Lutz, C. K., Novak, M. 2005. Environmental enrichment for nonhuman primates: Theory and application. ILAR Journal 46(2), 178-191.

Only social contact satisfies the goal of promoting a wide variety of species-typical activities while at the same time reducing or preventing the development of abnormal behavior... A number of toys should be provided initially and rotated on a regular basis to maintain interest... At present, the most effective form of enrichment for captive primates is social housing.