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In the United States, there is an unfortunate yet pervasive gender gap in wages: Women tend to make less than men for doing the same work. One prominent account for why this wage gap exists is that women and men negotiate differently. However, we currently do not know whether differences in negotiation are a product of extensive experience or are deeply rooted in development. Here, we brought data from children to bear on this important question. We gave 240 children between the ages of 4 and 9 years old a chance to negotiate for a bonus with a female or a male evaluator. Boys asked for the same bonus from a male and a female evaluator. Older girls, in contrast, asked for a smaller bonus from a male than a female evaluator. Our findings suggest that a gender gap in negotiation emerges surprisingly early in development, highlighting childhood as a key period for interventions.

Human adults use a range of social cues to obtain information about potential partners in cooperative contexts: we prefer partners who are competent, wealthy and generous, and those who abide by moral and social rules. One factor that carries particular weight is whether a prospective partner is fair. Here we ask whether children share this preference for fair partners and, if so, whether a prospective partner’s past fair behaviour influences children’s behaviour in a cooperative dilemma. Six- to nine-year-olds chose between partners who accepted or rejected resource allocations that were either strongly advantageously unequal, strongly disadvantageously unequal, or equal. They then played a one-shot Prisoner’s Dilemma Game with their chosen partner. Children overwhelmingly preferred to play with the partner who accepted rather than rejected allocations. Regardless of their partner choice decisions, children tended to defect in the Prisoner’s Dilemma Game, yet expected that their partners would be relatively more cooperative. Finally, children were more likely to cooperate with those they believed would cooperate. Together, these findings shed new light on the links between partner choice, fairness and cooperation in child development.

Risk and time preferences have often been viewed as reflecting inherent traits such as impatience and self-control. Here, we offer an alternative perspective, arguing that they are flexible and environmentally informed. In Study 1, we investigated risk and time preferences among children in the United States, India, and Argentina, as well as forager-horticulturalist Shuar children in Amazonian Ecuador. We find striking cross-cultural differences in behavior: children in India, the United States, and Argentina are more risk-seeking and future-oriented, whereas Shuar children are more risk-averse and exhibit more heterogeneous time preferences, on average preferring more today choices. To explore 1 of the socioecological forces that may be shaping these preferences, in Study 2, we compared the behavior of more and less market-integrated Shuar children, finding that those in market-integrated regions are more future-oriented and risk-seeking. These findings indicate that cross-cultural differences in risk and time preferences can be traced into childhood and may be influenced by the local environment. More broadly, our results contribute to a growing understanding of plasticity and variation in the development of behavior. (PsycINFO Database Record (c) 2020 APA, all rights reserved).

The coronavirus pandemic has had a significant influence on social interactions, introducing novel social norms such as mask-wearing and social distancing to protect people's health. Because these norms and associated practices are completely novel, it is unknown how children assess what kinds of interventions are appropriate under what circumstances and what principles they draw on in their decisions. We investigated children's reasoning about interventions against individuals who failed to adhere to COVID-19 norms. In this pre-registered study (N = 128), 4- to 7-year-olds heard stories about a norm violator, that is, a person who refuses to wear a mask in class (COVID condition) or wear indoor shoes in class when his or her shoes are muddy (Muddy Shoes condition). Children evaluated four different interventions-giving a mask/indoor shoes (Giving), preventing the person from entering (Exclusion), throwing a paper ball at the person (Throwing), and not intervening (Doing Nothing)-in terms of their rightness, niceness, and effectiveness. We found that across measures children evaluated Giving most positively, whereas they viewed Throwing most negatively. Doing Nothing and Exclusion received mixed evaluations across measures, revealing nuanced judgments of these interventions in children. In most measures, there was no difference between the COVID and Muddy Shoes conditions, suggesting that children's evaluations are not specific to the novel COVID-19 context. Together, our results show that children dynamically evaluate each intervention, taking multiple factors into account. The current study has implications for the development of interventions against norm violations.

We explore the developmental origins of intervention against gender-based pay inequality in 4- to 9-year-old children in the United States (N = 123; Study 1) and Peru (N = 115; Study 2), two countries characterized by different norms surrounding gender pay equity. We presented children with scenarios that featured gender-based pay inequality, and they could intervene at a cost to redistribute the earnings. We examined whether children favor equality or show gender bias in intervention depending on the direction of gender pay inequality. Across both societies, both girls and boys intervened against gender inequality regardless of its direction, a tendency that grew stronger with age. These findings suggest that despite developing in societies with existing gender pay inequalities, children strongly privilege equality.

Despite much recent empirical work on inequity aversion in nonhuman species, many questions remain about its distribution across taxa and the factors that shape its evolution and expression. Past work suggests that domestic dogs (Canis familiaris) and wolves (Canis lupus) are averse to inequitable resource distributions in contexts that call upon some degree of training such as ‘give paw’ and ‘buzzer press’ tasks. However, it is unclear whether inequity aversion appears in other canid species and in other experimental contexts. Using a novel inequity aversion task that does not require specific training, this study helps address these gaps by investigating inequity aversion in domestic dogs and a closely related but non-domesticated canid, the dingo (Canis dingo). Subjects were presented with equal and unequal reward distributions and given the opportunity to approach or refuse to approach allocations. Measures of interest were (1) subjects’ refusal to approach when getting no food; (2) approach latency; and (3) social referencing. None of these measures differed systematically across the inequity condition and control conditions in either dogs or dingoes. These findings add to the growing literature on inequity aversion in canids, providing data from a new species and a new experimental context. Additionally, they raise questions about the experimental features that must be in place for inequity aversion to appear in canids.

Much of human experience is informed by our ability to attribute mental states to others, a capacity known as theory of mind. While evidence for theory of mind in animals to date has largely been restricted to primates and other large-brained species, the use of ecologically-valid competitive contexts hints that ecological pressures for strategic deception may give rise to components of theory of mind abilities in distantly-related taxonomic groups. In line with this hypothesis, we show that cleaner wrasse (Labroides dimidiatus) exhibit theory of mind capacities akin to those observed in primates in the context of their cooperative cleaning mutualism. These results suggest that ecological pressures for strategic deception can drive human-like cognitive abilities even in very distantly related species., McAuliffe et al. demonstrate that in the context of cooperative cleaning, wild-caught female cleaner wrasse are more likely to cheat when their partners are out of view. This provides evidence that cleaner wrasse possess a building block of theory of mind: sensitivity to what others can and cannot see.

Forgiveness is a powerful feature of human social life, allowing for the restoration of positive cooperative relationships. Despite its importance, we know relatively little about how forgiveness develops during early life and the features that shape forgiveness decisions. Here, we investigated forgiveness behavior in children aged 5–10 years (N = 257) from the United States, varying transgressor intent and remorse in a behavioral task that pitted punishment against forgiveness. We found that baseline levels of forgiveness are high, suggesting that children assume the best of transgressors in the absence of information about intent and remorse. We also found age-related increases in sensitivity to intent but not remorse, such that older children are more likely to forgive accidental transgressions. Because forgiveness is an important tool in the human social toolkit, exploring the ways in which this ability develops across age can help us to better understand the early roots of human cooperation.

Recent research has shown that children’s sense of fairness is shaped in part by cultural practices, values, and norms. However, the specific social factors that motivate children’s fairness decisions remain poorly understood. The current study combined an ethnographic approach with experimental tests of fairness (the Inequity Game) in two Chinese schools with qualitatively different practices and norms. In the “University school,” children received explicit moral instruction on fairness reinforced by adults when supervising children’s activities. By contrast, in the “Community school,” children received less formal moral education and little adult supervision during play time, but norms of cooperation and fairness emerged through informal interactions with peers and other members of the community. Contrary to our predictions, children in both schools (N = 66) rejected both disadvantageous and advantageous allocations of resources in the test trials. However, in the very first practice trials, children from the Community school tended to reject all inequalities, whereas children from the University school tended to accept inequalities. We draw on the ethnographies of the schools to interpret these results, concluding that, despite the similarities in the experimental results, different motivations and social factors likely underlie the rejection of inequality in the two schools.

Ingroup favoritism influences third-party norm enforcement: Third-party punishers are more lenient when an ingroup member has been unfair. By contrast, in 2-party contexts, where punishers are the victims of unfairness, group bias effects are absent or inconsistent. Thus, group bias appears to be particularly influential when enforcing fairness among others, but less so when protecting oneself from unfairness. This would have implications for theories of how cooperation and intergroup cognition interact, but a more direct empirical test is lacking. To this end, developmental data are particularly useful as they can tell us whether and if so how group bias and fairness norm enforcement are related from their first emergence. Using a minimal groups manipulation, we induced ingroup bias in 6- to 9-year-olds and tested their willingness to reject disadvantageous (more for you) and advantageous (more for me) resource allocations when paired with in- and outgroup members. Group bias did not affect children's rejections of unfair allocations, although they reported that it was worse when an outgroup member had more. Our findings suggest that ingroup bias does not influence children's costly endorsement of equality, indicating that children perceive the equality norm to be indiscriminate and enforceable across group boundaries. (PsycInfo Database Record (c) 2020 APA, all rights reserved).

Children across diverse societies reject resource allocations that place them at a disadvantage (disadvantageous inequity aversion; DI). In certain societies, older children also reject advantageous allocations (advantageous inequity aversion; AI). Other work demonstrates that after collaboration, children reduce inequity by sharing. However, it is unknown whether collaboration leads to greater sharing because it encourages prosociality (Generosity Hypothesis) or because collaboration elicits stronger equitable tendencies (Equity Hypothesis). Here we use measures of inequity aversion that can disambiguate between these hypotheses. We tested 7- to 13-year-old children from rural India, a population that has shown DI but not AI, and 4- to 10-year-old children from rural Canada, a population that shows both AI and DI. Pairs of children worked either collaboratively or in parallel obtaining candy that was then used in a test of DI and AI. Results showed that in both societies collaboration did not encourage children to accept DI offers, providing evidence against the Generosity Hypothesis. However, in both societies older children demonstrated AI after collaboration but not after parallel work. For children in India AI emerged in ages where it had not been previously observed and children in Canada showed AI during early middle childhood. This suggests that collaboration can induce a willingness to sacrifice an advantage to achieve equity, consistent with the Equity Hypothesis.

Across a variety of contexts, adults tend to cooperate more with ingroup members than outgroup members. However, humans belong to multiple social groups simultaneously and we know little about how this cross-categorization affects cooperative decision-making. Nationality and gender are two social categories that are ripe for exploration in this regard: They regularly intersect in the real world and we know that each affects cooperation in isolation. Here we explore two hypotheses concerning the effects of cross-categorization on cooperative decision-making. First, theadditivity hypothesis(H1), which proposes that the effects of social categories are additive, suggesting that people will be most likely to cooperate with partners who are nationalityandgender ingroup members. Second, thecategory dominance hypothesis(H2), which proposes that one category will outcompete the other in driving decision-making, suggesting that either nationality or gender information will be privileged in cooperative contexts. Secondarily, we test whether identification with—and implicit bias toward—nationality and gender categories predict decision-making. Indian and US Americans (N= 479), made decisions in two cooperative contexts—the Dictator and Prisoner’s Dilemma Games—when paired with partners of all four social categories: Indian women and men, and US American women and men. Nationality exerted a stronger influence than gender: people shared and cooperated more with own-nationality partners and believed that own-nationality partners would be more cooperative. Both identification with—and implicit preferences for—own-nationality, led to more sharing in the Dictator Game. Our findings are most consistent with H2, suggesting that when presented simultaneously, nationality, but not gender, exerts an important influence on cooperative decision-making. Our study highlights the importance of testing cooperation in more realistic intergroup contexts, ones in which multiple social categories are in play.

Communication is a powerful tool for promoting cooperation in adults and is considered one of the most important solutions to social dilemmas. One feature that makes communication particularly useful in cooperative contexts is that it allows people to advertise their intentions to partners. Some work suggests that adults cooperate more after making nonbinding commitments to cooperate (i.e., commitments they do not need to uphold) than when they are not allowed to communicate their intentions to their partners. However, we know little about whether nonbinding commitments play a similar role in children. We addressed this gap by testing 6- to 9-year-old children in a simultaneous version of the iterated prisoner's dilemma game. In the communication condition, children could communicate their intended decision prior to their actual choice, whereas in the silent condition, they could not communicate. Overall, children in the communication condition were no more likely to cooperate than children in the silent condition. However, in the communication condition, but not in the silent condition, children's behavior was contingent on their previous decision; they were more likely to cooperate or defect when they had previously cooperated or defected, respectively. In addition, they rarely reversed their intended decisions in the game. Our findings suggest that, although nonbinding commitment does not promote children's cooperation in general, it may encourage children to stick to their chosen strategy, perhaps for the sake of appearing consistent. More broadly, these results contribute to our emerging understanding of the ways in which children solve social dilemmas.

An analysis of when children develop a sense of fairness (receiving less or more than a peer) is compared across seven different societies; aversion to receiving less emerges early in childhood in all societies, whereas aversion to receiving more emerges later in childhood and only in three of the seven societies studied. Previous research has shown that children develop a sense of fairness during childhood, but it is not known how the two components of this — aversion to being unfairly treated and aversion to seeing others unfairly treated — vary across cultures. These authors conducted experiments in seven cultures and find that aversion to being unfairly treated emerges early in childhood and consistently across cultures. Aversion to seeing others unfairly treated emerges later in childhood and is only seen in the experiment in three of the seven cultures studied. A sense of fairness plays a critical role in supporting human cooperation1,2,3. Adult norms of fair resource sharing vary widely across societies, suggesting that culture shapes the acquisition of fairness behaviour during childhood4,5. Here we examine how fairness behaviour develops in children from seven diverse societies, testing children from 4 to 15 years of age (n = 866 pairs) in a standardized resource decision task6,7. We measured two key aspects of fairness decisions: disadvantageous inequity aversion (peer receives more than self) and advantageous inequity aversion (self receives more than a peer). We show that disadvantageous inequity aversion emerged across all populations by middle childhood. By contrast, advantageous inequity aversion was more variable, emerging in three populations and only later in development. We discuss these findings in relation to questions about the universality and cultural specificity of human fairness.

Adults and children show ingroup favoritism in their 3rd-party punishment of cooperative norm violations, suggesting that group loyalty importantly shapes enforcement of cooperation. Ingroup favoritism additionally influences punishment of unfairness in the 2-party ultimatum game, in which people are directly affected by unfair behavior. However, the directionality of this relationship is unclear: In some cases, people are more forgiving of ingroup unfairness, whereas in others they are less forgiving. Here we aim to disambiguate this relationship by studying its origins in development, asking whether ingroup favoritism influences children's offers to others and whether it affects their responses to being treated unfairly. Six- to 10-year-olds played a group-based ultimatum game after being assigned to minimal groups and made proposals to-and responded to offers from-members of their in- and outgroups. We tested children's real bargaining behavior in the absence of deception. Results showed that, regardless of group membership, children's primary concern lay with fairness: Participants regularly offered equal splits and were more likely to reject unfair offers than fair offers. Consistent with past work, older children made more generous proposals than did younger children. Although our group manipulation successfully induced ingroup bias in participants, neither children's proposals nor responses were influenced by group membership. This suggests that second-party punishment of fairness norm violations is unbiased early in development and points to the potentially important role of experience with different groups in shaping later emerging bias in norm enforcement. We discuss implications for theories regarding when and to what extent group bias influences cooperation. (PsycINFO Database Record

The human sense of fairness entails sensitivity not just to equality, the equal division of resources, but also to merit, the relationship between an individual's share of resources and how hard they worked for their share. Recent evidence suggests that our sensitivity to equality has deep phylogenetic roots: several nonhuman animal species show an aversion to unequal reward distributions. However, the extent to which nonhuman animals share sensitivity to merit remains poorly understood, largely because previous studies have failed to properly manipulate work effort in inequity aversion tasks. Here, we tested whether cotton-top tamarins (Saguinus oedipus) would exhibit a differential response to inequity when acquiring rewards was either (1) effortful or (2) effortless. Subjects engaged in a pulling task in which they had an opportunity to deliver a disadvantageously unequal distribution of food to themselves and a partner (one piece for self, four pieces for partner). We made delivery effortful by adding a weight to the pulling handle. Critically, effort was calibrated to each individual. Results show that individuals varied markedly in their response to effort, highlighting the importance of manipulating work effort at the individual level. Overall, subjects showed little aversion to inequity. However, subjects were slightly less likely to accept inequity when doing so was effortful, although this effect was pronounced in only one individual. Our findings suggest a new method for capturing individual variation in effort and for studying the roots of the concept of merit in nonhuman animals.

Banded mongooses (Mungos mungo) extract encased food items by throwing them against anvils. Observations indicate that their chosen anvils are generally hard enough to crack open casings, suggesting an understanding of the physical properties that render an anvil suitable for cracking. We report results from two field experiments investigating spatial and physical aspects of anvil selection in a wild group of banded mongooses. Mongooses rapidly carried prey items to nearby anvils in their environment, without simply returning to the last anvil they passed, suggesting a detailed knowledge of anvil locations. Moreover, in choice experiments with hard or soft anvils, they always chose the appropriate anvil when both anvils were natural but chose indiscriminately when they were synthetic. These results support a recent suggestion that mongooses lack a generalized understanding of the functional properties of anvils but also indicate that they may mediate their decisions on the basis of familiarity. Together, our experiments suggest that mongooses employ simple rules of thumb that, in most cases, result in the selection of appropriate anvils. Where environmental problems are limited and predictable, selection will favor the evolution of such rules of thumb rather than a more generalized understanding of functional properties.

Identifying the motives underpinning punishment is crucial for understanding its evolved function. In principle, punishment of distributional inequality could be motivated by the desire to reciprocate losses ('revenge') or by the desire to reduce payoff asymmetries between the punisher and the target ('inequality aversion'). By separating these two possible motivations, recent work suggests that punishment is more likely to be motivated by disadvantageous inequality aversion than by a desire for revenge. Nevertheless, these findings have not consistently replicated across different studies. Here, we suggest that considering country of origin—previously overlooked as a possible source of variation in responses—is important for understanding when and why individuals punish one another. We conducted a two-player stealing game with punishment, using data from 2,400 subjects recruited from the USA and India. US-based subjects punished in response to losses and disadvantageous inequality, but seldom invested in antisocial punishment (defined here as punishment of non-stealing partners). India-based subjects, on the other hand, punished at higher levels than US-based subjects and, so long as they did not experience disadvantageous inequality, punished stealing and non-stealing partners indiscriminately. Nevertheless, as in the USA, when stealing resulted in disadvantageous inequality, India-based subjects punished stealing partners more than non-stealing partners. These results are consistent with the hypothesis that variation in punitive behavior varies across societies, and support the idea that punishment might sometimes function to improve relative status, rather than to enforce cooperation.

People across societies engage in costly sharing, but the extent of such sharing shows striking cultural variation, highlighting the importance of local norms in shaping generosity. Despite this acknowledged role for norms, it is unclear when they begin to exert their influence in development. Here we use a Dictator Game to investigate the extent to which 4- to 9-year-old children are sensitive to selfish (give 20%) and generous (give 80%) norms. Additionally, we varied whether children were told how much other children give (descriptive norm) or what they should give according to an adult (injunctive norm). Results showed that children generally gave more when they were exposed to a generous norm. However, patterns of compliance varied with age. Younger children were more likely to comply with the selfish norm, suggesting a licensing effect. By contrast, older children were more influenced by the generous norm, yet capped their donations at 50%, perhaps adhering to a pre-existing norm of equality. Children were not differentially influenced by descriptive or injunctive norms, suggesting a primacy of norm content over norm format. Together, our findings indicate that while generosity is malleable in children, normative information does not completely override pre-existing biases.

Human behaviour is influenced by social norms but norms can entail two types of information. Descriptive norms refer to what others do in this context, while injunctive norms refer to what ought to be done to ensure social approval. In many real-world situations these norms are often presented concurrently meaning that their independent effects on behaviour are difficult to establish. Here we used an online Dictator Game to test how descriptive and injunctive norms would influence dictator donations when presented independently of one another. In addition, we varied the cost of complying with the norm: By stating that $0.20 or $0.50 cent donations from a $1 stake were normal or suggested, respectively. Specifying a higher target amount was associated with increased mean donation size. In contrast to previous studies, descriptive norms did not seem to influence giving behaviour in this context, whereas injunctive norms were associated with increased likelihood to give at least the target amount to the partner. This raises the question of whether injunctive norms might be more effective than descriptive norms at promoting prosocial behaviour in other settings.

When enforcing norms for cooperative behavior, human adults sometimes exhibit in-group bias. For example, third-party observers punish selfish behaviors committed by out-group members more harshly than similar behaviors committed by in-group members. Although evidence suggests that children begin to systematically punish selfish behavior around the age of 6 y, the development of in-group bias in their punishment remains unknown. Do children start off enforcing fairness norms impartially, or is norm enforcement biased from its emergence? How does bias change over development? Here, we created novel social groups in the laboratory and gave 6- and 8-year-olds the opportunity to engage in costly third-party punishment of selfish sharing behavior. We found that by age 6, punishment was already biased: Selfish resource allocations received more punishment when they were proposed by out-group members and when they disadvantaged in-group members. We also found that although costly punishment increased between ages 6 and 8, bias in punishment partially decreased. Although 8-y-olds also punished selfish out-group members more harshly, they were equally likely to punish on behalf of disadvantaged in-group and out-group members, perhaps reflecting efforts to enforce norms impartially. Taken together, our results suggest that norm enforcement is biased from its emergence, but that this bias can be partially overcome through developmental change.

Human adults engage in costly third-party punishment of unfair behavior, but the developmental origins of this behavior are unknown. Here we investigate costly third-party punishment in 5- and 6-year-old children. Participants were asked to accept (enact) or reject (punish) proposed allocations of resources between a pair of absent, anonymous children. In addition, we manipulated whether subjects had to pay a cost to punish proposed allocations. Experiment 1 showed that 6-year-olds (but not 5-year-olds) punished unfair proposals more than fair proposals. However, children punished less when doing so was personally costly. Thus, while sensitive to cost, they were willing to sacrifice resources to intervene against unfairness. Experiment 2 showed that 6-year-olds were less sensitive to unequal allocations when they resulted from selfishness than generosity. These findings show that costly third-party punishment of unfair behavior is present in young children, suggesting that from early in development children show a sophisticated capacity to promote fair behavior.

Adults and children are willing to sacrifice personal gain to avoid both disadvantageous and advantageous inequity. These two forms of inequity aversion follow different developmental trajectories, with disadvantageous inequity aversion emerging around 4 years and advantageous inequity aversion emerging around 8 years. Although inequity aversion is assumed to be specific to situations where resources are distributed among individuals, the role of social context has not been tested in children. Here, we investigated the influence of two aspects of social context on inequity aversion in 4- to 9-year-old children: (1) the role of the experimenter distributing rewards and (2) the presence of a peer with whom rewards could be shared. Experiment 1 showed that children rejected inequity at the same rate, regardless of whether the experimenter had control over reward allocations. This indicates that children's decisions are based upon reward allocations between themselves and a peer and are not attempts to elicit more favorable distributions from the experimenter. Experiment 2 compared rejections of unequal reward allocations in children interacting with or without a peer partner. When faced with a disadvantageous distribution, children frequently rejected a smaller reward when a larger reward was visible, even if no partner would obtain the larger reward. This suggests that nonsocial factors partly explain disadvantageous inequity rejections. However, rejections of disadvantageous distributions were higher when the larger amount would go to a peer, indicating that social context enhances disadvantageous inequity aversion. By contrast, children rejected advantageous distributions almost exclusively in the social context. Therefore, advantageous inequity aversion appears to be genuinely social, highlighting its potential relevance for the development of fairness concerns. By comparing social and nonsocial factors, this study provides a detailed picture of the expression of inequity aversion in human ontogeny and raises questions about the function and evolution of inequity aversion in humans.

Humans involved in cooperative interactions willingly pay a cost to punish cheats. However, the proximate motives underpinning punitive behaviour are currently debated. Individuals who interact with cheats experience losses, but they also experience lower payoffs than the cheating partner. Thus, the negative emotions that trigger punishment may stem from a desire to reciprocate losses or from inequity aversion. Previous studies have not disentangled these possibilities. Here, we use an experimental approach to ask whether punishment is motivated by inequity aversion or by a desire for reciprocity. We show that humans punish cheats only when cheating produces disadvantageous inequity, while there is no evidence for reciprocity. This finding challenges the notion that punishment is motivated by a simple desire to reciprocally harm cheats and shows that victims compare their own payoffs with those of partners when making punishment decisions.

Studies of mate choice in vertebrates have focused principally on birds, in which male ornaments are often highly developed, and have shown that females commonly select mates on the basis of particular phenotypic characteristics that may reflect their genetic quality. Studies of female mate choice in mammals are less highly developed, and they have commonly focused on female mating preferences that are likely to be maintained by benefits to the female's own survival or breeding success. However, recent experimental studies of mate choice in mammals—especially rodents—provide increasing evidence of consistent female preferences that appear likely to generate benefits to the fitness of offspring. As yet, there is no compelling evidence that female mating preferences are less highly developed in female mammals than in female birds, although these preferences may more often be masked by the effects of male competition or of attempts by males to constrain female choice.