The study, titled Four- to 6-month-old infants use saliva-sharing interactions to infer social intimacy, was led by Brandon Woo, a researcher in UC Santa Barbara’s Department of Psychological & Brain Sciences. Woo, a developmental psychologist by training, has built his career around a deceptively simple question: what does the world look like from a baby’s point of view? How do infants, who cannot yet ask questions or explain themselves, make sense of other people’s actions, minds, and relationships? The new work extends a line of research suggesting that babies are not passive observers of the social scene around them but active interpreters of it, drawing inferences about who belongs to whom from surprisingly subtle behavioral evidence.

The inspiration for the research came from an observation drawn from anthropology and psychology across cultures: people in close relationships tend to be more comfortable sharing saliva with one another than strangers are. Kissing, sharing a fork, eating from the same bowl, licking the same ice cream cone — these behaviors are largely reserved for family members, romantic partners, and intimate friends. Woo frames the intuition with a vivid example: if you saw two people licking the same ice cream cone, you would probably guess that they know each other very well. The question he and his collaborators pursued was whether babies, who have had almost no time to learn such cultural conventions, make the same kind of guess.

The team had already established that older infants possess this ability. In previous work, Woo and his collaborators found that by around eight months of age, babies who watched two people engage in saliva-sharing behaviors — such as kissing, or sharing foods and utensils — inferred that those individuals would respond to each other’s distress. In other words, the infants seemed to understand that saliva sharing marks a thick relationship, one in which the participants are bound to help and comfort one another. What remained unknown was how early in development this capacity emerges, and that question mattered for reasons that go to the heart of a long-running debate about what babies know.

Four- to six-month-old infants present a demanding test case. Compared with the older babies studied previously, they have had far fewer opportunities to observe saliva-sharing interactions in their daily lives. They are also less likely to look at faces during social scenes, which limits the visual information they can take in. And they are typically fed foods — milk, purees — that are not shared with other people in the way a toddler might share a snack. If infants this young still draw the right conclusions from saliva-sharing scenes, the underlying expectation cannot plausibly be the product of extensive personal experience. It would instead point to an early-emerging, and possibly abstract, framework for thinking about human relationships.

To probe this, the researchers designed two experiments. In the first, a human protagonist placed an orange slice in their own mouth, took it out, and then inserted the fruit into the mouth of an actor puppet. This sequence represented a scenario of shared saliva, since the food had contacted both mouths. The contrasting condition involved the same kind of protagonist taking a ball and passing it to the puppet — an act of sharing, but one involving no saliva. After the interaction, the puppet expressed distress, and the researchers measured which person the infant looked at longer: the individual who had shared the orange slice or the individual who had shared the ball. Longer looking in infant studies is commonly interpreted as reflecting expectation, violation of expectation, or heightened interest, making it a standard window into the infant mind.

The second experiment was designed to separate saliva itself from food, a crucial methodological control. Here, a human protagonist put a finger into their own mouth, then into a puppet’s mouth, and then back into their own — a direct exchange of saliva with no food involved. In the contrast condition, the protagonist performed similar actions with a different puppet but touched foreheads instead, a form of intimate contact that does not involve saliva. The protagonist then began to cry, and the researchers observed which puppet the infants expected to come to the rescue. Woo notes that the finger-in-mouth scenario is not a common feature of children’s everyday lives, which made it a strong test of whether babies were truly tracking saliva exchange rather than simply responding to familiar mealtime routines.

The results were consistent across both experiments. When the actor expressed distress, the infants looked longer at the thick relationship partner — the individual who had shared saliva — rather than at the partner who had shared the ball or merely touched foreheads. In the food-sharing experiment, that meant babies expected the orange-slice sharer to respond to the puppet’s cries. In the finger experiment, it meant they expected the puppet whose mouth had received the protagonist’s finger to intervene. Woo describes the pattern as striking: even with less familiar actions, the babies were making inferences about closeness, and the knowledge they applied seemed to generalize across very different kinds of interactions.

That generalization is perhaps the most theoretically significant aspect of the findings. Woo emphasizes that it is fascinating that infants seem to possess abstract knowledge about relationships that applies across different actions. The babies were not simply learning that a specific behavior, such as food sharing, predicts a specific outcome. Instead, they appeared to extract a relational principle — that saliva sharing signals intimacy and mutual obligation — and to deploy that principle flexibly across scenarios ranging from the utterly familiar, like sharing food, to the unusual, like a finger placed in a puppet’s mouth. Abstraction of this kind, at an age when infants are still mastering basic motor control, poses a serious challenge to accounts of social development that place heavy weight on accumulated experience.

The work also speaks directly to a longstanding disagreement among scholars of human development about how much babies understand of the world. As Woo puts it, some scholars argue that babies are blank slates that know nothing, and this research is part of an effort to determine just how babies begin to make sense of the social world, of social relationships, and what role experience plays in that process. The evidence that four-month-olds already treat saliva sharing as a marker of a thick relationship suggests that at least some of the architecture of human social knowledge is present from very early in life. Because human survival and cooperation depend on understanding who will help whom, an early sensitivity to the signals of intimacy would make evolutionary sense, and the study’s publication in Evolution and Human Behavior reflects that framing. For parents, the image is a striking one: the infant in the stroller, seemingly absorbing nothing more than sights and sounds, may already be quietly charting the social network around them, deciding who is close to whom from a licked spoon, a shared bite, or a kiss.

Subject of Research: Infant social cognition and the use of saliva-sharing cues to infer relationship closeness

Article Title: Young infants infer the closeness of social relationships via saliva sharing interactions

Article References: Young infants infer the closeness of social relationships via saliva sharing interactions. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: infant cognition, social relationships, saliva sharing, developmental psychology, Evolution and Human Behavior, UC Santa Barbara, social networks, relationship inference, early development, distress response, human sociality, intimacy cues

The research, led by Oded Ritov, a psychology Ph.D. student in Berkeley’s Social Origins Lab, used an ingeniously simple setup involving pineapple, ropes, and two connected rooms. Over the course of more than 800 hours of observation, the team studied 26 chimpanzees at the Ngamba Island Chimpanzee Sanctuary in Uganda, documenting grooming patterns, daily interactions, and how regularly individuals stayed within arm’s reach of one another. From these observations, the researchers identified which chimps appeared to be closely bonded, effectively mapping the friendship networks of the sanctuary’s residents.

Once those relationships were established, the experiment began. One chimpanzee, designated the victim, was placed in a room containing three trays baited with pineapple. A rope attached to the trays ran into an adjacent room, where a second chimpanzee, the potential thief, could either leave the rope untouched or yank it to pull the fruit toward itself and steal the pineapple. The design allowed researchers to compare how the same individual behaved when paired with a close friend versus a non-friend, a within-subject approach that strengthens confidence that observed differences reflect the relationship itself rather than individual personality.

The results were striking. Chimpanzees stole in 70 percent of trials involving non-friends, but only 61 percent of trials involving friends. While a nine percentage point difference might appear modest at first glance, the methodological design gives it considerable weight. Because each chimp experienced both the friend and non-friend conditions, the researchers could be more confident that the reduction in stealing was a meaningful response to relationship status rather than random variation. According to Ritov, this is the first study to demonstrate that chimpanzees are able to hold back antisocial behavior specifically toward their friends.

Just as revealing were the emotional dynamics that unfolded after a theft. The researchers evaluated victims’ responses by measuring stomping, fist-pounding, and screeching, behaviors that serve as visible indicators of agitation in chimpanzees. Victims were initially angrier when a friend stole from them, an outcome that may reflect the violation of expectations that come with closeness. Yet they calmed down noticeably faster when the thief was a friend compared with when the thief was a non-friend, suggesting that the underlying bond helped de-escalate the emotional fallout of the transgression.

The thieves’ behavior changed as well. When the thief and victim were not friends, the thief expressed negative emotions for a much longer period, indicating that the interaction was perceived as more hostile. When stealing from a bonded partner, thieves showed less hostility, hinting that the act itself carried a different emotional texture depending on who was on the receiving end. Taken together, these patterns suggest that both parties in a chimpanzee friendship carry expectations about how they should be treated, and that violations of those expectations register emotionally on both sides.

“This suggests that there may be mutual expectations in chimps from both sides of the friendship, which makes it seem more like the complex friendships we see in humans,” Ritov said. The idea of mutual expectation is critical. Previous research has documented relationship-specific intentions in nonhuman primates, such as preferential grooming and selective food sharing, but intentions are only one component of what makes human friendships distinctive. Jan Engelmann, an associate professor of psychology and senior author of the paper, argues that the stronger ingredient is expectation.

“We expect more of our friends,” Engelmann said. “These findings really deepen our understanding of the evolutionary roots of friendships. This is, to my knowledge, the first evidence showing that also chimpanzees expect and maybe demand special treatment from their friends. These kinds of social expectations might form the basis of human moral principles.” In other words, the study hints that the normative glue binding human societies, the sense that certain people owe us certain treatment and that we owe it in return, may not have emerged from nothing but may instead trace back to social dynamics already present in the common ancestor we share with chimpanzees.

Scientists have long been cautious about labeling close chimpanzee relationships as friendships in the human sense, arguing that we cannot fully grasp the cognitive and emotional complexity behind their behavior. The new study addresses that skepticism by combining long-term observational data with a controlled experimental task, allowing researchers to move beyond correlation. The observation that closely bonded chimps groom together and share food more often had already suggested prosocial preferences; the theft experiment adds an antisocial dimension, showing that the same bonds restrain harmful behavior and shape emotional responses when harm occurs anyway.

The work fits into a broader research program by Ritov examining the evolution of human behavior and the possibility that morals and norms are part of our core being, traceable to other animals. He has recently investigated whether nonhuman animals possess a sense of fairness, and he is broadly interested in how emotions unfold across cultures and species. “We think that these expectations might be the basic building blocks of the larger-scale cooperation we see in humans,” Ritov said. “The expectations that chimpanzees have from their friends or close collaborative partners might have scaled up to more generalized expectations for how we should treat each other.”

That scaling hypothesis carries significant implications. If the expectations embedded in chimpanzee friendships represent the raw material from which human moral systems evolved, then the emotional underpinnings of our social lives, our sense of obligation, our outrage at betrayal, our capacity to forgive those we love, may be far older than our species. Collaborative relationships are important for our well-being, health, and survival, Ritov notes, and increasingly researchers are recognizing that this is not unique to humans. Other animals have friends as well, and with those friendships come expectations that, when broken, are felt on both sides of the relationship. The pineapple heists at Ngamba Island may be a small drama, but they illuminate something profound about where human morality may have begun.

Subject of Research: Experimental study of how chimpanzee friendships influence stealing behavior and emotional responses

Article Title: Chimps plunder less pineapple from friends — and expect them to do the same

Article References: Chimps plunder less pineapple from friends — and expect them to do the same. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: chimpanzees, friendship, social expectations, evolutionary psychology, antisocial behavior, emotions, cooperation, Ngamba Island, UC Berkeley, Evolution and Human Behavior, primates, moral evolution

The research addresses a long-standing puzzle in the science of cooperation. Human societies sustain remarkably high levels of collaboration among genetically unrelated individuals, and one of the most influential theoretical explanations is indirect reciprocity. Under indirect reciprocity, people decide whether to help someone based on that person’s reputation: those known to help others earn a good standing, which makes third parties more willing to assist them in the future, while defectors acquire bad standing and are shunned. Because reputational information circulates socially, an individual can benefit from a good reputation even with people they have never met. Decades of theoretical modeling and behavioral experiments have explored how such reputation-based systems can stabilize cooperation, but most of this work has focused on socially shared information—what everyone in a group supposedly knows about everyone else.

Everyday life, however, rarely presents reputation and experience in neat agreement. A coworker may be widely praised yet have treated you badly, or a neighbor may carry a poor reputation despite having personally helped you when you needed it. Yamamoto and his colleagues set out to examine precisely this conflict: how do people evaluate another person’s decision to cooperate or refuse when the public standing of the individual involved contradicts what the evaluator has directly experienced? The question matters because evaluations are the currency of indirect reciprocity. If people’s judgments are swayed by private history rather than shared reputation, the mechanics of reputation-based cooperation may be more complicated than standard models assume.

To investigate, the team conducted two scenario-based experiments with participants in Japan. The scenarios were deliberately grounded in familiar social settings: one involved coworkers at a restaurant, and the other involved neighbors in a residential community. In each scenario, participants read about a situation in which one person either cooperated with or refused a request made by another person. The researchers systematically varied two factors. The first was the reputation of the person making the request—whether that individual was publicly regarded as good or bad. The second was the participant’s own prior experience with the person responding to the request—whether that responder had previously cooperated with or refused to cooperate with the participant personally. Participants then evaluated the person who had responded, allowing the researchers to measure how reputation and personal experience each shaped moral judgment.

The results revealed a striking asymmetry between cooperation and non-cooperation. When the responder refused the request, evaluations of that refusal depended heavily on the participant’s own prior experience with the responder. Even when the responder carried a bad public reputation, the refusal was judged more negatively if that responder had previously helped the participant. Conversely, even when the responder enjoyed a good reputation, the refusal was judged more positively if that responder had previously refused to help the participant. In other words, when public reputation and personal experience pointed in opposite directions, it was the private, firsthand history that strongly colored how participants rated the act of non-cooperation. The shared social record, which much of the indirect reciprocity literature treats as the decisive input, was effectively pushed aside.

Cooperative behavior told a different story. Acts of cooperation were generally evaluated positively regardless of the other person’s reputation and regardless of how that person had previously treated the participant. This suggests that the human evaluative system does not process cooperation and defection symmetrically. Cooperation appears to be rewarded almost automatically, insulated from the evaluator’s private grievances and from the target’s public standing. Defection, by contrast, invites a more contextualized judgment—one in which the evaluator asks not simply whether the refusal was socially acceptable, but how the person on the receiving end had previously treated them personally.

Perhaps the most sobering finding concerned a scenario in which refusing seemed entirely justified. When the person being refused had both a bad public reputation and a personal history of refusing to cooperate with the participant—conditions under which there appeared to be good reason to withhold help—the refusal itself was still evaluated neutrally rather than positively. Refusing, in other words, was not converted into a virtuous act even when the circumstances seemed to warrant it. This indicates that while people may tolerate or excuse non-cooperation under certain conditions, they do not necessarily celebrate it as positively good. The moral ledger of everyday cooperation appears to reserve its positive marks for helping, with refusal occupying at best a neutral zone even when justified.

From a technical perspective, the findings challenge a simplifying assumption embedded in many formal models of indirect reciprocity. Standard models typically treat reputational information as a single, socially shared signal that determines who deserves help. The new results suggest that human evaluators integrate at least two distinct channels—socially transmitted reputation and privately accumulated experience—and that the weighting between these channels depends on the type of behavior being judged. For defection, the private channel dominates; for cooperation, the evaluation is largely insensitive to either channel. This kind of asymmetric integration could have important consequences for how reputation-based cooperation actually operates in groups, because it implies that shared reputational records may be less powerful in governing sanctions and forgiveness than theorists have often assumed. It also hints at why personal grudges and personal loyalties can persist within communities even when everyone has access to the same public information about one another.

The study also clarifies the scope of previous research. Earlier work on indirect reciprocity concentrated mainly on how socially shared reputational information guides decisions about whom to cooperate with, leaving open the question of what happens when that shared information conflicts with lived experience. By manipulating reputation and personal experience independently within controlled scenarios, Yamamoto and his colleagues were able to isolate the contribution of each source and demonstrate that direct experience is not merely one input among many but can override reputation altogether in judgments of defection. The use of two distinct social contexts—workplace and neighborhood—suggests the pattern is not confined to a single type of relationship, although the scenarios were hypothetical and conducted with participants in Japan, so further work will be needed to test how general the effect is across cultures and real-world interactions.

Professor Yamamoto framed the work as a step toward understanding how people combine heterogeneous information when judging others. “In everyday life, we judge others by combining many different kinds of information, including what we hear about them from others and what we have experienced ourselves,” he said. “Our study provides one clue to understanding how people make judgments when these sources of information do not agree.” He also outlined where the research program is headed next: “In future research, we would like to examine how people use reputational information in environments where direct personal experience is limited, such as online rating systems and AI-mediated judgments.” That direction is increasingly urgent. On digital platforms, ratings and reviews function as public reputations, yet users often have little or no direct experience with the sellers, drivers, or service providers they evaluate. If human judgment naturally privileges firsthand experience where it exists, the design of reputation systems for environments that lack such experience becomes a genuinely open scientific and engineering question—one that this study helps to define.

Subject of Research: How direct personal experience and public reputation interact in human evaluations of cooperation and defection in indirect reciprocity

Article Title: “Public reputation” or “Personal experience”?

Article References: “Public reputation” or “Personal experience”?. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: indirect reciprocity, cooperation, reputation, defection, social evaluation, human behavior, evolutionary psychology, moral judgment, personal experience, scenario experiments, Evolution and Human Behavior, prosocial behavior