Signs of generalized anxiety and compulsive disorders in chimpanzees

Fear and anxiety have been studied extensively in humans and other animals. However, far less attention has been focused on the clinical and ethical implications of nonhuman animals’ susceptibility to psychological disorders. Behavioral signs of psychopathology in nonhuman animals, including our closest phylogenetic relatives, are rarely described as clinical syndromes. In this study, we drew on approaches described in child psychiatry, veterinary medicine, and primatology, to identify behavioral clusters in chimpanzees comparable with human anxiety disorders, such as generalized anxiety disorder and obsessive–compulsive disorder.In phase 1 of this study, we accessed published case reports of 20 chimpanzees subjected to maternal separation, social isolation, experimentation, or similar experiences. We tested the inter-rater reliability of the Diagnostic and Statistical Manual of Mental Disorders (fourth edition; DSM-IV) criteria for generalized anxiety disorder and obsessive–compulsive disorder when applied to chimpanzees in these case studies. Additionally, based on the DSM-IV, veterinary approaches, and ethograms, we defined behaviorally anchored alternative criteria, which proved more reliable than the DSM-IV criteria in phase 1. In phase 2, the new behaviorally anchored criteria were applied to chimpanzees living in wild sites in Africa (n = 196) and those with previous histories of experimentation, orphanage, illegal seizure, or violent human conflict (n = 168) living in sanctuaries. In phase 2, 18% of chimpanzees living in sanctuaries met the set of behaviorally anchored criteria for generalized anxiety disorder, compared with 0.5% of those in the wild (P = 0.036), and 19% of chimpanzees in sanctuaries met the set of behaviorally anchored criteria for obsessive–compulsive disorder, compared with 0% of those in the wild (P = 0.071).Chimpanzees display behavioral clusters similar to anxiety disorders described in humans, underscoring the importance of ethical considerations regarding their use in experimentation and other captive settings.