Social relationships in nonhuman primates: Potential models of pervasive disorders

The diagnosis of autism in humans is currently based on behavioral criteria, including impairments in social behavior and communicative skills, and the presence of stereotypic behavior. In this chapter, we draw on behavioral parallels between humans and nonhuman primates, and propose the use of chimpanzees as a noninvasive model for understanding the development of behavioral traits associated with autism. We first examined the literature on behavioral deficits in apes. Then, using the Hominoid Personality Questionnaire (HPQ), we developed an Autism Scale for chimpanzees. This was accomplished by first asking six raters with experience of autism to rate each trait from the HPQ (a total of 54 items) on how strongly it was associated with autism. Each trait was weighted based on its mean rating, and the sum of scores on these traits, for 176 chimpanzees, was then used to produce an overall Autism Score for each chimpanzee. On this scale, we found that males scored significantly higher than females (b = 0.33, df = 1, 174, p < 0.05). We also examined mean trait differences for individuals in the upper and lower quartiles of the Autism Scale. We found low concurrence between the trait loadings and our predictions—which built on previous findings of stereotypic behavior induced by the environment; this suggests that the traits exhibited may be associated with neuro-developmental deficits, and not simply a result of environmental trauma. We discuss the application of the Autism Scale to understanding neurodevelopmental disorders in nonhuman primates at the behavioral level.