The Origin of Illusory Correlations.

If the distributions of two binary variables are skewed, people erroneously perceive a correlation even if the variables are actually uncorrelated. Specifically, people perceive a correlation between the variables' infrequent (vs. frequent) levels. While such systematic Illusory Correlations (ICs) can account for important phenomena, including erroneous stereotypes linking minority groups with infrequent attributes, the theoretical explanation is still a matter of debate. As proposed in the distinctiveness-based account, ICs arise due to a memory advantage for infrequent events. Alternatively, it has been proposed that ICs reflect ad hoc inferences from univariate base rates. In two experiments, we tested the accounts' predictions using a standard IC paradigm followed by (1) a contingency judgment task, asking participants to directly report contingencies, and (2) a trial-by-trial source assignment task of group membership and attribute. Bayesian-hierarchical model analysis revealed that ICs in contingency judgments were positively correlated to ad hoc inferences, but uncorrelated to exemplar memory in participants' source assignments. Moreover, inducing a focus on infrequent events improved memory for distinct events, but did not result in stronger ICs. Thus, our results support the explanation of ICs as ad hoc inferences from base rates, rather than a memory advantage for distinctive item combinations.