Evidence for a midlife crisis in great apes consistent with the U-shape in human well-being.

Recently, economists and behavioral scientists have studied the pattern of human well-being over the lifespan. In dozens of countries, and for a large range of well-being measures, including happiness and mental health, well-being is high in youth, falls to a nadir in midlife, and rises again in old age. The reasons for this U-shape are still unclear. Present theories emphasize sociological and economic forces. In this study we show that a similar U-shape exists in 508 great apes (two samples of chimpanzees and one sample of orangutans) whose well-being was assessed by raters familiar with the individual apes. This U-shaped pattern or "midlife crisis" emerges with or without use of parametric methods. Our results imply that human well-being's curved shape is not uniquely human and that, although it may be partly explained by aspects of human life and society, its origins may lie partly in the biology we share with great apes. These findings have implications across scientific and social-scientific disciplines, and may help to identify ways of enhancing human and ape well-being.

The majority of genetic variation in orangutan personality and subjective well-being is nonadditive.

The heritability of human personality is well-established. Recent research indicates that nonadditive genetic effects, such as dominance and epistasis, play a large role in personality variation. One possible explanation for the latter finding is that there has been recent selection on human personality. To test this possibility, we estimated additive and nonadditive genetic variance in personality and subjective well-being of zoo-housed orangutans. More than half of the genetic variance in these traits could be attributed to nonadditive genetic effects, modeled as dominance. Subjective well-being had genetic overlap with personality, though less so than has been found in humans or chimpanzees. Since a large portion of nonadditive genetic variance in personality is not unique to humans, the nonadditivity of human personality is not sufficient evidence for recent selection of personality in humans. Nonadditive genetic variance may be a general feature of the genetic structure of personality in primates and other animals.

Happy orang-utans live longer lives.

Nonhuman primate ageing resembles its human counterpart. Moreover, ratings of subjective well-being traits in chimpanzees, orang-utans and rhesus macaques are similar to those of humans: they are intercorrelated, heritable, and phenotypically and genetically related to personality. We examined whether, as in humans, orang-utan subjective well-being was related to longer life. The sample included 184 zoo-housed orang-utans followed up for approximately 7 years. Age, sex, species and number of transfers were available for all subjects and 172 subjects were rated on at least one item of a subjective well-being scale. Of the 31 orang-utans that died, 25 died a mean of 3.4 years after being rated. Even in a model that included, and therefore, statistically adjusted for, sex, age, species and transfers, orang-utans rated as being "happier" lived longer. The risk differential between orang-utans that were one standard deviation above and one standard deviation below baseline in subjective well-being was comparable with approximately 11 years in age. This finding suggests that impressions of the subjective well-being of captive great apes are valid indicators of their welfare and longevity.

Assessing chimpanzee personality and subjective well-being in Japan.

We tested whether the cultural background of raters influenced ratings of chimpanzee personality. Our study involved comparing personality and subjective well-being ratings of 146 chimpanzees in Japan that were housed in zoos, research institutes, and a retirement sanctuary to ratings of chimpanzees in US and Australian zoos. Personality ratings were made on a translated and expanded version of a questionnaire used to rate chimpanzees in the US and Australia. Subjective well-being ratings were made on a translated version of a questionnaire used to rate chimpanzees in the US and Australia. The mean interrater reliabilities of the 43 original adjectives did not markedly differ between the present sample and the original sample of 100 zoo chimpanzees in the US. Interrater reliabilities of these samples were highly correlated, suggesting that their rank order was preserved. Comparison of the factor structures for the Japanese sample and for the original sample of chimpanzees in US zoos indicated that the overall structure was replicated and that the Dominance, Extraversion, Conscientiousness, and Agreeableness domains clearly generalized. Consistent with earlier studies, older chimpanzees had higher Dominance and lower Extraversion and Openness scores. Correlations between the six domain scores and subjective well-being were comparable to those for chimpanzees housed in the US and Australia. These findings suggest that chimpanzee personality ratings are not affected by the culture of the raters.

Disequilibrium in behavior analysis: A disequilibrium theory redux.

Disequilibrium theory is an approach to reinforcement that reconsiders the putative response strengthening prowess of stimuli. This disequilibrium approach-the pinnacle of the response deprivation hypothesis-reliably predicts changes in behavior without reference to a response strengthening process. While the strengthening model of reinforcement has received renewed and critical appraisal in behavior analysis, its appraisers have not fully considered the role that a disequilibrium conceptualization might play in their respective theories of reinforcement. In this essay we celebrate William Timberlake's legacy by elucidating the assumptions of disequilibrium theory and by exploring its predictions and implications within behavior analysis. We treat the disequilibrium approach to reinforcement as the theory of reinforcement in behavior analysis, and in doing so, we distinguish disequilibrium conditions from motivating operations and explore future directions regarding the potential to predict generalization and maintenance outcomes. The disequilibrium approach to reinforcement is not a mere deprivation operation used for the purposes of establishing a stimulus as a "reinforcer," as it is a general theory of behavior.

Aping humans: age and sex effects in chimpanzee (Pan troglodytes) and human (Homo sapiens) personality.

Ratings of 202 chimpanzees on 43 personality descriptor adjectives were used to calculate scores on five domains analogous to the human Five-Factor Model and a chimpanzee-specific Dominance domain. Male and female chimpanzees were divided into five age groups ranging from juvenile to old adult. Internal consistencies and interrater reliabilities of factors were stable across age groups and approximately 6.8 year retest reliabilities were high. Age-related declines in Extraversion and Openness and increases in Agreeableness and Conscientiousness paralleled human age differences. The mean change in absolute standardized units for all five factors was virtually identical in humans and chimpanzees after adjustment for different developmental rates. Consistent with their aggressive behavior in the wild, male chimpanzees were rated as more aggressive, emotional, and impulsive than females. Chimpanzee sex differences in personality were greater than comparable human gender differences. These findings suggest that chimpanzee and human personality develop via an unfolding maturational process.

Personality links with lifespan in chimpanzees.

Life history strategies for optimizing individual fitness fall on a spectrum between maximizing reproductive efforts and maintaining physical health over time. Strategies across this spectrum are viable and different suites of personality traits evolved to support these strategies. Using data from 538 captive chimpanzees (Pan troglodytes) we tested whether any of the dimensions of chimpanzee personality - agreeableness, conscientiousness, dominance, extraversion, neuroticism, and openness - were associated with longevity, an attribute of slow life history strategies that is especially important in primates given their relatively long lives. We found that higher agreeableness was related to longevity in males, with weaker evidence suggesting that higher openness is related to longer life in females. Our results link the literature on human and nonhuman primate survival and suggest that, for males, evolution has favored the protective effects of low aggression and high quality social bonds.

Predicting the Effects of Interventions: A Tutorial on the Disequilibrium Model.

The disequilibrium approach to reinforcement and punishment, derived from the probability-differential hypothesis and response deprivation hypothesis, provides a number of potentially useful mathematical models for practitioners. The disequilibrium approach and its accompanying models have proven effective in the prediction and control of behavior, yet they have not been fully espoused and integrated into clinical practice. The purpose of this tutorial is to detail the disequilibrium approach and adapt its mathematical models for use as a tool in applied settings. The disequilibrium models specify how to arrange contingencies and predict the effects of those contingencies. We aggregate these models, and provide them as a single tool, in the form of a Microsoft Excel® spreadsheet that calculates the direction and magnitude of behavior change based on baseline measures and a practitioner's choice of intervention parameters. How practitioners take baseline measures and select intervention parameters in accordance with disequilibrium models is explicated. The proposed tool can be accessed and downloaded for use at https://osf.io/knf7x/.

Personality and subjective well-being in orangutans (Pongo pygmaeus and Pongo abelii).

Orangutans (Pongo pygmaeus and Pongo abelii) are semisolitary apes and, among the great apes, the most distantly related to humans. Raters assessed 152 orangutans on 48 personality descriptors; 140 of these orangutans were also rated on a subjective well-being questionnaire. Principal-components analysis yielded 5 reliable personality factors: Extraversion, Dominance, Neuroticism, Agreeableness, and Intellect. The authors found no factor analogous to human Conscientiousness. Among the orangutans rated on all 48 personality descriptors and the subjective well-being questionnaire, Extraversion, Agreeableness, and low Neuroticism were related to subjective well-being. These findings suggest that analogues of human, chimpanzee, and orangutan personality domains existed in a common ape ancestor.

Great ape origins of personality maturation and sex differences: a study of orangutans and chimpanzees.

Human personality development evinces increased emotional stability, prosocial tendencies, and responsibility. One hypothesis offered to explain this pattern is Social-Investment Theory, which posits that culturally defined social roles, including marriage and employment, are responsible for the increased maturity. Alternatively, Five-Factor Theory emphasizes the role of biological factors, such as those governing physical development, which may predate the emergence of humans. Five-Factor Theory, unlike Social-Investment Theory, predicts that all or some of the human personality developmental trends should be present in great apes, our closest evolutionary relatives. To test this prediction and to better understand the evolutionary origins of sex differences, we examined age and sex differences in the chimpanzee and orangutan personality domains Extraversion, Dominance, Neuroticism, and Agreeableness. We also examined the Activity and Gregariousness facets of Extraversion and the orangutan Intellect domain. Extraversion and Neuroticism declined across age groups in both species, in common with humans. A significant interaction indicated that Agreeableness declined in orangutans but increased in chimpanzees, as it does in humans, though this may reflect differences in how Agreeableness was defined in each species. Significant interactions indicated that male chimpanzees, unlike male orangutans, displayed higher Neuroticism scores than females and maintained higher levels of Activity and Dominance into old age than female chimpanzees, male orangutans, and female orangutans. Personality-age correlations were comparable across orangutans and chimpanzees and were similar to those reported in human studies. Sex differences were stronger in chimpanzees than in humans or orangutans. These findings support Five-Factor Theory, suggest the role of gene-culture coevolution in shaping personality development, and suggest that sex differences evolved independently in different species.

Subjective well-being is heritable and genetically correlated with dominance in chimpanzees (Pan troglodytes).

The hypothesis that subjective well-being (SWB) is heritable and genetically correlated with Dominance was tested using 128 zoo chimpanzees. Dominance was a chimpanzee-specific personality factor including items reflecting Extraversion and low Neuroticism. SWB was measured with a 4-item scale. The best behavior genetic model included additive genetic and nonshared environmental effects for SWB and Dominance, marginal matemal effects for SWB, a high genetic correlation, and a low nonshared environmental correlation. Results indicated that the shared variance between SWB and Dominance was a consequence of common genes and that the unique variance between SWB and Dominance was a consequence of the nonshared environment. These findings indicate that common genes may underlie the correlation between human personality factors and SWB.

Laterality in spontaneous motor activity of chimpanzees and squirrel monkeys.

Varieties of nonmanipulative motor responses were observed in chimpanzees and squirrel monkeys. Chimpanzees displayed a right hand preference for touching their inanimate environments but used their right and left hands equally for touching their faces and their bodies. The latter result was not consistent with previous reports of a left hand preference for face touching in apes. The right hand preference for environmental touching was stronger in male than in female chimpanzees. Squirrel monkeys had a right preference for combined hand and foot responses directed to their bodies, but expressed no handedness for environmentally directed touching. These limb preferences in chimpanzees and squirrel monkeys indicate that neither precise, complex manipulation nor postural instability are necessary conditions for population level hand preferences. Factor analysis of the chimpanzee manual responses showed distinct self and environmentally directed factors. Analysis of the squirrel monkey data also showed self and environmental factors, except that body scratching had a negative loading on the environmental factor. This latter result suggests that self-scratching by squirrel monkeys is a displacement activity that suppresses manual exploration of the environment. © 1992 Wiley-Liss, Inc.

Parsimonious explanations and wider evolutionary consequences.

The uncertainty response adds an important new dimension to conventional animal learning and memory studies. Although the uncertainty response by monkeys and dolphins resembled that of humans, parsimony alone does not necessarily indicate that the monkeys and dolphins had a full self-awareness. However, the uncertain response may be an index of an evolutionary precursor to full self-awareness of uncertainty and a theory of mind.

Chimpanzee facial symmetry: a biometric measure of chimpanzee health.

This paper reports a study of fitness indicator theory in chimpanzees. First, it establishes a theoretical perspective for the study of fitness indicator theory and the relationships among indicators of fitness in humans and other animals. Second, it describes a methodology for assessing facial fluctuating asymmetry (FA) in a sample (N = 21) of zoo chimpanzees (Pan troglodytes). Third, associations among chimpanzee facial FA and health are described. FA was positively associated with negative health symptoms, and negatively associated with general health. Results are discussed under the framework of good genes theory.

A cross-setting study of chimpanzee (Pan troglodytes) personality structure and development: zoological parks and Yerkes National Primate Research Center.

This study addressed whether personality ratings using a 43 adjective questionnaire based on the Five-Factor Model generalized from a sample of 202 zoo-housed chimpanzees to a sample of 175 chimpanzees housed in Yerkes National Primate Research Center. Mean interrater reliabilities of adjectival ratings were lower for the chimpanzees housed in Yerkes. In addition, rank order of the interrater reliabilities of items differed between settings. To compare factor structure, we first examined whether we could replicate the original six factor structure found in an earlier study of 100 zoo chimpanzees using principal factors analysis in the Yerkes sample and 102 new zoo chimpanzees. The dominance, extraversion, conscientiousness, and agreeableness factors were clearly replicated in the Yerkes sample and the 102 new zoo chimpanzees. The Neuroticism and Openness factors did not replicate in the Yerkes sample, but they also did not replicate in the new zoo chimpanzees. These findings suggest the need to sample more adjectives representing neuroticism and openness in future versions of the questionnaire. We next sought to determine whether factor structure, as determined by principal components analysis, remained invariant across the two settings. This analysis revealed dominance, extraversion, conscientiousness, and agreeableness factors in both settings and a high level of congruence between the zoo and Yerkes samples for these factors. Finally, we tested whether factor scores in the two samples were similarly related to age and sex. With the exception of differences in age effects for dominance and agreeableness, age, and sex effects were consistent across samples. These findings suggest that, whereas there may be differences in the ease with which ratings are made, personality structure, and development are largely consistent across widely differing settings.

A chimpanzee (Pan troglodytes) analogue of cross-national generalization of personality structure: zoological parks and an African sanctuary.

Six personality factors, including five resembling the human Big Five, had previously been identified in a separate group of zoo-housed chimpanzees. Comparability of chimpanzee personality factor structure was examined in two highly contrasting habitats: zoos and a large African sanctuary. Questionnaires for the zoo chimpanzees were in English, while most for the chimpanzees in the sanctuary were in French. Differences between the two settings were sufficiently extensive to make them analogous to cross-national human personality studies. Internal consistencies for five of the six factors did not differ between the two samples. The patterns of correlations between the unit-weighted factors were also similar for the two samples. Data from these two samples were pooled and factor analyzed. The resulting factor structure was then rotated to the factor structure described in the original study of chimpanzee personality. Dominance, Extraversion, Dependability, and Agreeableness had high congruences. Emotionality and Openness did not, but the items that had the highest loadings were consistent with the factors' definitions. Finally, sex and age effects for all factors generalized across habitats.