We know that we don't know: Children's understanding of common ignorance in a coordination game.

Common ground is the knowledge, beliefs, and suppositions shared between partners in an interaction. Previous research has focused extensively on what partners know they know together, that is, "common knowledge." However, another important aspect of common ground is what partners know they do not know together, that is, "common ignorance." A new coordination game was designed to investigate children's use of common ignorance. Without communicating or seeing each other's decisions, 4- to 8-year-olds needed to make the same decision as their partner about whether to try to retrieve a reward. To retrieve it, at least one of them needed to know a secret code. The knowledge/ignorance of both partners was ostensively manipulated by showing one partner, both partners, or neither partner the secret code in four conditions: common knowledge (both knew the code), common ignorance (neither partner knew the code), common privileged self knowledge (only children knew the code), and common privileged other knowledge (only their partner knew the code). Children's decisions, latency, and uncertainty were coded. Results showed that the common ignorance states were generally more difficult than the common knowledge states. Unexpectedly, children at all ages had difficulty with coordinating when their partner knew the code but they themselves did not (common privileged other knowledge). This study shows that, along with common knowledge, common ignorance and common privileged self knowledge and other knowledge also play important roles in coordinating with others but may develop differently.

Young children use imitation communicatively.

There is growing evidence that children imitate not just to learn from others but also to affiliate socially with them. However, although imitation can convey a wealth of affiliative information to others, it is not yet known whether imitators intend for this to be the case. In particular, we do not know whether children imitate communicatively in some contexts, expending extra effort to make sure that the demonstrator sees their imitation. Here, in two experiments (N = 20 and N = 48, respectively), we tested whether preschool-age children modify their imitation when needed to ensure that the demonstrator sees it. In each trial, children were shown a demonstration. Then, for their response, in one condition a barrier obscured the demonstrator's view of children's imitation unless children raised their arms above the barrier while imitating. In the other condition the demonstrator was able to see children's imitation without any additional effort from children. Results from both experiments showed that children were significantly more likely to imitate with their arms raised when their actions would otherwise be obscured from view. In the second experiment, we also coded for other communicative behaviors (e.g., social smiles, eye contact, showing gestures) and found that children often displayed communicative behaviors while imitating, as expected, in both conditions. Thus, young children actively use imitation communicatively in some contexts.

When does it pay to follow the crowd? Children optimize imitation of causally irrelevant actions performed by a majority.

Cultural evolutionary theory posits that human cultural complexity rests on a set of adaptive learning biases that help to guide functionality and optimality in social learning, but this sits in contrast with the commonly held view that children are unselective "over-imitators." Here, we tested whether 4- and 6-year-old children use social learning biases flexibly to fine-tune their copying of irrelevant actions. Children watched a video of a majority demonstrating causally irrelevant actions and a minority demonstrating only causally relevant actions. In one condition observers approved of the majority and disapproved of the minority, and in the other condition observers watched the majority and minority neutrally. Results showed that both 4- and 6-year-olds copied the inefficient majority more often than the efficient minority when the observers had approved of the majority's actions, but they copied the efficient minority significantly more when the observers had watched neutrally. We discuss the implications of children's optimal selectivity in copying and the importance of integrating social approval into majority-biased learning when acquiring norms and conventions and in broader processes of cultural evolution.

Common knowledge that help is needed increases helping behavior in children.

Although there is considerable evidence that at least some helping behavior is motivated by genuine concern for others' well-being, sometimes we also help solely out of a sense of obligation to the persons in need. Our sense of obligation to help may be particularly strong when there is common knowledge between the helper and the helpee that the helpee needs help. To test whether children's helping behavior is affected by having common knowledge with the recipient about the recipient's need, 6-year-olds faced a dilemma: They could either collect stickers or help an experimenter. Children were more likely to help when they and the experimenter had common knowledge about the experimenter's plight (because they heard it together) than when they each had private knowledge about it (because they heard it individually). These results suggest that already in young children common knowledge can heighten the sense of obligation to help others in need.

Selective copying of the majority suggests children are broadly "optimal-" rather than "over-" imitators.

Human children, in contrast to other species, are frequently cast as prolific "over-imitators". However, previous studies of "over-imitation" have overlooked many important real-world social dynamics, and may thus provide an inaccurate account of this seemingly puzzling and potentially maladaptive phenomenon. Here we investigate this topic using a cultural evolutionary approach, focusing particularly on the key adaptive learning strategy of majority-biased copying. Most "over-imitation" research has been conducted using consistent demonstrations to the observer, but we systematically varied the frequency of demonstrators that 4- to 6-year-old children observed performing a causally irrelevant action. Children who "over-imitate" inflexibly should copy the majority regardless of whether the majority solution omits or includes a causally irrelevant action. However, we found that children calibrated their tendency to acquire the majority behavior, such that copying did not extend to majorities that performed irrelevant actions. These results are consistent with a highly functional, adaptive integration of social and causal information, rather than explanations implying unselective copying or causal misunderstanding. This suggests that our species might be better characterized as broadly "optimal-" rather than "over-" imitators.

The Early Emergence of Guilt-Motivated Prosocial Behavior.

Guilt serves vital prosocial functions: It motivates transgressors to make amends, thus restoring damaged relationships. Previous developmental research on guilt has not clearly distinguished it from sympathy for a victim or a tendency to repair damage in general. The authors tested 2- and 3-year-old children (N = 62 and 64, respectively) in a 2 × 2 design, varying whether or not a mishap caused harm to someone and whether children themselves caused that mishap. Three-year-olds showed greatest reparative behavior when they had caused the mishap and it caused harm, thus showing a specific effect of guilt. Two-year-olds repaired more whenever harm was caused, no matter by whom, thus showing only an effect of sympathy. Guilt as a distinct motivator of prosocial behavior thus emerges by at least 3 years.

I won't tell: Young children show loyalty to their group by keeping group secrets.

Group loyalty is highly valued. However, little is known about young children's loyal behavior. This study tested whether 4- and 5-year-olds (N=96) remain loyal to their group even when betraying it would be materially advantageous. Children and four puppets were allocated to novel groups. Two of these puppets (either in-group or out-group members) then told children a group secret and urged them not to disclose the secret. Another puppet (not assigned to either group) then bribed children with stickers to tell the secret. Across ages, children were significantly less likely to reveal the secret in the in-group condition than in the out-group condition. Thus, even young children are willing to pay a cost to be loyal to their group.

Young children show the bystander effect in helping situations.

Much research in social psychology has shown that otherwise helpful people often fail to help when bystanders are present. Research in developmental psychology has shown that even very young children help and that the presence of others can actually increase helping in some cases. In the current study, in contrast, 5-year-old children helped an experimenter at very high levels when they were alone but helped significantly less often in the presence of bystanders who were potentially available to help. In another condition designed to elucidate the mechanism underlying the effect, children's helping was not reduced when bystanders were present but confined behind a barrier and thus unable to help (a condition that has not been run in previous studies with adults). Young children thus show the bystander effect, and it is due not to social referencing or shyness to act in front of others but, rather, to a sense of a diffusion of responsibility.

Children infer affiliative and status relations from watching others imitate.

We investigated whether young children are able to infer affiliative relations and relative status from observing others' imitative interactions. Children watched videos showing one individual imitating another and were asked about the relationship between those individuals. Experiment 1 showed that 5-year-olds assume that individuals imitate people they like. Experiment 2 showed that children of the same age assume that an individual who imitates is relatively lower in status. Thus, although there are many advantages to imitating others, there may also be reputational costs. Younger children, 4-year-olds, did not reliably make either inference. Taken together, these experiments demonstrate that imitation conveys valuable information about third party relationships and that, at least by the age of 5, children are able to use this information in order to infer who is allied with whom and who is dominant over whom. In doing so, they add a new dimension to our understanding of the role of imitation in human social life.

The effects of collaboration and minimal-group membership on children's prosocial behavior, liking, affiliation, and trust.

Recent theoretical work has highlighted potential links between interpersonal collaboration and group membership in the evolution of human sociality. Here we compared the effects of collaboration and minimal-group membership on young children's prosocial behavior (i.e., helping and resource allocation), liking, affiliation, and trust. In a design that matched as closely as possible these two ways of connecting with others, we showed that 5-year-olds' behavior was affected similarly by collaboration and minimal-group membership; both increased children's preference for their partners on multiple dimensions and produced overall effects of a similar magnitude. In contrast, 3.5-year-olds did not have a strong preference for either collaborators or minimal in-group members. Thus, both collaboration and minimal-group membership are similarly effective in their influence on children's prosocial behavior and social preferences.

Stick with your group: young children's attitudes about group loyalty.

For adults, loyalty to the group is highly valued, yet little is known about how children evaluate loyalty. We investigated children's attitudes about loyalty in a third-party context. In the first experiment, 4- and 5-year-olds watched a video of two groups competing. Two members of the losing group then spoke. The disloyal individual said she wanted to win and therefore would join the other group. The loyal individual said she also wanted to win but would stay with her group. Children were then asked five forced-choice questions about these two individuals' niceness, trustworthiness, morality, and deservingness of a reward. The 5-year-olds preferred the loyal person across all questions; results for the 4-year-olds were considerably weaker but in the same direction. The second experiment investigated the direction of the effect in 5-year-olds. In this experiment, children answered questions about either a loyal individual, a disloyal individual, or a neutral individual. Children rated both the loyal and neutral individuals more positively than the disloyal individual across a number of measures. Thus, whereas disloyal behavior is evaluated unfavorably by children, loyal behavior is the expected norm. These results suggest that, at least from 5 years of age, children understand that belonging to a group entails certain commitments. This marks an important step in their own ability to negotiate belonging and become trustworthy and reliable members of their social groups.

Eighteen-month-olds understand false beliefs in an unexpected-contents task.

Recent studies suggest that infants understand that others can have false beliefs. However, most of these studies have used looking time measures, and the few that have used behavioral measures are all based on the change-of-location paradigm, leading to claims that infants might use behavioral rules instead of mental state understanding to pass these tests. We investigated infants' false-belief reasoning using a different paradigm. In this unexpected-contents helping task, 18-month-olds were familiarized with boxes for blocks that contained blocks. When an experimenter subsequently reached for a box for blocks that now contained a spoon, infants based their choice of whether to give her a spoon or a block on her true or false belief about which object the block box contained. These results help to demonstrate the flexibility of infants' false-belief understanding.

Limitations to the cultural ratchet effect in young children.

Although many animal species show at least some evidence of cultural transmission, broadly defined, only humans show clear evidence of cumulative culture. In the current study, we investigated whether young children show the "ratchet effect," an important component of cumulative culture--the ability to accumulate efficient modifications across generations. We tested 16 diffusion chains--altogether consisting of 80 children--to see how they solved an instrumental task (i.e., carrying something from one location to another). We found that when the chain was seeded with an inefficient way of solving the task, 4-year-olds were able to innovate and transmit these innovations so as to reach a more efficient solution. However, when it started out with relatively efficient solutions already (i.e., the ones that children in a control condition discovered for themselves), there were no further techniques invented and/or transmitted beyond that. Thus, young children showed the ratchet effect to a limited extent, accumulating efficient modifications but not going beyond the inventive level of the individual.

Being mimicked increases prosocial behavior in 18-month-old infants.

Most previous research on imitation in infancy has focused on infants' learning of instrumental actions on objects. This study focused instead on the more social side of imitation, testing whether being mimicked increases prosocial behavior in infants, as it does in adults (van Baaren, Holland, Kawakami, & van Knippenberg, 2004). Eighteen-month-old infants (N = 48) were either mimicked or not by an experimenter; then either that experimenter or a different adult needed help. Infants who had previously been mimicked were significantly more likely to help both adults than infants who had not been mimicked. Thus, even in infancy, mimicry has positive social consequences: It promotes a general prosocial orientation toward others.

Selective imitation of in-group over out-group members in 14-month-old infants.

Recent research has shown that infants are more likely to engage with in-group over out-group members. However, it is not known whether infants' learning is influenced by a model's group membership. This study investigated whether 14-month-olds (N = 66) selectively imitate and adopt the preferences of in-group versus out-group members. Infants watched an adult tell a story either in their native language (in-group) or a foreign language (out-group). The adult then demonstrated a novel action (imitation task) and chose 1 of 2 objects (preference task). Infants did not show selectivity in the preference task, but they imitated the in-group model more faithfully than the out-group model. This suggests that cultural learning is beginning to be truly cultural by 14 months of age.

Young children's understanding of cultural common ground.

Human social interaction depends on individuals identifying the common ground they have with others, based both on personally shared experiences and on cultural common ground that all members of the group share. We introduced 3- and 5-year-old children to a culturally well-known object and a novel object. An experimenter then entered and asked, 'What is that?', either as a request for information or in a recognitory way. When she was requesting information, both 3- and 5-year-olds assumed she was asking about the novel object. When she seemed to recognize an object, 5-year-olds assumed she was referring to the culturally well-known object. Thus, by 3 years of age, children are beginning to understand that they share cultural common ground with other members of their group.

One-year-old infants follow others' voice direction.

We investigated 1-year-old infants' ability to infer an adult's focus of attention solely on the basis of her voice direction. In Studies 1 and 2, 12- and 16-month-olds watched an adult go behind a barrier and then heard her verbally express excitement about a toy hidden in one of two boxes at either end of the barrier. Even though they could not see the adult, infants of both ages followed her voice direction to the box containing the toy. Study 2 showed that infants could do this even when the adult was positioned closer to the incorrect box while she vocalized toward the correct one (and thus ruled out the possibility that infants were merely approaching the source of the sound). In Study 3, using the same methods as in Study 2, we found that chimpanzees performed the task at chance level. Our results show that infants can determine the focus of another person's attention through auditory information alone-a useful skill for establishing joint attention.

Great apes infer others' goals based on context.

In previous studies claiming to demonstrate that great apes understand the goals of others, the apes could potentially have been using subtle behavioral cues present during the test to succeed. In the current studies, we ruled out the use of such cues by making the behavior of the experimenter identical in the test phase of both the experimental and control conditions; the only difference was the preceding "context." In the first study, apes interpreted a human's ambiguous action as having the underlying goal of opening a box, or not, based on that human's previous actions with similar boxes. In the second study, chimpanzees learned that when a human stood up she was going to go get food for them, but when a novel, unexpected event happened, they changed their expectation-presumably based on their understanding that this new event led the human to change her goal. These studies suggest that great apes do not need concurrent behavioral cues to infer others' goals, but can do so from a variety of different types of cues-even cues displaced in time.

Twelve-month-olds' comprehension and production of pointing.

This study explored whether infants aged 12 months already recognize the communicative function of pointing gestures. Infants participated in a task requiring them to comprehend an adult's informative pointing gesture to the location of a hidden toy. They mostly succeeded in this task, which required them to infer that the adult was attempting to direct their attention to a location for a reason - because she wanted them to know that a toy was hidden there. Many of the infants also reversed roles and produced appropriate pointing gestures for the adult in this same game, and indeed there was a correlation such that comprehenders were for the most part producers. These findings indicate that by 12 months of age infants are beginning to show a bidirectional understanding of communicative pointing.

Showing and giving: from incipient to conventional forms.

Understanding humans' motivation and capacity for social interaction requires understanding communicative gestures. Gestures are one of the earliest means that infants employ to communicate with others, and showing and giving are among the earliest-emerging gestures. However, there are limited data on the processes that lead up to the emergence of conventional showing and giving gestures. This study aimed to provide such data. Twenty-five infants were assessed longitudinally at monthly intervals from 6 to 10 months of age using a variety of methods (elicitation procedures, free play observations and maternal interviews), as well as via questionnaires conducted at 11-12 months. A particular focus was on pre-conventional, incipient gestures, behaviours that involved some components of conventional gestures, but lacked other important components. We present observational evidence that at least some of these behaviours (observed as early as 7 months of age) were communicative and make the case for how conventional showing and giving may emerge gradually in the context of social interactions. We also discuss the influence of maternal interpretations of these early behaviours on their development. Overall, the study seeks to draw attention to the importance of understanding the cognitive, motor and interactional processes that lead to the emergence of infants' earliest communicative gestures. This article is part of the theme issue 'Revisiting the human 'interaction engine': comparative approaches to social action coordination'.