Categorization is a fundamental property of the human brain. We used an imagemorphing procedure to investigate the categorical perception of facial gender information. Three experiments, an identification
and two matching tasks, were reported. First, we showed that, even when facial image information changes linearly across unfamiliar male and female faces, gender is perceived categorically. This holds only
when faces are presented in an upright orientation. Second, subjects discriminated more easily two unknown morphed faces presenting a gender change as compared to two unknown morphed faces belonging to
the same gender, even when the physical distance between the pairs was identical. We discuss the results in terms of how representations of faces are encoded and stored in long-term memory.