The experience of pain appears to be associated, from early infancy and across pain stimuli, with a consistent facial expression in humans. A social function is proposed for this: the communication of pain and the need for help to observers, to whom information about danger is of value, and who may provide help within a kin or cooperative relationship. Some commentators have asserted that the evidence is insufficient to account for the consistency of the face, as judged by technical means or in the perceptions of observers, or that facial expression is epiphenomenal to a gross behavioural defensive response to pain. The major criticism is that it is unnecessary to invoke evolutionary mechanisms beyond the emergence of an unconditioned facial response to pain in neonates, subsequently shaped and maintained by instrumental and social reinforcement throughout life. These criticisms are addressed, acknowledging the need for further data to address some, and elaborating the areas in which evolutionary and operant mechanisms would predict different behavioural interactions, rather than acting synergistically. Several supportive commentaries propose extending evolutionarily-based hypotheses to sex differences, the complexities of others' responses within the social relationship, and the role of empathy. A number of commentators provided valuable suggestions for experimental paradigms or methodological issues. Overall, addressing these issues indicates the need for further conceptual development and for collection of data specifically in relation to these hypotheses.