Categorical Perception of Male and Female Faces and the Single-Route Hypothesis

The cognitive process of categorizing perceptually similar stimuli into qualitatively different categories is fundamental to any systematic acting upon the world, as it helps to reduce the immense number of entities to more manageable fragments and spares us from learning anew each time we encounter an unknown individual. Categories are evident in all sensory modalities and range from relatively simple (e. g., color perception) to the most abstract human concepts, as for example faces. Categorical perception (CP) has been shown for face identity (e. g., Beale Keil 1995), ethnicity (Levin Beale, 2000), and facial expression (Calder et al., 1996). Astonishingly, for sex, a natural facial characteristic consisting of only two biologically relevant categories, con icting results have been reported so far. CP for sex has been shown (Campanella et al., 2001) when sex information was varied linearly (by morphing) between male and female face identities, thus intermixing identity and sex information. When sex continua were created based on single face identities (Bultho and Newell, 2004), no CP for sex was found in nave participants. So the question remained open whether or not there is CP for the perception of sex as a facial dimension or if processing of the sex of a face is directly linked to processing of the face's identity, as proposed by the "`single-route hypothesis"' (e. g., by Rossion, 2002, Ganel Goshen Gottstein, 2002; Bultho Newell, 2004). To overcome one potential constraint of earlier studies, i. e., 'asymmetric' sex morph continua, we performed extensive ratings of faces and sex morphs from our face database, to create 'controlled' male and female faces with similar perceived degrees of 'maleness' and 'femaleness'. We then examined CP of sex for these faces with classical discrimination and classication experiments. Critically, we manipulated the degree of familiarization of the faces prior to testing, as follows. Observers were either nave, or familiarized with the average male and female face of all faces, or the endpoint identities of the morph continua, or with other male and female faces with the same perceived degree of maleness and femaleness than the test faces. Our results conrm the lack of naturally occurring CP for sex and provide more evidence for the linked processing of sex and identity, as participants showed clear CP only after familiarization with the test face identities