Seyfarth, Robert M., and Dorothy L. Cheney. 2012. “The evolutionary origins of friendship.” Annual Review of Psychology 63:153–77.
119
Silk, Joan B. 2003. “Cooperation without counting: The puzzle of friendship.” In P. Hammerstein, ed. Genetic and Cultural Evolution of Cooperation. Dahlem Workshop Reports. Cambridge, MA: MIT Press, pp. 37–54.
