1. (1975). Play fighting. New York: Academic Press.
2. (1945). Vom ursprung and von den grenzen der freiheit, eine deutung des spieles bei tier und mensch. Basel: Schwabe.
3. & (1981). A critical reanalysis of the ontogeny and phylogeny of mammalian social and locomotor play: An ethological hornet's nest. In & (Eds.), Behavioral development: The Bielefeld interdisciplinary project Cambridge: Cambridge University Press, pp. 296-337.
4. Vigilance during play in squirrel monkeys