: Facial expression is one of the core issues in the ethological approach to the study of human behaviour. This study discusses sex-specific aspects of the recognition of the facial expression of fear using results from our previously published experimental study. We conducted an experiment in which 201 participants judged seven different facial expressions: anger, contempt, disgust, fear, happiness, sadness and surprise (Trnka et al. 2007). Participants were able to recognize the facial expression of fear significantly better on a male face than on a female face. Females also recognized fear generally better than males. The present study provides a new interpretation of this sex difference in the recognition of fear. We interpret these results within the paradigm of human ethology, taking into account the adaptive function of the facial expression of fear. We argue that better detection of fear might be crucial for females under a situation of serious danger in groups of early hominids. The crucial role of females in nurturing and protecting offspring was fundamental for the reproductive potential of the group. A clear decoding of this alarm signal might thus have enabled the timely preparation of females for escape or defence to protect their health for successful reproduction. Further, it is likely that males played the role of guardians of social groups and that they were responsible for effective warnings of the group under situations of serious danger. This may explain why the facial expression of fear is better recognizable on the male face than on the female face.