Inconsistencies among independent sources of information about psychological constructs are widely documented, but not adequately explained. Measurement error as the primary explanation, though historically popular, is no longer tenable. Yet, even as assessors acknowledge that various measures of the same construct are not necessarily interchangeable, there are no agreed upon frameworks to discern the unique contribution of each measure in multiinformant and multimethod assessment protocols. In this study, we focus on the relevance of the target trait in its measured contexts and on the functional equivalence of the trait across its measures (similar self-regulatory requirements for trait expression) as driving relations between scores. These 2 considerations enabled prediction of informant differences in mean ratings and of patterns of divergences and convergences between parent and teacher ratings of kindergarteners’ social competence (SC) and executive functioning (EF) and between informant-based and performance-based measures of executive functioning (N = 73).