reference-group effects (discovered in cross-cultural settings) occur when responses to self-report items are based not on respondents’ absolute level of a construct but rather on their level relative to a salient comparison group. In this article, we examine the impact of reference-group effects on the assessment of self-reported personality and attitudes. Two studies illustrate that a reference-group effect can be induced by small changes to instruction sets, changes that mirror the instruction sets of commonly used measures of personality. Scales that specified different reference groups showed substantial reductions in criterion-related validities for academic performance, self-reported counterproductive behaviors, and self-reported health outcomes relative to reference-group-free versions of those scales.