Children and adults' intuitions of what people can believe.

Two preregistered studies tested how 5- to 6-year-olds, 7- to 8-year-olds, and adults judged the possibility of holding alternative beliefs (N = 240, 110 females, U.S. sample, mixed ethnicities, data collected from September 2020 through October 2021). In Study 1, children and adults thought people could not hold different beliefs when their initial beliefs were supported by evidence (but judged they could without this evidential constraint). In Study 2, children and adults thought people could not hold different beliefs when their initial beliefs were moral beliefs (but judged they could without this moral constraint). Young children viewed moral beliefs as more constrained than adults. These results suggest that young children already have sophisticated intuitions of the possibility of holding various beliefs and how certain beliefs are constrained.

Disagreement reduces overconfidence and prompts exploration in young children.

Can the experience of disagreement lead young children to reason in more sophisticated ways? Across two preregistered studies, four- to six-year-old US children (N = 136, 50% female, mixed ethnicities, data collected 2020-2022) experienced either a disagreement or an agreement with a confederate about a causal mechanism after being presented with ambiguous evidence. We measured (1) children's confidence in their belief before and after the (dis)agreement, and (2) how long children searched for information about the correct answer. Disagreement, especially with an expert (Experiment 2), reduced overconfidence and prompted children to search longer for information, compared to agreement. Together, our findings suggest possibilities for interventions aimed at fostering humility and learning across the lifespan.

Chimpanzee and Human Risk Preferences Show Key Similarities.

Risk preference impacts how people make key life decisions related to health, wealth, and well-being. Systematic variations in risk-taking behavior can be the result of differences in fitness expectations, as predicted by life-history theory. Yet the evolutionary roots of human risk-taking behavior remain poorly understood. Here, we studied risk preferences of chimpanzees (86 Pan troglodytes; 47 females; age = 2-40 years) using a multimethod approach that combined observer ratings with behavioral choice experiments. We found that chimpanzees' willingness to take risks shared structural similarities with that of humans. First, chimpanzees' risk preference manifested as a traitlike preference that was consistent across domains and measurements. Second, chimpanzees were ambiguity averse. Third, males were more risk prone than females. Fourth, the appetite for risk showed an inverted-U-shaped relation to age and peaked in young adulthood. Our findings suggest that key dimensions of risk preference appear to emerge independently of the influence of human cultural evolution.

Chimpanzees prepare for alternative possible outcomes.

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.

Evidence for a developmental shift in the motivation underlying helping in early childhood.

We investigated children's positive emotions as an indicator of their underlying prosocial motivation. In Study 1, 2-, and 5-year-old children (N = 64) could either help an individual or watch as another person provided help. Following the helping event and using depth sensor imaging, we measured children's positive emotions through changes in postural elevation. For 2-year-olds, helping the individual and watching another person help was equally rewarding; 5-year-olds showed greater postural elevation after actively helping. In Study 2, 5-year-olds' (N = 59) positive emotions following helping were greater when an audience was watching. Together, these results suggest that 2-year-old children have an intrinsic concern that individuals be helped whereas 5-year-old children have an additional, strategic motivation to improve their reputation by helping.

Children's developing ability to adjust their beliefs reasonably in light of disagreement.

Two preregistered experiments (N = 218) investigated children's developing ability to respond reasonably to disagreement. U.S. children aged 4-9, and adults (50% female, mostly white) formed an initial belief, and were confronted with the belief of a disagreeing other, whose evidence was weaker, stronger than, or equal to participants' evidence. With age, participants were increasingly likely to maintain their initial belief when their own evidence was stronger, adopt the other's belief when their evidence was weaker, and suspend judgment when both had equally strong evidence. Interestingly, 4- to 6-year-olds only suspended judgment reliably when this was assessed via the search for additional information (Experiment 2). Together, our experiments suggest that the ability to respond reasonably to disagreement develops over the preschool years.

Do chimpanzees reason logically?

Psychologists disagree about the development of logical concepts such as or and not. While some theorists argue that infants reason logically, others maintain that logical inference is contingent on linguistic abilities and emerges around age 4. In this Registered Report, we conducted five experiments on logical reasoning in chimpanzees. Subjects (N = 16; 10 females; M = 24 years) participated in the same setup that has been administered to children: the two-, three-, and four-cup-task. Chimpanzees performed above chance in the two-cup-, but not in the three-cup-task. Furthermore, chimpanzees selected the logically correct option more often in the test than the control condition of the four-cup-task. We discuss possible interpretations of these findings and conclude that our results are most consistent with non-deductive accounts.

Chimpanzees consider freedom of choice in their evaluation of social action.

Judgements of wrongdoing in humans often hinge upon an assessment of whether a perpetrator acted out of free choice: whether they had more than one option. The classic inhibitors of free choice are constraint (e.g. having your hands tied together) and ignorance (e.g. being unaware that an alternative exists). Here, across two studies, we investigate whether chimpanzees consider these factors in their evaluation of social action. Chimpanzees interacted with a human experimenter who handed them a non-preferred item of food, either because they were physically constrained from accessing the preferred item (Experiment 1) or because they were ignorant of the availability of the preferred item (Experiment 2). We found that chimpanzees were more likely to accept the non-preferred food and showed fewer negative emotional responses when the experimenter was physically constrained compared with when they had free choice. We did not, however, find an effect of ignorance on chimpanzee's evaluation. Freedom of choice factors into chimpanzees' evaluation of how they are treated, but it is unclear whether mental state reasoning is involved in this assessment.

How children revise their beliefs in light of reasons.

We investigate how the ability to respond appropriately to reasons provided in discourse develops in young children. In Study 1 (N = 58, Germany, 26 girls), 4- and 5-, but not 3-year-old children, differentiated good from bad reasons. In Study 2 (N = 131, Germany, 64 girls), 4- and 5-year-old children considered both the strength of evidence for their initial belief and the quality of socially provided reasons for an alternative view when deciding whether to change their minds. Study 3 (N = 80, the United States, 42 girls, preregistered) shows that 4- and 5-year-old children also consider meta-reasons (reasons about reasons) in their belief revision. These results suggest that by age 4, children possess key critical thinking capacities for participating in public discourse.

The influence of friendship and merit on children's resource allocation in three societies.

Recent work has suggested that principles of fairness that seem like natural laws to the Western mind, such as sharing more of the spoils with those who contributed more, can in fact vary significantly across populations. To build a better understanding of the developmental roots of population differences with respect to fairness, we investigated whether 7-year-old children (N = 432) from three cultural backgrounds-Kenya, China, and Germany-consider friendship and merit in their distribution of resources and how they resolve conflicts between the two. We found that friendship had considerable and consistent influence as a cross-culturally recurrent motivation: children in all three cultures preferentially shared with a friend rather than with a neutral familiar peer. On the other hand, the role of merit in distribution seemed to differ cross-culturally: children in China and Germany, but not in Kenya, selectively distributed resources to individuals who worked more. When we pitted friendship against merit, there was an approximately even split in all three cultures between children who favored the undeserving friend and children who shared with the hard-working neutral individual. These results demonstrate commonalities and variability in fairness perceptions across distinct cultures and speak to the importance of cross-cultural research in understanding the development of the human mind.

Human children but not chimpanzees make irrational decisions driven by social comparison.

Human evolutionary success is often argued to be rooted in specialized social skills and motivations that result in more prosocial, rational and cooperative decisions. One manifestation of human ultra-sociality is the tendency to engage in social comparison. While social comparison studies typically focus on cooperative behaviour and emphasize concern for fairness and equality, here we investigate the competitive dimension of social comparison: a preference for getting more than others, expressed in a willingness to maximize relative payoff at the cost of absolute payoff. Chimpanzees and human children (5-6- and 9-10-year-olds) could decide between an option that maximized their absolute payoff (but put their partner at an advantage) and an option that maximized their relative payoff (but decreased their own and their partner's payoff). Results show that, in contrast to chimpanzees and young children, who consistently selected the rational and payoff-maximizing option, older children paid a cost to reduce their partner's payoff to a level below their own. This finding demonstrates that uniquely human social skills and motivations do not necessarily lead to more prosocial, rational and cooperative decision-making.

Children engage in competitive altruism.

Humans cultivate their reputations as good cooperators, sometimes even competing with group mates, to appear most cooperative to individuals during the process of selecting partners. To investigate the ontogenetic origins of such "competitive altruism," we presented 5- and 8-year-old children with a dyadic sharing game in which both children simultaneously decided how many rewards to share with each other. The children were either observed by a third-person peer or not. In addition, the children either knew that one of them would be picked for a subsequent collaborative game or had no such knowledge. We found that by 8 years of age, children were more generous in the sharing game not only when their behavior was observed by a third party but also when it could affect their chances of being chosen for a subsequent game. This is the first demonstration of competitive altruism in young children, and as such it underscores the important role of partner choice (and individual awareness of the process) in encouraging human cooperation from an early age.

Concern for Group Reputation Increases Prosociality in Young Children.

The motivation to build and maintain a positive personal reputation promotes prosocial behavior. But individuals also identify with their groups, and so it is possible that the desire to maintain or enhance group reputation may have similar effects. Here, we show that 5-year-old children actively invest in the reputation of their group by acting more generously when their group's reputation is at stake. Children shared significantly more resources with fictitious other children not only when their individual donations were public rather than private but also when their group's donations (effacing individual donations) were public rather than private. These results provide the first experimental evidence that concern for group reputation can lead to higher levels of prosociality.

Social disappointment explains chimpanzees' behaviour in the inequity aversion task.

Chimpanzees' refusal of less-preferred food when an experimenter has previously provided preferred food to a conspecific has been taken as evidence for a sense of fairness. Here, we present a novel hypothesis-the social disappointment hypothesis-according to which food refusals express chimpanzees' disappointment in the human experimenter for not rewarding them as well as they could have. We tested this hypothesis using a two-by-two design in which food was either distributed by an experimenter or a machine and with a partner present or absent. We found that chimpanzees were more likely to reject food when it was distributed by an experimenter rather than by a machine and that they were not more likely to do so when a partner was present. These results suggest that chimpanzees' refusal of less-preferred food stems from social disappointment in the experimenter and not from a sense of fairness.

The impact of choice on young children's prosocial motivation.

The current study explored how freedom of choice affects preschoolers' prosocial motivation. Children (3- and 5-year-olds) participated in either a choice condition (where they could decide for themselves whether to help or not) or a no-choice condition (where they were instructed to help). Prosocial motivation was subsequently assessed by measuring the amount children helped an absent peer in the face of an attractive alternative game. The 5-year-olds provided with choice helped more than the children not provided with choice, and this effect was stronger for girls than for boys. There was no difference between conditions for the 3-year-olds. These results highlight the importance of choice in young children's prosocial development.

The effects of being watched on resource acquisition in chimpanzees and human children.

Animals react in many different ways to being watched by others. In the context of cooperation, many theories emphasize reputational effects: Individuals should cooperate more if other potential cooperators are watching. In the context of competition, individuals might want to show off their strength and prowess if other potential competitors are watching. In the current study, we observed chimpanzees and human children in three experimental conditions involving resource acquisition: Participants were either in the presence of a passive observer (observed condition), an active observer who engaged in the same task as the participant (competition condition), or in the presence of but not directly observed by a conspecific (mere presence condition). While both species worked to acquire more resources in the competition condition, children but not chimpanzees also worked to acquire more resources in the observer condition (compared to the mere presence condition). These results suggest evolutionary continuity with regard to competition-based observer effects, but an additional observer effect in young children, potentially arising from an evolutionary-based concern for cooperative reputation.

Chimpanzees trust conspecifics to engage in low-cost reciprocity.

Many of humans' most important social interactions rely on trust, including most notably among strangers. But little is known about the evolutionary roots of human trust. We presented chimpanzees (Pan troglodytes) with a modified version of the human trust game--trust in reciprocity--in which subjects could opt either to obtain a small but safe reward on their own or else to send a larger reward to a partner and trust her to reciprocate a part of the reward that she could not access herself. In a series of three studies, we found strong evidence that in interacting with a conspecific, chimpanzees show spontaneous trust in a novel context; flexibly adjust their level of trust to the trustworthiness of their partner and develop patterns of trusting reciprocity over time. At least in some contexts then, trust in reciprocity is not unique to humans, but rather has its evolutionary roots in the social interactions of humans' closest primate relatives.

Young children care more about their reputation with ingroup members and potential reciprocators.

Human cooperation depends on individuals caring about their reputation, and so they sometimes attempt to manage them strategically. Here we show that even 5-year-old children strategically manage their reputation. In an experimental setting, children shared significantly more resources with an anonymous recipient when (1) the child watching them could reciprocate later, and (2) the child watching them was an ingroup rather than an outgroup member (as established by minimal group markers). This study is not only the first to show that young children selectively invest in their reputation with specific individuals, but also the first to show that we care more about our reputation with ingroup than with outgroup members.

From outcome to process: A developmental shift in judgments of good reasoning.

What does it mean to reason well? One might argue that good reasoning means that the outcome of the reasoning process is correct: reaching the right belief. Alternatively, good reasoning might refer to the reasoning process itself: following the right epistemic procedures. In a preregistered study, we investigated children's (4-9-year-olds) and adults' judgments of reasoning in China and the US (N = 256). Participants of all age groups evaluated the outcome when the process was kept constant - they favored agents who reached correct over incorrect beliefs, and they evaluated the process when the outcome was kept constant - they preferred agents who formed their beliefs using valid over invalid procedures. Developmental changes emerged when we pitted outcome against process: young children weighed outcome more heavily than process; older children and adults showed the reverse preference. This pattern was constant across the two cultural contexts, with the switch from outcome to process happening earlier in development in China. These results suggest that children initially value what someone believes, but, with development, come to increasingly value how beliefs are formed.