In four experiments, participants made speeded same-different responses to pairs of face photographs showing the same woman or different women with the same expression or different expressions. Compared with responses to positive pairs, negative pairs were matched more slowly on identity than on expression. A secondary finding showed that face expressions (same, different) influenced identity responses, and identities influenced expression responses, equally for positive and negative pairs. The independence of this irrelevant-dimension effect from the contrast effect supports the conclusion required by the main finding that negation slows perceptual encoding of surface-based information used for identification more than it does encoding of edge-based information used for expression recognition.