Psychological essentialism is an important strand of theorising within the study of categorisation, according to which people believe, and act as if, category membership is determined by the possession of an essence. Many authors have developed this position by arguing that people believe and behave as if essentialism is true of social categories. This paper reports the results of a study investigating peoples categorization of sexual orientation. Contrary to essentialism, behavioural and not genetic factors were deemed more important in categorizing sexual orientation. These results suggest that the extent of essentialising about such social categories may be over-estimated, and that a similarity-based view may be more appropriate.