a1 Liverpool Hope University and British Academy Centenary Research Project, Liverpool L16 9JD, United Kingdom. Lyonsm2@hope.ac.uk http://hopelive.hope.ac.uk/psychology/evolutiongroup
In this commentary, the idea of reciprocity potential indicators is tied in with ultimate accounts on sex differences in social sensitivity. It is proposed that, rather than crying, smiling is a more likely cooperative signal. The possibility of coevolution and polymorphism in perceptual and signalling systems are also discussed briefly, with a reference to Theory of Mind and Machiavellianism.