Many authors consider cooperation as a binding force holding human society together. More than other animals, human beings would be able to abandon immediate benefits to others, in the hope of future reciprocity. This hypothesis is, however, refuted by simulation: less cooperative agents always prevail eventually. I consider an alternative hypothesis, inspired by the study and modelling of natural language. Seemingly cooperative acts would rather be signals. Showing prosocial attitudes is a way to advertise qualities that are crucial, in our species, to attract friends and thus build up one’s social network.