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Intergroup bias has been a pervasive phenomenon throughout human history, but its psychological underpinnings are still the subject of debate. The present work tests whether intergroup attitudes and behaviors are motivated by ingroup positivity, outgroup negativity, or both, across the first few years of life. In two studies (total N = 128), children were introduced to an ingroup doll and an outgroup doll, and interacted with each one independently in a resource allocation task. Toddlers showed both ingroup positivity and outgroup negativity (Study 1). Preschoolers shifted from this pattern, showing positivity and avoiding negativity toward both ingroup and outgroup members (Study 2). Together, these studies suggest that outgroup negativity plays a stronger role in motivating early intergroup bias than previously thought.

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These studies investigate the influence of adults' explicit attention to commonalities of appearance on children's preference for individuals resembling themselves. Three findings emerged: (1) An adult's identification of two dolls' respective similarity to and difference from the child led 3-year-olds to prefer the similar doll (study 1, n = 32). (2) When the adult did not comment on similarity, children age 6 years but not younger preferred physically similar individuals (study 2, n = 68), suggesting that a spontaneous preference for physically similar others does not emerge before school age. (3) Four- but not 3-year-olds generalized an adult's pedagogical cues about similarity, leading them to prefer a self-resembling doll in a new context (study 3, n = 80). These findings collectively suggest that the preference for individuals resembling ourselves develops through a process of internalizing adults' attention to, and messages about, similarities of appearance.

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Two studies examined whether children (5- and 6-year-olds; 8- and 9-year-olds, n = 214) and adults (n = 72) consider social relationship when evaluating unhelpful or helpful actions. Participants learned about a person-in-need who was (or was not) helped by someone they knew (a friend) and someone they did not know (a stranger). Older children and adults judged an unhelpful friend as meaner than an unhelpful stranger, and judged a helpful stranger as nicer than a helpful friend. Younger children did not judge an unhelpful friend as any meaner than an unhelpful stranger, and they judged a helpful friend as nicer than a helpful stranger. These findings suggest that a mature appreciation of how social relationship matters for evaluation emerges relatively late in development.

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Previous research has demonstrated that toddlers are willing to punish those who harm others. This work, however, has predominantly focused on punishment in the form of resource reduction-taking away a resource or withholding access to a resource from an antisocial other. Here, in two studies, we examined whether 4- to 7-year-old children (N = 141) engage in direct, corporal punishment against antisocial others in third-party contexts. Children were given the opportunity to press buttons so that antisocial and prosocial puppets would be hit with a hammer. In Study 1, younger children (∼4-year-olds) hit the antisocial and prosocial puppets indiscriminately, whereas older children (∼7-year-olds) tended to preferentially hit the antisocial puppet. In Study 2, we tested a larger sample of 4- to 7-year-olds, and found that none of the children engaged in corporal punishment. Collapsing across both Studies 1 and 2 also indicated a null effect-children did not engage in third-party corporal punishment. We observed these findings even though children evaluated the antisocial puppet as mean and understood that pressing the hit button hurt the puppets. These findings suggest that children lack a strong desire to corporally punish third-party social wrongdoers. Our results illustrate the importance of considering different types of punishment in assessing the development of third-party punishment, and raise questions about the development of corporal third-party punishment.

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Learning what to eat is a critical problem that humans must solve over the course of ontogeny. Recent research underscores the importance of social learning processes to the development of food preferences in infancy and early childhood, but research investigating how (and whether) learned edibility information is generalized remains inconclusive. Here we investigate whether 18-month-olds generalize socially learned information about plant edibility. Across two experiments, infants watched an adult eat fruit from one type of plant and then were presented with a choice between two new plants: one was the same type of plant the adult had eaten from, and the other was a different type of plant. Infants' reaching and eating behavior was assessed during the choice phase. The results showed that 18-month-olds generalize edibility to the same type of plant. These findings provide new insights into the nature of human food learning processes early in development.

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Many scholars draw on evidence from evolutionary biology, behavioral economics, and infant research to argue that humans are "noble savages", endowed with indiscriminate kindness. We believe this is mistaken. While there is evidence for an early-emerging moral sense - even infants recognize and favor instances of fairness and kindness amongst third parties - altruistic behaviors are selective from the start. Babies and young children favor those who have been kind to them in the past, and favor familiar individuals over strangers. They hold strong biases for ingroup over outgroup and for self over other, and indeed are more unequivocally selfish than older children and adults. Much of what is most impressive about adult morality arises not through inborn capacities, but a fraught developmental process that involves exposure to culture and the exercise of rationality.

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When object A moves adjacent to a stationary object, B, and in that instant A stops moving and B starts moving, people irresistibly see this as an event in which A causes B to move. Real-world causal collisions are subject to Newtonian constraints on the relative speed of B following the collision, but here we show that perceptual constraints on the relative speed of B (which align imprecisely with Newtonian principles) define two categories of causal events in perception. Using performance-based tasks, we show that triggering events, in which B moves noticeably faster than A, are treated as being categorically different from launching events, in which B does not move noticeably faster than A, and that these categories are unique to causal events (Experiments 1 and 2). Furthermore, we show that 7- to 9-month-old infants are sensitive to this distinction, which suggests that this boundary may be an early-developing component of causal perception (Experiment 3).

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Differences in vocal fundamental (F0) and average formant (Fn) frequencies covary with body size in most terrestrial mammals, such that larger organisms tend to produce lower frequency sounds than smaller organisms, both between species and also across different sex and life-stage morphs within species. Here we examined whether three-month-old human infants are sensitive to the relationship between body size and sound frequencies. Using a violation-of-expectation paradigm, we found that infants looked longer at stimuli inconsistent with the relationship-that is, a smaller organism producing lower frequency sounds, and a larger organism producing higher frequency sounds-than at stimuli that were consistent with it. This effect was stronger for fundamental frequency than it was for average formant frequency. These results suggest that by three months of age, human infants are already sensitive to the biologically relevant covariation between vocalization frequencies and visual cues to body size. This ability may be a consequence of developmental adaptations for building a phenotype capable of identifying and representing an organism's size, sex and life-stage.

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When children's self-interests are at odds with their moral considerations, what do they do? In the current study of 5- and 6-year-olds (N=160), we asked (a) whether children would select the offering of a do-gooder over a neutral individual at a personal cost, (b) whether they would reject the offering of a wrongdoer over a neutral individual at a personal cost, and (c) whether these two types of decisions involve comparable levels of conflict. In the absence of material considerations, children preferred a nice character to a neutral one, but this preference was easily overcome for material gain; children accepted a larger offering from a neutral source over a smaller offering from a nice source. In contrast, children's aversion to negative characters was largely unaffected by the same material consideration; they rejected a larger offering from a mean source in favor of a smaller offering from a neutral source. In addition, children's response times indicated that deciding whether or not to "sell out" to a wrongdoer for personal gain engenders conflict but that deciding whether to take a lesser gain from a do-gooder does not. These findings indicate that children weigh both their own material interests and others' social behaviors when selecting social partners and, importantly, that an aversion to wrongdoers is a more powerful influence on these choices than an attraction to do-gooders.

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Background: At birth, the release of surfactant from alveolar type II cells (ATIIs) is stimulated by increased activity of the beta-adrenergic/adenylyl cyclase/cyclic 3'-5' adenosine monophosphate-signaling cascade. Atrial natriuretic peptide (ANP) stimulates surfactant secretion through natriuretic peptide receptor A (NPR-A). ANP inhibits adenylyl cyclase activity through its binding to NPR-C. We wished to further understand the role of the NPR-C in perinatal transition. Methods: We studied ATII expression of NPR-C in fetal and newborn sheep using immunohistochemistry, and surfactant secretion in isolated ATIIs by measuring 3[H] choline release into the media. Results: ANP induced surfactant secretion, and, at higher doses, it inhibits the stimulatory effect of the secretagogue terbutaline. ATII NPR-C expression decreased significantly after birth. Premature delivery also markedly decreased ANP and NPR-C in ATIIs. Co-incubation of terbutaline (10-4 M) with ANP (10-6 M) significantly decreased 3[H] choline release from isolated newborn ATII cells when compared with terbutaline alone; this inhibitory effect was mimicked by the specific NPR-C agonist, C-ANP (10-10 M). Conclusion: ANP may act as an important epithelial-derived inhibitor of surfactant release in the fetal lung, and downregulation of ANP and NPR-C following birth may sensitize ATII cells to the effects of circulating catecholamines, thus facilitating surfactant secretion.

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How unappealing are individuals who behave badly towards others? We show here that children and even infants, although motivated by material rewards, are nonetheless willing to incur costs to avoid "doing business" with a wrongdoer. When given the choice to accept a smaller offering from a do-gooder or a larger offering from a wrongdoer, children and infants chose to accept the smaller offering. It was only when the difference between the offerings was very large that their aversion to the wrongdoer was overcome by personal incentives. These findings show that a willingness to forgo self-interests when faced with wrongdoers is a fundamental aspect of human nature.

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It is human nature to like those who are like us. Even babies prefer individuals who share their tastes, and dislike those with contrasting views. However, our pluralistic society requires accepting differences and tolerating those who disagree. Can findings in infant research inform strategies to encourage acceptance of diversity?

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Generosity is greatly valued and admired, but can it sometimes be unappealing? The current study investigated 8- to 10-year-old children's (N = 128) preference for generous individuals, and the effects of social comparison on their preferences. In Experiment 1, children showed a strong preference for a generous to a stingy child; however, this preference was significantly reduced in a situation that afforded children a comparison of their own (lesser) generosity to that of another child. In Experiment 2, children's liking for a generous individual was not reduced when that individual was an adult, suggesting that similarity in age influences whether a child engages in social comparison. These findings indicate that, by middle childhood, coming up short in comparison with a peer can decrease one's liking for a generous individual.

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Recent research underscores the importance of social learning to the development of food preferences. Here, we explore whether social information about edibility--an adult placing something in his or her mouth--can be selectively tied to certain types of entities. Given that humans have relied on gathered plant resources across evolutionary time, and given the costs of trial-and-error learning, we predicted that human infants may possess selective social learning strategies that rapidly identify edible plants. Evidence from studies with 6- and 18-month-olds demonstrated that infants selectively identify plants, over artifacts, as food sources after seeing the same food-relevant social information applied to both object types. These findings are the first evidence for content-specific social learning mechanisms that facilitate the identification of edible plant resources. Evolved learning mechanisms such as these have enabled humans to survive and thrive in varied and changing environments.

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Young children dislike getting less than others, which might suggest a general preference for equal outcomes. However, young children are typically not averse to others receiving less than themselves. These results are consistent with two alternatives: young children might not have any preferences about others receiving less than themselves, or they might have preferences for others receiving less than themselves. We test these alternatives with 5- to 10-year-old children. We replicate previous findings that children will take a cost to avoid being at a relative disadvantage, but also find that 5- and 6-year-olds will spitefully take a cost to ensure that another's welfare falls below their own. This result suggests that the development of fairness includes overcoming an initial social comparison preference for others to get less relative to oneself.

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Plants have been central to human life as sources of food and raw materials for artifact construction over evolutionary time. But plants also have chemical and physical defenses (such as harmful toxins and thorns) that provide protection from herbivores. The presence of these defenses has shaped the behavioral strategies of non-human animals. Here we report evidence that human infants possess strategies that would serve to protect them from dangers posed by plants. Across two experiments, infants as young as eight months exhibit greater reluctance to manually explore plants compared to other entities. These results expand the growing literature showing that infants are sensitive to certain ancestrally recurrent dangers, and provide a basis for further exploration.

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Adults tend to like individuals who are similar to themselves, and a growing body of recent research suggests that even infants and young children prefer individuals who share their attributes or personal tastes over those who do not. In this study, we examined the nature and development of attitudes toward similar and dissimilar others in human infancy. Across two experiments with combined samples of more than 200 infant participants, we found that 9- and 14-month-old infants prefer individuals who treat similar others well and treat dissimilar others poorly. A developmental trend was observed, such that 14-month-olds' responses were more robust than were 9-month-olds'. These findings suggest that the identification of common and contrasting personal attributes influences social attitudes and judgments in powerful ways, even very early in life.

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A central feature of human psychology is our pervasive tendency to divide the social world into "us" and "them". We prefer to associate with those who are similar to us over those who are different, preferentially allocate resources to similar others, and hold more positive beliefs about similar others. Here we investigate the developmental origins of these biases, asking if preference for similar others occurs prior to language and extensive exposure to cultural norms. We demonstrate that, like adults, prelinguistic infants prefer those who share even trivial similarities with themselves, and these preferences appear to reflect a cognitive comparison process ("like me"/"not like me"). However, unlike adults, infants do not appear to prefer others with an utterly arbitrary similarity to themselves. Together, these findings suggest that the phenomena of ingroup bias, and enhanced interpersonal attraction toward those who resemble ourselves, may be rooted in an inherent preference for similarity to self, which itself may be enhanced during development by the influence of cultural values.

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Although adults generally prefer helpful behaviors and those who perform them, there are situations (in particular, when the target of an action is disliked) in which overt antisocial acts are seen as appropriate, and those who perform them are viewed positively. The current studies explore the developmental origins of this capacity for selective social evaluation. We find that although 5-mo-old infants uniformly prefer individuals who act positively toward others regardless of the status of the target, 8-mo-old infants selectively prefer characters who act positively toward prosocial individuals and characters who act negatively toward antisocial individuals. Additionally, young toddlers direct positive behaviors toward prosocial others and negative behaviors toward antisocial others. These findings constitute evidence that the nuanced social judgments and actions readily observable in human adults have their foundations in early developing cognitive mechanisms.

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