The Simon effect is mostly explained in terms of dual-route models, which imply unidirectional activation processes from stimulus features to response features. However, there is also evidence that these preactivated response features themselves prime corresponding stimulus features. From this perspective, the Simon effect should only occur whenever the response is unequivocally mapped to just 1 stimulus feature (one-to-one mapping). If, however, more than 1 stimulus feature is mapped to each spatial response (many-to-one mapping), priming activation should spread and thus reduce or eliminate the Simon effect. In a series of 4 Simon task experiments, the authors compared many-to-one with one-to-one mapping, holding stimulus set size constant. As was expected, the Simon effect was only present with one-to-one mapping but was eliminated with many-to-one mapping. The authors therefore suggest that the Simon effect also depends on priming from response features to stimulus features. Theoretical implications of these findings are discussed.