Publications

Abstract

Previous research has shown significant effects of race and accent on children’s developing social preferences. Accounts of the primacy of accent biases in the evolution and ontogeny of discriminant cooperation have been proposed, but lack systematic cross-cultural investigation. We report three controlled studies conducted with 5−10 year old children across four towns in the Brazilian Amazon, selected for their variation in racial and accent homogeneity/heterogeneity. Study 1 investigated participants’ (N = 289) decisions about friendship and sharing across color-contrasted pairs of target individuals: Black-White, Black-Pardo (Brown), Pardo-White. Study 2 (N = 283) investigated effects of both color and accent (Local vs Non-Local) on friendship and sharing decisions. Overall, there was a significant bias toward the lighter colored individual. A significant preference for local accent mitigates but does not override the color bias, except in the site characterized by both racial and accent heterogeneity. Results also vary by participant age and color. Study 3 (N = 235) reports results of an accent discrimination task that shows an overall increase in accuracy with age. The research suggests that cooperative preferences based on accent and race develop differently in response to locally relevant parameters of racial and linguistic variation.

Abstract

Comparative thanatology encompasses the study of death-related responses in non-human animals and aspires to elucidate the evolutionary origins of human behavior in the context of death. Many reports have revealed that humans are not the only species affected by the death of group members. Non-human primates in particular show behaviors such as congregating around the deceased, carrying the corpse for prolonged periods of time (predominantly mothers carrying dead infants), and inspecting the corpse for signs of life. Here, we extend the focus on death-related responses in non-human animals by exploring whether chimpanzees are inclined to console the bereaved: the individual(s) most closely associated with the deceased. We report a case in which a chimpanzee (Pan troglodytes) mother experienced the loss of her fully developed infant (presumed stillborn). Using observational data to compare the group members’ behavior before and after the death, we found that a substantial number of group members selectively increased their affiliative expressions toward the bereaved mother. Moreover, on the day of the death, we observed heightened expressions of species-typical reassurance behaviors toward the bereaved mother. After ruling out several alternative explanations, we propose that many of the chimpanzees consoled the bereaved mother by means of affiliative and selective empathetic expressions.

Abstract

Intergroup variation (IGV) refers to variation between different groups of the same species. While its existence in the behavioural realm has been expected and evidenced, the potential effects of IGV are rarely considered in studies that aim to shed light on the evolutionary origins of human socio-cognition, especially in our closest living relatives—the great apes. Here, by taking chimpanzees as a point of reference, we argue that (i) IGV could plausibly explain inconsistent research findings across numerous topics of inquiry (experimental/behavioural studies on chimpanzees), (ii) understanding the evolutionary origins of behaviour requires an accurate assessment of species' modes of behaving across different socio-ecological contexts, which necessitates a reliable estimation of variation across intraspecific groups, and (iii) IGV in the behavioural realm is increasingly likely to be expected owing to the progressive identification of non-human animal cultures. With these points, and by extrapolating from chimpanzees to generic guidelines, we aim to encourage researchers to explicitly consider IGV as an explanatory variable in future studies attempting to understand the socio-cognitive and evolutionary determinants of behaviour in group-living animals.

Abstract

Cultural evolution is the product of the psychological mechanisms that underlie individual decision making. One commonly studied learning mechanism is a disproportionate preference for majority opinions, known as conformist transmission. While most theoretical and experimental work approaches the majority in terms of the number of individuals that perform a behaviour or hold a belief, some recent experimental studies approach the majority in terms of the number of instances a behaviour is performed. Here, we use a mathematical model to show that disagreement between these two notions of the majority can arise when behavioural variants are performed at different rates, with different salience or in different contexts (variant overrepresentation) and when a subset of the population act as demonstrators to the whole population (model biases). We also show that because conformist transmission changes the distribution of behaviours in a population, how observers approach the majority can cause populations to diverge, and that this can happen even when the two approaches to the majority agree with regards to which behaviour is in the majority. We discuss these results in light of existing findings, ranging from political extremism on twitter to studies of animal foraging behaviour. We conclude that the factors we considered (variant overrepresentation and model biases) are plausibly widespread. As such, it is important to understand how individuals approach the majority in order to understand the effects of majority influence in cultural evolution.

Abstract

Farine and Aplin (1) question the validity of our study reporting group-specific social dynamics in chimpanzees (2). As alternative to our approach, Farine and Aplin advance a “prenetwork permutation” methodology that tests against random assortment (3). We appreciate Farine and Aplin’s interest and applied their suggested approaches to our data. The new analyses revealed highly similar results to those of our initial approach. We further dispel Farine and Aplin’s critique by outlining its incompatibility to our study system, methodology, and analysis.First, when we apply the suggested prenetwork permutation to our proximity dataset, we again find significant population-level differences in association rates, while controlling for population size [as derived from Farine and Aplin’s script (4); original result, P < 0.0001; results including prenetwork permutation, P < 0.0001]. Furthermore, when we … ↵1To whom correspondence may be addressed. Email: ejcvanleeuwen{at}gmail.com.

Abstract

In bonobos, strong bonds have been documented between unrelated females and between mothers
and their adult sons, which can have important fitness benefits. Often age, sex or kinship similarity
have been used to explain social bond strength variation. Recent studies in other species also stress
the importance of personality, but this relationship remains to be investigated in bonobos. We used
behavioral observations on 39 adult and adolescent bonobos housed in 5 European zoos to study the
role of personality similarity in dyadic relationship quality. Dimension reduction analyses on individual
and dyadic behavioral scores revealed multidimensional personality (Sociability, Openness, Boldness,
Activity) and relationship quality components (value, compatibility). We show that, aside from
relatedness and sex combination of the dyad, relationship quality is also associated with personality
similarity of both partners. While similarity in Sociability resulted in higher relationship values, lower
relationship compatibility was found between bonobos with similar Activity scores. The results of this
study expand our understanding of the mechanisms underlying social bond formation in anthropoid
apes. In addition, we suggest that future studies in closely related species like chimpanzees should
implement identical methods for assessing bond strength to shed further light on the evolution of this
phenomenon.

Abstract

In the Smaldino et al. study ‘Sigmoidal Acquisition Curves are Good Indicators of Conformist Transmission’, our original findings regarding the conditional validity of using population-level sigmoidal acquisition curves as means to evidence individual-level conformity are contested. We acknowledge the identification of useful nuances, yet conclude that our original findings remain relevant for the study of conformist learning mechanisms. Replying to: Smaldino, P. E., Aplin, L. M. & Farine, D. R. Sigmoidal Acquisition Curves Are Good Indicators of Conformist Transmission. Sci. Rep. 8, https://doi.org/10.1038/s41598-018-30248-5 (2018).

Abstract

Although there is good evidence that social animals show elaborate cognitive skills to deal with others, there are few reports of animals physically using social agents and their respective responses as means to an end—social tool use. In this case study, we investigated spontaneous and repeated social tool use behavior in chimpanzees (Pan troglodytes). We presented a group of chimpanzees with an apparatus, in which pushing two buttons would release juice from a distantly located fountain. Consequently, any one individual could only either push the buttons or drink from the fountain but never push and drink simultaneously. In this scenario, an adult male attempted to retrieve three other individuals and push them toward the buttons that, if pressed, released juice from the fountain. With this strategy, the social tool user increased his juice intake 10-fold. Interestingly, the strategy was stable over time, which was possibly enabled by playing with the social tools. With over 100 instances, we provide the biggest data set on social tool use recorded among nonhuman animals so far. The repeated use of other individuals as social tools may represent a complex social skill linked to Machiavellian intelligence.

Abstract

Social information use is a pivotal characteristic of the human species. Avoiding the cost of individual exploration, social learning confers substantial fitness benefits under a wide variety of environmental conditions, especially when the process is governed by biases toward relative superiority (e.g., experts, the majority). Here, we examine the development of social information use in children aged 4–14 years (n = 605) across seven societies in a standardised social learning task. We measured two key aspects of social information use: general reliance on social information and majority preference. We show that the extent to which children rely on social information depends on children’s cultural background. The extent of children’s majority preference also varies cross-culturally, but in contrast to social information use, the ontogeny of majority preference follows a U-shaped trajectory across all societies. Our results demonstrate both cultural continuity and diversity in the realm of human social learning.

Abstract

Understanding intraspecific variation in sociality is essential for characterizing the flexibility and evolution of social systems, yet its study in nonhuman animals is rare. Here, we investigated whether chimpanzees exhibit population-level differences in sociality that cannot be easily explained by differences in genetics or ecology. We compared social proximity and grooming tendencies across four semiwild populations of chimpanzees living in the same ecological environment over three consecutive years, using both linear mixed models and social network analysis. Results indicated temporally stable, population-level differences in dyadic-level sociality. Moreover, group cohesion measures capturing network characteristics beyond dyadic interactions (clustering, modularity, and social differentiation) showed population-level differences consistent with the dyadic indices. Subsequently, we explored whether the observed intraspecific variation in sociality could be attributed to cultural processes by ruling out alternative sources of variation including the influences of ecology, genetics, and differences in population demographics. We conclude that substantial variation in social behavior exists across neighboring populations of chimpanzees and that this variation is in part shaped by cultural processes.

Abstract

Social learning is predicted to evolve in socially living animals provided the learning process is not random but biased by certain socio-ecological factors. One bias of particular interest for the emergence of (cumulative) culture is the tendency to forgo personal behaviour in favour of relatively better variants observed in others, also known as the “copy-if-better” strategy. We investigated whether chimpanzees employ copy-if-better in a simple token-exchange paradigm controlling for individual and random social learning. After being trained on one token-type, subjects were confronted with a conspecific demonstrator who either received the same food reward as the subject (control condition) or a higher value food reward than the subject (test condition) for exchanging another token-type. In general, the chimpanzees persisted in exchanging the token-type they were trained on individually, indicating a form of conservatism consistent with previous studies. However, the chimpanzees were more inclined to copy the demonstrator in the test compared to the control condition, indicating a tendency to employ a copy-if-better strategy. We discuss the validity of our results by considering alternative explanations and relate our findings to the emergence of cumulative culture.

Abstract

The ‘grooming handclasp’ is one of the most well-established cultural traditions in chimpanzees. A recent study by Wrangham et al. [1] reduced the cultural scope of grooming-handclasp behavior by showing that grooming-handclasp style convergence is “explained by matrilineal relationship rather than conformity” [1]. Given that we previously reported cultural differences in grooming-handclasp style preferences in captive chimpanzees [2], we tested the alternative view posed by Wrangham et al. [1] in the chimpanzee populations that our original results were based on. Using the same outcome variable as Wrangham et al. [1] — the proportion of high-arm grooming featuring palm-to-palm clasping — we found that matrilineal relationships explained neither within-group homogeneity nor between-group heterogeneity, thereby corroborating our original conclusion that grooming-handclasp behavior can represent a group-level cultural tradition in chimpanzees.

Abstract

ct For the first time, chimpanzees have been observed using tools to clean the corpse of a deceased group member. A female chimpanzee sat down at the dead body of a young male, selected a firm stem of grass, and started to intently remove debris from his teeth. This report contributes novel behaviour to the chimpanzee's ethogram, and highlights how crucial information for reconstructing the evolutionary origins of human mortuary practices may be missed by refraining from developing adequate observation techniques to capture non-human animals' death responses

Abstract

Conformist transmission, defined as a disproportionate likelihood to copy the majority, is considered a potent mechanism underlying the emergence and stabilization of cultural diversity. However, ambiguity within and across disciplines remains as to how to identify conformist transmission empirically. In most studies, a population level outcome has been taken as the benchmark to evidence conformist transmission: a sigmoidal relation between individuals’ probability to copy the majority and the proportional majority size. Using an individual-based model, we show that, under ecologically plausible conditions, this sigmoidal relation can also be detected without equipping individuals with a conformist bias. Situations in which individuals copy randomly from a fixed subset of demonstrators in the population, or in which they have a preference for one of the possible variants, yield similar sigmoidal patterns as a conformist bias would. Our findings warrant a revisiting of studies that base their conformist transmission conclusions solely on the sigmoidal curve. More generally, our results indicate that population level outcomes interpreted as conformist transmission could potentially be explained by other individual-level strategies, and that more empirical support is needed to prove the existence of an individual-level conformist bias in human and other animals.

Abstract

The social behavior of chimpanzees has been extensively studied, yet not much is known about how they behave in response to the death of a group member. Here, we provide a detailed report of the reactions of a group of chimpanzees to finding the dead body of a 9-year-old male group member. The behavior of the group was characterized by quiet attendance and close inspections punctuated by rare displays. Moreover, the body was continuously attended and closely inspected by several adults and juveniles, including an adult male who formed a close social bond with the deceased individual after the deceased individual’s mother died 4 years earlier. When considered with observations of how chimpanzees respond to dead infants and adults in this group and in others, these observations suggest that chimpanzees’ responses to death may be mediated by social bonds with the deceased individual. The results are discussed in light of recent reports on chimpanzees’ reactions to dead community members and more general primate thanatology.

Abstract

In the wild, chimpanzees (Pan troglodytes) are often faced with clumped food resources that they may know how to access but abstain from doing so due to social pressures. To better understand how social settings influence resource acquisition, we tested fifteen semi-wild chimpanzees from two social groups alone and in the presence of others. We investigated how resource acquisition was affected by relative social dominance, whether collaborative problem solving or (active or passive) sharing occurred amongst any of the dyads, and whether these outcomes were related to relationship quality as determined from six months of observational data. Results indicated that chimpanzees, regardless of rank, obtained fewer rewards when tested in the presence of others compared to when they were tested alone. Chimpanzees demonstrated behavioral inhibition; chimpanzees who showed proficient skill when alone often abstained from solving the task when in the presence of others. Finally, individuals with close social relationships spent more time together in the problem solving space, but collaboration and sharing were infrequent and sessions in which collaboration or sharing did occur contained more instances of aggression. Group living provides benefits and imposes costs, and these findings highlight that one cost of group living may be diminishing productive individual behaviors.

Abstract

Recent debates have questioned the extent to which culturally-transmitted norms drive behavioral variation in resource sharing across human populations. We shed new light on this discussion by examining the group-level variation in the social dynamics and resource sharing of chimpanzees, a species that is highly social and forms long-term community associations but differs from humans in the extent to which cultural norms are adopted and enforced. We rely on theory developed in primate socioecology to guide our investigation in four neighboring chimpanzee groups at a sanctuary in Zambia. We used a combination of experimental and observational approaches to assess the distribution of resource holding potential in each group. In the first assessment, we measured the proportion of the population that gathered in a resource-rich zone, in the second we assessed naturally occurring social spacing via social network analysis, and in the third we assessed the degree to which benefits were equally distributed within the group. We report significant, stable group-level variation across these multiple measures, indicating that group-level variation in resource sharing and social tolerance is not necessarily reliant upon human-like cultural norms.

Abstract

Social learning in chimpanzees has been studied extensively and it is now widely accepted that chimpanzees have the capacity to learn from conspecifics through a multitude of mechanisms. Very few studies, however, have documented the existence of spontaneously emerged 'traditions' in chimpanzee communities. While the rigor of experimental studies is helpful to investigate social learning mechanisms, documentation of naturally occurring traditions is necessary to understand the relevance of social learning in the real lives of animals. In this study, we report on chimpanzees spontaneously copying a seemingly non-adaptive behaviour ("grass-in- ear behaviour"). The behaviour entailed chimpanzees selecting a stiff, straw-like blade of grass, inserting the grass into one of their own ears, adjusting the position, and then leaving it in their ear during subsequent activities. Using a daily focal follow procedure, over the course of one year, we observed 8 (out of 12) group members engaging in this peculiar behaviour. Importantly, in the 3 neighbouring groups of chimpanzees (n=82), this behaviour was only observed once, indicating that ecological factors were not determiners of the prevalence of this behaviour. These observations show that chimpanzees have a tendency to copy each other's behaviour, even when the adaptive value of the behaviour is presumably absent.

Abstract

In this review, we explore the extent to which the recent evidence for conformity in nonhuman animals may alternatively be explained by the animals' preference for social information regardless of the number of individuals demonstrating the respective behaviour. Conformity as a research topic originated in human psychology and has been described as the phenomenon in which individuals change their behaviour to match the behaviour displayed by the majority of group members. Recent studies have aimed to investigate the same process in nonhuman animals; however, most of the adopted designs have not been able to control for social influences independent of any majority influence and some studies have not even incorporated a majority in their designs. This begs the question to what extent the ‘conformity interpretation’ is preliminary and should be revisited in light of animals' general susceptibility to social influences. Similarly, demarcating social from majority influences sheds new light on the original findings in human psychology and motivates reinterpretation of the reported behavioural patterns in terms of social instead of majority influences. Conformity can have profound ramifications for individual fitness and group dynamics; identifying the exact source responsible for animals' behavioural adjustments is essential for understanding animals' learning biases and interpreting cross-species data in terms of evolutionary processes.

Abstract

In a highly social species like chimpanzees, the process by which individuals become attuned to their social environment may be of vital importance to their chances of survival. Typically, this socializing process, defined by all acquisition experiences and fine-tuning efforts of social interaction patterns during ontogeny, occurs in large part through parental investment. In this study, we investigated whether maternal presence would enhance the socializing process in chimpanzees by comparing the social interactions of orphaned and mother-reared individuals at the Chimfunshi Wildlife Orphanage Trust in Zambia. As response variables, we selected social interactions during which an elaborate level of fine-tuning is assumed to be necessary for sustaining the interaction and preventing escalation: social play. Comparing orphaned (n=8) to sex- and age-matched mother-reared juvenile chimpanzees (n=9), we hypothesized that the orphaned juveniles would play less frequently than the mother-reared and would be less equipped for fine-tuning social play (which we assayed by rates of aggression) because of the lack of a safe and facilitating social environment provided by the mother. First, contrary to our hypothesis, results showed that the orphaned juveniles engaged in social play more frequently than the mother-reared juveniles, although for significantly shorter amounts of time. Second, in support of our hypothesis, results showed that social play of the orphaned juveniles more often resulted in aggression than social play of the mother-reared juveniles. In conjunction, these results may indicate that, just like in humans, chimpanzee mothers provide their offspring with adequate social skills that might be of pivotal importance for future challenges like successful group-living and securing competitive fitness advantages.

Abstract

Human societies are characterized by more cultural diversity than chimpanzee communities. However, it is currently unclear what mechanism might be driving this difference. Because reliance on social information is a pivotal characteristic of culture, we investigated individual and social information reliance in children and chimpanzees. We repeatedly presented subjects with a reward-retrieval task on which they had collected conflicting individual and social information of equal accuracy in counterbalanced order. While both species relied mostly on their individual information, children but not chimpanzees searched for the reward at the socially demonstrated location more than at a random location. Moreover, only children used social information adaptively when individual knowledge on the location of the reward had not yet been obtained. Social information usage determines information transmission and in conjunction with mechanisms that create cultural variants, such as innovation, it facilitates diversity. Our results may help explain why humans are more culturally diversified than chimpanzees

Abstract

We here review existing evidence for majority influences in children under the age of ten years and comparable studies with animals ranging from fish to apes. Throughout the review, we structure the discussion surrounding majority influences by differentiating the behaviour of individuals in the presence of a majority and the underlying mechanisms and motivations. Most of the relevant research to date in both developmental psychology and comparative psychology has focused on the behavioural outcomes, where a multitude of mechanisms could be at play. We further propose that interpreting cross-species differences in behavioural patterns is difficult without considering the psychology of the individual. Some attempts at this have been made both in developmental psychology and comparative psychology. We propose that physiological measures should be used to subsidize behavioural studies in an attempt to understand the composition of mechanisms and motivations underlying majority influence. We synthesize the relevant evidence on human brain function in order to provide a framework for future investigation in this area. In addition to streamlining future research efforts, we aim to create a conceptual platform for productive exchanges across the related disciplines of developmental and comparative psychology.

Abstract

Majority influences have long been a subject of great interest for social psychologists and, more recently, for researchers investigating social influences in nonhuman primates. Although this empirical endeavor has culminated in the conclusion that some ape and monkey species show “conformist” tendencies, the current approach seems to suffer from two fundamental limitations: (a) majority influences have not been operationalized in accord with any of the existing definitions, thereby compromising the validity of cross-species comparisons, and (b) the results have not been systematically scrutinized in light of alternative explanations. In this review, we aim to address these limitations theoretically. First, we will demonstrate how the experimental designs used in nonhuman primate studies cannot test for conformity unambiguously and address alternative explanations and potential confounds for the presented results in the form of primacy effects, frequency exposure, and perception ambiguity. Second, we will show how majority influences have been defined differently across disciplines and, therefore, propose a set of definitions in order to streamline majority influence research, where conformist transmission and conformity will be put forth as operationalizations of the overarching denominator majority influences. Finally, we conclude with suggestions to foster the study of majority influences by clarifying the empirical scope of each proposed definition, exploring compatible research designs and highlighting how majority influences are inherently contingent on situational trade-offs.

Abstract

Chimpanzees have been shown to be adept learners, both individually and socially. Yet, sometimes their conservative nature seems to hamper the flexible adoption of superior alternatives, even to the extent that they persist in using entirely ineffective strategies. In this study, we investigated chimpanzees’ behavioural flexibility in two different conditions under which social animals have been predicted to abandon personal preferences and adopt alternative strategies: i) under influence of majority demonstrations (i.e. conformity), and ii) in the presence of superior reward contingencies (i.e. maximizing payoffs). Unlike previous nonhuman primate studies, this study disentangled the concept of conformity from the tendency to maintain one’s first-learned strategy. Studying captive (n=16) and semi-wild (n=12) chimpanzees in two complementary exchange paradigms, we found that chimpanzees did not abandon their behaviour in order to match the majority, but instead remained faithful to their first-learned strategy (Study 1a and 1b). However, the chimpanzees’ fidelity to their first-learned strategy was overridden by an experimental upgrade of the profitability of the alternative strategy (Study 2). We interpret our observations in terms of chimpanzees’ relative weighing of behavioural options as a function of situation-specific trade-offs. More specifically, contrary to previous findings, chimpanzees in our study abandoned their familiar behaviour to maximize payoffs, but not to conform to a majority.

Abstract

Although laughter forms an important part of human non-verbal communication, it has received rather less attention than it deserves in both the experimental and the observational literatures. Relaxed social (Duchenne) laughter is associated with feelings of wellbeing and heightened affect, a proximate expla- nation for which might be the release of endorphins. We tested this hypothesis in a series of six experimental studies in both the laboratory (watching videos) and naturalistic contexts (watching stage performances), using change in pain threshold as an assay for endorphin release. The results show that pain thresholds are significantly higher after laughter than in the control condition. This pain-tolerance effect is due to laughter itself and not simply due to a change in positive affect. We suggest that laughter, through an endorphin-mediated opiate effect, may play a crucial role in social bonding.

Abstract

Grooming handclasp (GHC) behaviour was originally advocated as the first evidence of social culture in chimpanzees owing to the finding that some populations engaged in the behaviour and others do not. To date, however, the validity of this claim and the extent to which this social behaviour varies between groups is unclear. Here, we measured (i) variation, (ii) durability and (iii) expansion of the GHC behaviour in four chimpanzee communities that do not systematically differ in their genetic backgrounds and live in similar ecological environments. Ninety chimpanzees were studied for a total of 1029 h; 1394 GHC bouts were observed between 2010 and 2012. Critically, GHC style (defined by points of bodily contact) could be systematically linked to the chimpanzee’s group identity, showed temporal consistency both withinand between-groups, and could not be accounted for by the arm-length differential between partners. GHC has been part of the behavioural repertoire of the chimpanzees under study for more than 9 years (surpassing durability criterion) and spread across generations (surpassing expansion criterion). These results strongly indicate that chimpanzees’ social behaviour is not only motivated by innate predispositions and individual inclinations, but may also be partly cultural in nature.

Abstract

The mother-offspring bond is one of the strongest and most essential social bonds. Following is a detailed behavioral report of a female chimpanzee two days after her 16-month-old infant died, on the first day that the mother is observed to create distance between her and the corpse. A series of repeated approaches and retreats to and from the body are documented, along with detailed accounts of behaviors directed toward the dead infant by the mother and other group members. The behavior of the mother toward her dead infant not only highlights the maternal contribution to the mother-infant relationship but also elucidates the opportunities chimpanzees have to learn about the sensory cues associated with death, and the implications of death for the social environment.

Abstract

The aim of this study was to examine the unique contribution of two aspects of emotion regulation (awareness and coping) to the development of internalizing problems in 11-year-old high-functioning children with an autism spectrum disorder (HFASD) and a control group, and the moderating effect of group membership on this. The results revealed overlap between the two groups, but also significant differences, suggesting a more fragmented emotion regulation pattern in children with HFASD, especially related to worry and rumination. Moreover, in children with HFASD, symptoms of depression were unrelated to positive mental coping strategies and the conviction that the emotion experience helps in dealing with the problem, suggesting that a positive approach to the problem and its subsequent emotion experience are less effective in the HFASD group.

Abstract

Humans respond to unfair situations in various ways. Experimental research has revealed that non-human species also respond to unequal situ- ations in the form of inequity aversions when they have the disadvantage. The current study focused on play fights in gorillas to explore for the first time, to our knowledge, if/how non-human species respond to inequities in natural social settings. Hitting causes a naturally occurring inequity among individuals and here it was specifically assessed how the hitters and their partners engaged in play chases that followed the hitting. The results of this work showed that the hitters significantly more often moved first to run away immediately after the encounter than their partners. These findings provide evidence that non-human species respond to inequities by trying to maintain their competitive advantages. We conclude that non-human pri- mates, like humans, may show different responses to inequities and that they may modify them depending on if they have the advantage or the disadvantage.