Response disequilibrium theory, an extension of the response deprivation hypothesis, proposes that behavior-change is due to disruptions in an organism's baseline levels of responding. This theory challenges the notion of distinct stimuli as reinforcers or punishers and instead emphasizes the role of instrumental and contingent activities. The present study aimed to test the predictions of response disequilibrium theory by replicating Heth and Warren’s, Animal Learning Behavior, 6, 294–300, (1978) experiment using a modernized apparatus. Sixteen undergraduate psychology students participated in the experiment, which included a familiarization period, baseline periods, and a contingency period. During the contingency period, the instrumental activity was the activation of a video that resulted in a deficit or excess of audio relative to baseline. According to response disequilibrium theory, a deficit will produce a reinforcement effect whereas an excess will produce a punishment effect. The results supported the predictions of response disequilibrium theory, as instrumental video activity increased for participants in the deficit condition and decreased for participants in the excess condition. In addition, the study revealed a strong correlation between the size of disequilibrium and the magnitude of behavior change. These findings further support the use of disequilibrium models to a priori identify effective contingency arrangements in both research and practice. Informal applications of the Premack principle may be supplanted with the precision of a formal model of reinforcement and punishment in response disequilibrium theory.

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.

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.

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/.

Over the past 20 years, the study of personality has blossomed within primatology. Rating inventories have been extensively used across species to identify personality factor structures for different species. Chimpanzees share a common six-factor structure, composed of Dominance, Extraversion, Agreeableness, Conscientiousness, Neuroticism, and Openness. Conscientiousness, Agreeableness, and Extraversion, have been tied to positive effects on overall health and longevity in humans, while Neuroticism is associated with shorter life span. In captive gorillas, Extraversion alone is associated with longevity. We undertook a study of all captive, personality questionnaire-rated chimpanzees; the Chimpanzee and Hominoid Personality Questionnaires have been used to assess almost 600 chimpanzees from Europe, Australia, Japan, and the United States. With these data we investigated which personality dimensions and other life history characteristics might predict all-cause mortality. In an accelerated failure survival analysis, we found a strong effect of Extraversion (b = -4.74, p<0.0001), as well as lesser but significant effects of Agreeableness (b = 2.87, p<0.01) and Dominance (b = 1.64, p<0.05). Female and wild born chimpanzees appear to also live significantly longer (ps<0.01). The robustness of an animal's social network and the quality of its social interactions play a role in the health of social organisms. This is reflected in the importance of Extraversion for gorilla and chimpanzee health, and Agreeableness and Dominance for chimpanzees. Yet, the effect of Agreeableness is also similar to what has been found in humans.

University students partitioned in four groups were treated to a four-phase resurgence preparation in which three response sequences were sequentially acquired in the first three phases and tested for serial positioning effects and response variability in terminal resurgence phase. For the three test groups, three colored backgrounds served as the contextual stimuli that corresponded to the acquisition of the response sequences in the initial three phases. In the final resurgence phase, all response sequences were placed on extinction. The three test groups were re-presented a respective colored background found in one of the earlier acquisition phases. The fourth group did not involve any colored background throughout the entire experiment. Across all three test groups, the highest acquired response sequence proportion was correlated with the discriminative stimulus present in the resurgence phase. The no-color group without contextual change across phases showed recency and primacy effects consistent with those found in serial position studies. Moreover, the presence of the discriminative stimulus in the resurgence phase resulted in less variable response sequences relative to the no-color group. These findings support the role of discriminative stimuli in the manner with which response patterns are organized in resurgence.

The Rowlands target article makes a case, based on John Locke's metaphysical argument, that personhood can be applied to many species beyond a few nonhuman primates. The problem with this argument is that personhood has an open-ended list of defining attributes and can, at best only be partially applied to nonhuman species. Therefore, a dichotomous distinction cannot be made between qualifying and non-qualifying species. Instead, between-species comparisons must be based on observational and experimental procedures to demonstrate the extent to which human mental attributes are shared with other species. This would allow a more informed view about the appropriateness of personhood in nonhumans.

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.

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.

Ratings of chimpanzee, Pan troglodytes, and orang-utan, Pongo pygmaeus and Pongo abelii, personality reveal dimensions resembling those found in humans. Critics have argued that this similarity derives from anthropomorphic projection or other rater-based effects. We developed two forms of data reduction analyses to determine whether these dimensions can best be explained by the inherent tendencies of the animals (e.g. orang-utans that are curious are playful) or anthropomorphic projections of raters (e.g. believing that orang-utans that are curious should be playful). We found that personality dimensions derived after differences between rater means and rater*item interactions had been removed from ratings replicated the previously discovered dimensions. Conversely, we found a different set of dimensions when analysing items from which differences between animal means and animal*item interactions had been removed. Finally, we used multilevel factor analysis to examine whether the published structure was replicated when we extracted factors based on the within-level animal differences in item scores effects while allowing between-rater differences to covary freely. Again, the personality dimensions were similar to those described in previous studies. These analyses can be used in combination with interrater reliability, temporal stability, and correlations between personality and other external variables to validate animal personality ratings. These analyses confirmed that personality similarities between humans and great apes are best explained by genetic and phylogenetic affinity and not by anthropomorphic artefacts. (c) 2012 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved.

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.

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, orangutans 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.

Although concerns about anthropomorphism and subjectivity have limited the widespread use of subjective personality ratings in primate research, recent developments show that subjective personality ratings of primates have acceptable interrater reliabilities and display good evidence of construct validity. The role of human personality dimensions or factors has typically been one of defining the taxonomy of human personality and examining relationships between those factors and multiple human traits and outcomes. We describe three additional areas in which subjective personality ratings may be useful within the context of primatology. First, when human personality factors are extended to nonhuman primates, interspecies differences in factor structure afford opportunities to infer changes that have occurred in the underlying behavioral correlations during evolutionary development. In other words, personality factors can be interpreted as evolutionary characters. Our studies show a close correspondence between the personality factor structure of chimpanzees and orangutans with one notable exception. We suspect that a factor in chimpanzees reflecting an association between aggression, emotionality, and unpredictability is homologous to the Conscientiousness factor in humans. Such a Conscientiousness factor is completely absent in orangutans. This chimpanzee factor may be a consequence of intense intragroup competition in male chimpanzees. Second, the highly aggressive disposition of wild male chimpanzees may also reflect a spectrum of personality differences between male and female chimpanzees. Male chimpanzees score higher than females on traits related to aggression, unpredictability, and emotionality, a pattern unlike that displayed by humans. Third, subjective personality ratings are potentially useful in addressing the disparity between current measures of psychological well-being in nonhuman primates that focus on low levels of well-being including pathological behaviors, and measures of happiness or subjective well-being in humans that focus on positive subjective states.

Many aspects of personality are honestly signaled on the human face, as shown by accurate identification of personality traits from static images of unknown faces with neutral expressions. Here, we examined the evolutionary history of this signal system. In four studies, we found that untrained human observers reliably discriminated characteristics related to extraversion solely from nonexpressive facial images of chimpanzees (Pan troglodytes). In chimpanzees, as in humans, there is therefore information in the static, nonexpressive face that signals aspects of an individual's personality. We suggest that this performance is best explained by shared personality structure and signaling in the two species.

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. Am. J. Primatol. 71:283–292, 2009. © 2009 Wiley‐Liss, Inc.

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. Am. J. Primatol. 71:283‐292, 2009. r 2009 Wiley-Liss, Inc.