Humans differ from one another in numerous ways. How we react to events. How we interpret our environment. How we interact with others. And what we are drawn to. Those who live with, have interacted with, or have worked with animals have made similar observations throughout millennia.
These differences are consequential. The five dimensions—neuroticism, extraversion, openness, agreeableness, and conscientiousness—that describe human personality (McCrae & John, 1992) are associated with a range of outcomes, among them psychological well-being and health (Deary et al., 2010). One important question is “Why are there five dimensions, and why these five?” Another is “Why are some personality dimensions, namely extraversion and conscientiousness, related to psychological well-being and health?”
Comparative psychologists have studied these questions by examining personality in closely related species, namely other great apes, which include chimpanzees, bonobos, gorillas, and orangutans. They have found variations of the human dimensions in great apes (Weiss, 2018). Extraversion in chimpanzees, bonobos, and humans, for example, showed both overlaps and divergences (King & Figueredo, 1997; Weiss et al., 2015). Researchers also have found clear differences:
- Great apes have a dominance factor, yet humans don’t (Weiss, 2022);
- Humans and most great apes have distinct extraversion and openness dimensions, yet orangutans don’t appear to (Weiss et al., 2006);
- Chimpanzees and bonobos (Freeman et al., 2013; King & Figueredo, 1997; Weiss et al., 2015) have conscientiousness dimensions, yet orangutans and wild mountain gorillas don’t appear to (Eckardt et al., 2015; Weiss et al., 2006).
Also, similar associations between personality dimensions and psychological well-being and health can be found in great apes (e.g., King & Landau, 2003) and other primates (e.g., in common marmosets; Inoue-Murayama et al., 2018).
A paper published in the Journal of Comparative Psychology (Weiss et al., 2026) describes a study of these questions in gibbons, a species of “small apes” (although we think they’re pretty “great”!). Gibbons are ideal for studying these questions, as they are evolutionarily closer to orangutans than to other great apes (Locke et al., 2011). They also are similar to orangutans in that they live in canopies of Asian forests, and they are less social than the African great apes (Cunningham & Mootnick, 2009). Thus, if gibbons resemble orangutans in their personality or how personality dimensions relate to their psychological well-being or health, these features are likely remnants “left in the dust” by the African great apes. However, if gibbons resemble African great apes, then orangutans are “evolutionary oddballs” among great apes.
This study benefited from an international team, including Michaela Masilkova (Czechia); Marieke Gartner (United States); Blake Morton, Lewis Rowden, and Susan Cheyne (United Kingdom); and Miho Inoue-Murayama (Japan). Moreover, people at zoos, research centers, and field sites from countries around the world contributed ratings on the Hominoid Personality Questionnaire (Weiss, 2017; Weiss et al., 2009); a well-being scale (King & Landau, 2003); and a question concerning overall health, which was adapted from a measure used in research on humans (Benyamini & Idler, 1999). The study collected data on at least one of these forms on nearly 400 gibbons. We even had enough data on three of the four types of gibbons to see how much personality varied among such closely related species. We verified that the ratings were consistent across raters and then used factor analysis to see how groups of possibly related variables fit together.
Overall, gibbon personality was much like that of orangutans. First, the blended extraversion and openness dimension, as such, appears to be an ancient trait in apes. It may have been selected against, perhaps because of the needs relating to the larger social groups of African apes. Second, we found a conscientiousness dimension in data from two of the three gibbon genera and the entire sample. Thus, the absence of a conscientiousness dimension (i.e., the items related to this dimension did not group together) was unique to orangutans. Perhaps selection for high conscientiousness was uniform in orangutans because carelessness in the canopy could lead to falls with and, consequently, major injuries—or worse. This finding also pushes back the timeline for the evolution of conscientiousness in our species to 20 million years ago. The findings concerning personality and psychological well-being and health were consistent with orangutans, the other great apes, humans, and other primates. These connections are truly ancient.
One study implication concerns a view held by some that personality factors (the dimensions) are abstractions; they are not real except in a mathematical sense. This study, and others (e.g., Adams et al., 2015; Masilkova et al., 2020), however, shows that the strong version of this argument is incorrect. The configurations among traits we found when we performed factor analyses on these data reflect evolved adaptations of populations of species. A second implication concerns animal welfare and conservation. It is clear now that, when a species goes extinct, it also means the loss of individuals.