Journal of Personality and Social Psychology 1979, Vol. 37, No. 11, 2025-2043 Idiosyncratic Weighting of Trait Information in Impression Formation Thomas M. Ostrom Ohio State University Deborah Davis University of Nevada, Reno An analysis is offered of the role of unequal weighting in the averaging model of information integration. A distinction is made between unequal weighting at the normative level (which has been referred to as "differential weighting") and unequal weighting at the level of the individual subject (which we refer to as "idiosyncratic weighting"). Two studies are reported that examine the prevalence of idiosyncratic weighting in the trait-judgment impression forma- tion task. Whereas most past research on the question of unequal weighting in this task involved averaging responses across both subjects and stimulus replications, the present studies were analyzed at the level of an individual subject's repeated responses to separate stimulus replications. Clear evidence of idiosyncratic weighting was obtained from about 50% of the 120 subjects; only 20% of the subjects indicated absolutely no tendency toward unequal weighting. There was no evidence that idiosyncratic weighting was restricted to just a subset of stimuli, since all of the 20 stimulus replications showed idiosyncratic weighting effects. In contrast to previous findings, negative traits did not always receive more weight than positive traits. In more than 20% of the instances of unequal weighting, the more positive trait was accorded a higher weight. Information integration theory (Anderson, 1974) offers an approach for understanding how people combine stimulus information when making judgments and decisions. The theory seeks to determine the nature of the integration rule (e.g., adding, averaging, mul- tiplying) employed by people in various re- sponse domains. In addition, it provides a way to determine whether all stimulus items in the domain contribute equally to the overall judg- ment or whether they carry different weights. Integration theory provides no a priori basis for predicting which integration rule or weighting assumption is correct for any par- ticular response domain. It does, however, provide an array of conceptual alternatives This research was supported by National Science Foundation Grant GS-38604. The authors are grate- ful to Sarah Boysen for her assistance with collection of the data. Requests for reprints should be sent to Thomas M. Ostrom, 404C W. 17th Avenue, Columbus, Ohio 43210. along with a diagnostic methodology (func- tional measurement). With a comprehensive series of studies, it is possible to determine which conceptual alternatives best describe a response domain. Hence, one of the great strengths of integration theory is its ability to uncover different integration rules for differ- ent response domains. In order to apply integration theory to a response domain, it is necessary to specify two features of that domain, the population of information or stimulus items and the na- ture of the subjective judgment continuum. It is necessary to specify these two features because a shift in either may affect the inte- gration rule or parameter values. For example, one integration rule may apply when traits are combined with traits (e.g., averaging), and a different rule might apply when traits are combined with adverbs (e.g., multiplication). Parameter values may also vary as the nature of the judgment continuum shifts. Whereas friendly may have a higher scale value than Copyright 1979 by the American Psychological Association, Inc. 0022-3514/79/3711-2025$00.75 2025