Categorical perception of sex occurs in familiar but not unfamiliar faces

We investigated whether male and female faces are discrete categories at the perceptual level and whether familiarization plays a role in the categorical perception of sex. We created artificial sex continua between male and female faces using a 3-D morphing algorithm and used classical categorization and discrimination tasks to investigate categorical perception of sex. In Experiments I and 2, 3-D morphs were computed between individual male and female faces. In Experiments 3 and 4, we used face continua in which only the sex of the facial features changed, while the identity characteristics of the facial features remained constant. When the faces were unfamiliar (Experiments I and 3), we failed to find evidence for categorical perception of sex. In Experiments 2 and 4, we familiarized participants with the individual face images by instructing participants to learn the names of the individuals in the endpoint face images (Experiment 2) or to classify face images along a continuum as male or female using a feedback procedure (Experiment 4). In both these experiments we found evidence for a categorical effect for sex after familiarization. Our findings suggest that despite the importance of face perception in our everyday world, sex information present in faces is not naturally perceived categorically. Categorical perception of sex was only found after training with the face stimulus set. Our findings have implications for functional models of face processing which suggest two independent processing routes, one for facial expression and one for identity: We propose that sex perception is closely linked with the processing of facial identity.