Roberts, J. A. 1992. Behavioral observation in a macaque breeding colony: Assessment of enrichment strategies. Congress of the International Primatological Society, 167-168 (Abstract).

While the percentage of animals demonstrating one abnormal behavior in one month observation may reach 30%, the frequency of deleterious abnormal behavior [self-aggression, hair pulling, bizarre stereotypical behaviors] is low at 2.2% of the total population (N=1303). These animals are then selected for increased enrichment strategies.