Terry, R. L. 1970. Primate grooming as a tension reduction mechanism. Journal of Psychology 76, 129-136.

In this paper grooming was considered as a response to tension generated by an event which threatens the integration of a specific member of the group. In response to this tension, grooming is engaged in with the result that the individual member's integration is restored, tension is reduced, and the solidarity of the group is increased. An immediate result of grooming is a reduction in tension and a restoration of integration; an ultimate result of several acts of grooming involving many group members is an increase in total group solidarity.