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Pragmatic theories assume that during communicative exchanges humans strive to be optimally informative and spontaneously adjust their communicative signals to satisfy their addressee’s epistemic needs. To investigate this ability in infants, we designed a task in which 18-month-olds had to point at the target object they wanted to receive. In Experiment 1, we found that when the target was placed behind a distractor object, infants appropriately modified their pointing to avoid mistakenly indicating the distractor to their partner. When the objects were covered, and their communicative partner had no information (Experiment 2) or incorrect information (Experiment 3) about the target’s location – as opposed to being knowledgeable about it – infants pointed at the target more often and employed modified pointing more frequently when it was necessary. This demonstrates that 18-month-olds can take into account their communicative partner’s epistemic states and provide her with relevant information through optimally informative deictic gestures.

When facing uncertainty, humans often build mental models of alternative outcomes. Considering diverging scenarios allows agents to respond adaptively to different actual worlds by developing contingency plans (covering one's bases). In a pre-registered experiment, we tested whether chimpanzees (Pan troglodytes) prepare for two mutually exclusive possibilities. Chimpanzees could access two pieces of food, but only if they successfully protected them from a human competitor. In one condition, chimpanzees could be certain about which piece of food the human experimenter would attempt to steal. In a second condition, either one of the food rewards was a potential target of the competitor. We found that chimpanzees were significantly more likely to protect both pieces of food in the second relative to the first condition, raising the possibility that chimpanzees represent and prepare effectively for different possible worlds.

Central to rationality is the ability to think about our beliefs, and make sure they are based on good reasons. If we have several conflicting reasons to believe something, we wait for more information before making a decision, perhaps rechecking our original reasons before proceeding. In these studies, we presented great apes and young children with conflicting evidence about the location of a reward. We found that apes double-checked the evidence for their original choice before making a final decision, revealing an awareness of their own beliefs and reasoning not hitherto documented. Young children, in contrast, were more sensitive to peer disagreement than conflicting physical evidence, illustrating the distinctively social nature of human rationality.

Humans are ultra-social: they spontaneously incorporate others’ mental states into their action-planning (Kaminski et al.,2008), and altercentric: their behavior is influenced by others’ perspectives, even perspectives irrelevant to their instrumental goal (Kampis & Southgate, 2020). Recent evidence suggests that similarly to human infants, non-human great apes anticipate others’ actions based on their beliefs (Krupenye et al.,2016; Kano et al, 2019); raising the critical question whether altercentrism is uniquely human. In two experiments, we tested chimpanzees, bonobos, and orangutans in a manual search paradigm adapted from Mendes et al. (2008). These experiments replicated findings demonstrating apes’ first-person object-tracking abilities. Experiment 1 found no evidence for altercentrism: apes’ search behavior was not spontaneously modulated by another agent’s beliefs (unlike 14-month-old human infants; Kampis & Kovács, 2020). Experiment 2 found tentative evidence that apes inferred a person’s actions based on her beliefs, and adapted their own actions based on this information.

Effective social co-ordination benefits from mentally representing a partner’s actions. Chimpanzees can successfully work together, but the cognitive mechanisms they employ to aid social co-ordination remain unclear. Studies of action planning show that, like humans, chimpanzees demonstrate the end-state-comfort effect; considering the end of an action sequence during motor-planning, e.g. using an initially awkward grasp when handling an overturned glass to facilitate how it will end up being held. Human research shows that we extend this to a partner; we pass objects in a way that facilitates the action to be performed with them. Here, we assessed the location in which chimpanzees passed a tool to an experimenter to investigate action accommodation. We manipulated experimenter hand location and their ease of access to locations and found some effect on passing behaviour indicating that, under certain conditions, chimpanzees consider a partner’s actions when planning their own actions in a co-ordination context.

A crucial step in language evolution was likely joint attention with alternating gaze between vocalizing individualsand an object. This triadic interaction likely formed a foundation for labeling of objects. We have argued that vocalizationsused for “social glue” – flexible low intensity and low arousal vocalizations given during e.g. grooming, keeping in contact withthe group, etc. – are a probable source of raw material for first labels. It is critical that these vocalizations be socially directed,by gaze contact. We longitudinally investigated directed gaze during vocalizations in low arousal interactions during the firstyear in three bonobo mother-infant pairs and compared them with 9 human mother-infant pairs. We found that bonobo infantsdirected their gaze to a conspecific during vocalizations only 8% of the time while human infants directed it 44% of the time.

Cognition presents evolutionary research with one of its greatest challenges. Cognitive evolution has been explained at the proximate level by shifts in absolute and relative brain volume and at the ultimate level by differences in social and dietary complexity. However, no study has integrated the experimental and phylogenetic approach at the scale required to rigorously test these explanations. Instead, previous research has largely relied on various measures of brain size as proxies for cognitive abilities. We experimentally evaluated these major evolutionary explanations by quantitatively comparing the cognitive performance of 567 individuals representing 36 species on two problem-solving tasks measuring self-control. Phylogenetic analysis revealed that absolute brain volume best predicted performance across species and accounted for considerably more variance than brain volume controlling for body mass. This result corroborates recent advances in evolutionary neurobiology and illustrates the cognitive consequences of cortical reorganization through increases in brain volume. Within primates, dietary breadth but not social group size was a strong predictor of species differences in self-control. Our results implicate robust evolutionary relationships between dietary breadth, absolute brain volume, and self-control. These findings provide a significant first step toward quantifying the primate cognitive phenome and explaining the process of cognitive evolution.

Recognition and memory of familiar conspecifics provides the foundation for complex sociality and is vital to navigating an unpredictable social world [Tibbetts and Dale, Trends Ecol. Evol. 22, 529-537 (2007)]. Human social memory incorporates content about interactions and relationships and can last for decades [Sherry and Schacter, Psychol. Rev. 94, 439-454 (1987)]. Long-term social memory likely played a key role throughout human evolution, as our ancestors increasingly built relationships that operated across distant space and time [Malone et al., Int. J. Primatol. 33, 1251-1277 (2012)]. Although individual recognition is widespread among animals and sometimes lasts for years, little is known about social memory in nonhuman apes and the shared evolutionary foundations of human social memory. In a preferential-looking eye-tracking task, we presented chimpanzees and bonobos (N = 26) with side-by-side images of a previous groupmate and a conspecific stranger of the same sex. Apes' attention was biased toward former groupmates, indicating long-term memory for past social partners. The strength of biases toward former groupmates was not impacted by the duration apart, and our results suggest that recognition may persist for at least 26 y beyond separation. We also found significant but weak evidence that, like humans, apes may remember the quality or content of these past relationships: apes' looking biases were stronger for individuals with whom they had more positive histories of social interaction. Long-lasting social memory likely provided key foundations for the evolution of human culture and sociality as they extended across time, space, and group boundaries. [ABSTRACT FROM AUTHOR]

In the handful of existing comparative false belief studies, chimpanzees have consistently failed tests that 5- to 6-year-old children have passed. However, those tests were either explicitly cooperative communicativeor competitive, both of which are problematic for different reasons. We therefore devised a new change-of-contents false belief test for children and chimpanzees that did not include these problematic elements. Nevertheless, chimpanzees showed no evidence of understanding false beliefs (consistent with past research, however, children showed a clear improvement in test performance from 3.5 to 4.5 years of age). Our results suggest that the cooperative-communicative or competitive nature of previous false belief tests was not solely responsible for chimpanzees’ failure. That chimpanzees have now also failed a more socially neutral test supports the conclusion that chimpanzees may simply not recognize false belief states in others. Additionally, our test departs from a near exclusive reliance on the change-of-location paradigm in false belief research. It therefore expands the repertoire of methods available for testing false belief understanding with minimally verbal and nonverbal procedures.