Base rate effects on the IAT.

We investigated the influence of stimulus base rates on the Implicit Association Test (IAT). Using an East/West-German attitude-IAT, we demonstrated that both overall response speed and differential response speed underlying IAT effects depend on the relative frequencies of the stimulus categories. First, when those stimuli that are more common in reality also occurred more frequently in the stimulus list, response speed generally increased. Second, IAT effects increased when congruent blocks profited from the compatibility of frequency-based response biases (i.e., frequent target stimuli and frequent valence stimuli mapped onto the same response key), whereas IAT effects decreased when incongruent trial blocks profited from response compatibility. These findings demonstrate that the stimulus context moderates the magnitude of the IAT effect. Simultaneously, they highlight the need to explore the extent to which implicit measures reflect properties of the task or the environment rather than attributes of test-takers.