Scientific reports of personality in nonhuman primates are now appearing with increasing frequency across a wide range of disciplines, including psychology, anthropology, endocrinology, and zoo management. To identify general patterns of research and summarize the major findings to date, we present a comprehensive review of the literature, allowing us to pinpoint the major gaps in knowledge and determine what research challenges lay ahead. An exhaustive search of five scientific databases identified 210 relevant research reports. These articles began to appear in the 1930s, but it was not until the 1980s that research on primate personality began to gather pace, with more than 100 articles published in the last decade. Our analyses of the literature indicate that some domains (e.g., sex, age, rearing conditions) are more evenly represented in the literature than are others (e.g., species, research location). Studies examining personality structure (e.g., with factor analysis) have identified personality dimensions that can be divided into 14 broad categories, with Sociability, Confidence/Aggression, and Fearfulness receiving the most research attention. Analyses of the findings pertaining to inter-rater agreement, internal consistency, test-retest reliability, generally support not only the reliability of primate personality ratings scales but also point to the need for more psychometric studies and greater consistency in how the analyses are reported. When measured at the level of broad dimensions, Extraversion and Dominance generally demonstrated the highest levels of inter-rater reliability, with weaker findings for the dimensions of Agreeableness, Emotionality, and Conscientiousness. Few studies provided data with regard to convergent and discriminant validity; Excitability and Dominance demonstrated the strongest validity coefficients when validated against relevant behavioral criterion measures. Overall, the validity data present a somewhat mixed picture, suggesting that high levels of validity are attainable, but by no means guaranteed. Discussion focuses on delineating major theoretical and empirical questions facing research and practice in primate personality.

One effective method for measuring personality in primates is to use personality trait ratings to distill the experience of people familiar with the individual animals. Previous rating instruments were created using either top-down or bottom-up approaches. Top-down approaches, which essentially adapt instruments originally designed for use with another species, can unfortunately lead to the inclusion of traits irrelevant to chimpanzees or fail to include all relevant aspects of chimpanzee personality. Conversely, because bottom-up approaches derive traits specifically for chimpanzees, their unique items may impede comparisons with findings in other studies and other species. To address the limitations of each approach, we developed a new personality rating scale using a combined top-down/bottom-up design. Seventeen raters rated 99 chimpanzees on the new 41-item scale, with all but one item being rated reliably. Principal components analysis, using both varimax and direct oblimin rotations, identified six broad factors. Strong evidence was found for five of the factors (Reactivity/Undependability, Dominance, Openness, Extraversion, and Agreeableness). A sixth factor (Methodical) was offered provisionally until more data are collected. We validated the factors against behavioral data collected independently on the chimpanzees. The five factors demonstrated good evidence for convergent and predictive validity, thereby underscoring the robustness of the factors. Our combined top-down/ bottom-up approach provides the most extensive data to date to support the universal existence of these five personality factors in chimpanzees. This framework, which facilitates cross-species comparisons, can also play a vital role in understanding the evolution of personality and can assist with husbandry and welfare efforts.

Chimpanzees appear to understand something about the attentional states of others; in the present experiment, we investigated whether they understand that the attentional state of a human is based on eye gaze. In all, 116 adult chimpanzees were offered food by an experimenter who engaged in one of the four experimental manipulations: eyes closed, eyes open, hand over eyes, and hand over mouth. The communicative behavior of the chimpanzees was observed. More visible behaviors were produced when the experimenter's eyes were visible than when the experimenter's eyes were not visible. More vocalizations were produced when the experimenter's eyes were closed than when they were open, but there were no differences in other attention getting behaviors. There was no effect of age or rearing history. The results suggest that chimpanzees use the presence of the eyes as a cue that their visual gestures will be effective.