Much variability in social behavior is intra-individual; people vary their actions strategically across contexts. Yet most personality assessment methods are inter-individual; they chart between-person differences in average behavioral tendencies. The present research advanced novel idiographic assessment methods to predict intra-individual variability in a behavioral domain of long-standing interest to personality psychologists: humor. Individuals participated in three assessment sessions. Personal beliefs about reasons for using humor were assessed either idiographically or with respect to a nomothetic system of humor-related reasons for action. The subjective relevance of nomothetically- and idiographically-identified reasons to everyday social situations was assessed. Subsequently, participants reported their perceived likelihood of using humor in specific social contexts. As predicted, likelihood of humor use varied substantially as a function of the relevance of situations to idiographically-identified reasons, but did not vary as a function of the relevance of situations to reasons identified nomothetically. Implications for personality and assessment are discussed.