Date of Award


Document Type


Degree Name

Doctor of Philosophy (PhD)

First Advisor

Sarah Laszlo


Most theoretical accounts of psychological similarity maintain that similarity judgments are based on shared features (and shared relations among those features, e.g., the commonalities between spatula and ladle). Accounts rarely include associations between targets of comparison (e.g., the association between egg and spatula) as a contributor to similarity judgments. This position is taken despite the fact that people will often choose associates over things with shared features and relations in similarity judgment tasks. So-called dual-process models - where thematic integration and feature (and relation) based comparison are component processes of perceived human similarity - have been proposed to handle this apparent failure to account for human similarity judgments. The present experiments were designed to further explore the thematic association effect on similarity with the goal to test the hypothesis that confusion about similarity and association (rather than a radical theoretical redirection, e.g., the dual-process model) is the cause of the reported thematic association influence on similarity judgments. Experiment 1 introduces a novel task for collecting similarity judgments of real world concepts - the Anti-Thematic Intrusion (ATI) task - and tests alternative task instructions as a possible driver of thematic intrusion on similarity. Experiment 2 examines the effect of the isolated components of the ATI task as compared to the classic two-alternative, forced choice similarity judgment task to determine what changes from the classic task are most influential for reducing thematic intrusion. Experiment 3 was conducted to confirm that the concept sets used in Experiments 1 and 2 did not produce biased responding. Having explored task, instruction and concept-based effects, Experiment 4 investigated behavioral and electrophysiological differences among individuals to attempt to clarify how differences between individuals correspond to similarity judgment behavior. The results were not expected in that the strength of the thematic association effect on similarity was weaker than predicted; Experiments 1, 2, and 4 show that overall association-based preferences were only present in situations strongly biased toward producing that response type. It was also found that taxonomic pair matching reliably increased across the time course of the task. Changes in the properties of the task and the instructions attenuate the effect, suggesting that the intrusion of thematic relationships on similarity judgments is not an unyielding feature of the similarity judgment process (as dual-process accounts propose) but instead (at least in part) due to interpretation of the task goal and confusion about similarity and association-based relatedness. Finally, this confusion is identifiable by less differentiation in the EEG signal elicited by these competing semantic relations, where people who produce more similarity-based responding also produce more distinctive ERP waveforms for taxonomic and thematic category members.