Joint actions require co-actors to coordinate their individual contributions to achieve joint goals. One proposal is that such coordination is supported by joint action plans. However, it remains unclear whether joint action planning is generally prioritized, or whether it is flexibly engaged depending on task demands. Here, we investigated the factors that influence co-actors' propensity to use the individual- and joint-level planning. Pairs of participants performed a joint task in which pre-cues provided information about individual contributions, the relation between contributions, or both. We manipulated the flexibility required to map actions onto outcomes, the order in which individual- and joint-level information was provided, and the relation between individual contributions. When mapping actions onto their outcomes required little flexibility, cues about the relation between individual contributions provided a performance benefit, indicating a tendency to prioritize joint action planning. In contrast, when action-outcome mappings were variable and required more flexibility, co-actors refrained from a general use of joint action plans and relied primarily on individual planning. Under these conditions, joint cues improved performance only when they allowed participants to infer their own required contribution from information about their partner's contribution. The order in which individual and joint-level information was specified and the relation between individual contributions did not influence participants propensity to engage in joint action planning. Together, these findings show that joint action planning is not generally prioritized but is flexibly recruited as a function of task demands, specifically the flexibility required to map actions onto outcomes.

Understanding how individuals plan, predict and adapt their actions is central to understanding the high degree of coordination involved in joint action. While previous research has examined the role of joint action plans in linking individual action representations in a two-person joint action plan, the specific conditions and representations associated with these plans are less well understood. In this study, we investigated when and how joint action plans that specify the relations between individual contributions facilitate performance in a joint task. We hypothesized that joint action plans serve to organize individual contributions to a joint action by specifying the relations between them. Pairs of participants performed a joint computer task that required flipping blocks to produce a joint pattern. We manipulated the availability and information provided by two types of pre-cues: joint cues specified the relation between individual contributions and individual cues specified each participant’s individual part. The results of three experiments showed that information regarding the relation between individual contributions helped co-actors integrate representations of their individual contributions. This is evidenced by faster response times when a joint cue was presented together with a cue specifying the partner’s action. Moreover, joint cues could be used to compensate for the lack of information about individual contributions, as evidenced by faster response times when a joint cue was presented in trials without information about individual contributions. We conclude that joint action plans that specify the relations between individual contributions serve to organize representations of individual contributions and thereby help co-actors infer their own underspecified contribution to a joint action.

Across cultures, people have long sung while working together-yet potential functional benefits of singing work songs have rarely been investigated experimentally. Rhythmic joint actions such as rowing, hammering or clapping tend to accelerate over time, a phenomenon known as joint rushing, which can undermine coordinated joint performance. Here, we examine whether features typical of traditional work songs help prevent joint rushing. In three experiments, pairs of participants performed rhythmic tapping tasks under conditions that simulated key aspects of work songs: solo vocalizations and metric subdivisions. When one person vocalized a subdivided rhythm, joint rushing disappeared completely, timing variability was reduced and interpersonal coordination improved. These findings reveal how vocalizations can act as a real-time scaffold for joint performance, linking basic cognitive timing mechanisms to the evolution of collective human practices. Work songs, and perhaps other forms of shared vocalization, may thus represent culturally evolved tools that harness cognitive mechanisms to enhance coordination in collaborative tasks.

Coordination requires anticipating and adapting to others' actions in real time. While research has shown that observers predict others’ solo actions, little is known about how predictive processes change when observing joint actions. We tested whether, for joint actions, observers form (1) stronger predictions because they expect others to act more reliably due to social considerations (e.g., commitment), or (2) weaker predictions because they expect coordination to introduce additional sources of error (e.g. due to miscommunication). To this end, we presented participants with image sequences depicting either one or two actors building a block tower. The solo actor added blocks, alternating between her left and right hands, while the joint actors took turns adding blocks with one hand each. Participants made speeded responses to identify the side from which the block came. Of interest was how accurately participants identified the block’s side when it violated the alternating pattern, with errors indicating anticipation for the alternating pattern to continue. Participants were less likely to make anticipatory errors on violation trials for joint actions, indicating weaker action predictions (Study 1). Hierarchical drift-diffusion modelling (Study 2) revealed that this was due to participants accumulating sensory evidence more rapidly and responding more cautiously on joint action trials, relying less on their prior expectations about an action’s identity and more on incoming sensory information. This strategy might serve to enable observers to flexibly update their predictions about unfolding actions in the face of coordination-induced errors.

People synchronize their behavioral and physiological rhythms with each other during social interaction. While interpersonal synchronization has largely been associated with positive effects such as social bonding, some evidence suggests that it may also impair self-regulation and disrupt intrapersonal coordination. Because respiration and heart rhythms are weakly coupled within individuals, we investigated whether synchronizing breathing with another person alters intrapersonal cardiorespiratory coupling. Across two experiments, participants synchronized their breathing reciprocally (bidirectional interaction), unidirectionally with a confederate, or with prerecorded breathing signals, while respiration and electrocardiography were continuously measured. Relative phase analyses revealed that bidirectional breathing synchronization induced in-phase synchronization of heart rhythms between individuals. Critically, interpersonal synchronization coincided with cardiorespiratory decoupling: respiration and heart rhythms became more out-of-phase during interaction compared to resting baselines and the unidirectional condition. Moreover, stronger interpersonal respiratory synchronization predicted greater intrapersonal cardiorespiratory decoupling, particularly when participants adapted their own breathing to another person's. These findings provide evidence that interpersonal physiological synchronization entails a trade-off with intrapersonal physiological coupling, perturbing the phase relationship within one's own physiological system. We propose that aligning one's physiological rhythms with others strengthens self-other coupling but weakens intrapersonal coupling, pointing to a physiological mechanism of self-decoupling during social interaction.

Evidence from joint action research suggests that a suite of spontaneous cognitive and perceptual processes underpins our ability to coordinate with others. We tested whether people spontaneously perceive affordances (or action possibilities) for the dyad when a co-actor is present. To test this hypothesis, we asked participants to complete an object-based response time task either alone (individual condition) or while seated next to a partner who completed the complementary task (joint condition). Participants in both conditions made unimanual go/no-go keypresses to objects that afforded either a bimanual (e.g., a laundry hamper) or unimanual (e.g., a bucket) grasp. In total, we conducted seven experiments (total N =293). An initial experiment revealed an affordance × task interaction effect on response times, such that responses to bimanually graspable objects were faster in the joint than in the individual condition. However, several follow-up experiments failed to replicate this finding, providing no overall evidence for spontaneously perceived joint affordances.

Optimal foraging requires agents to strike a balance between potential costs and rewards of interacting with stimuli in the environment. Research on human and animal foraging shows that the value an agent assigns to a stimulus is correlated with the speed of their reaching movement towards that stimulus (Shadmehr et al., 2019). Humans and other animals learn about the value of stimuli in their environment by observing others acting (Pyke, 1984; Boyd, Richerson & Henrich, 2011). Considering that humans are able to derive specific mental states such as intentions, emotions or confidence from specific movement parameters (Becchio et al., 2012), we aimed to investigate whether observers can use an actor's movement speed to: 1) infer the value of a foraging stimulus; and 2) use such cues to inform their own foraging behavior. The current study first replicated the effect of stimulus value on reaching movements in a novel foraging task (Exp. 1, N = 34). In three further experiments, we demonstrate that, depending on the speed by which an actor reaches for stimuli, observers infer the value of these stimuli (Exp. 2, N = 54), express foraging preferences (Exp. 3, N = 54), and invest time and effort to forage (Exp. 4, N = 105). This demonstrates that observers optimize their own explore-exploit decisions by inferring the value of a stimulus from the manner by which an actor approaches it, highlighting the fundamental role that action understanding plays in successful foraging.

For two decades, the joint Simon effect (JSE) has been a cornerstone of joint action research. Initially presented as evidence that humans spontaneously represent others’ actions in a functionally equivalent way to their own, the JSE now serves as an interdisciplinary measure of self-other integration across research on social cognition, human-robot interaction, and animal cognition. However, whether the JSE is better explained by non-social mechanisms remains an open question. The present meta-analytic review sought to provide an overview of the JSE literature and to evaluate its suitability as a measure of self-other integration. Our meta-analysis of 274 JSEs from 80 studies (7042 participants across 186 samples) revealed a significant overall effect of gD = .60. Analysis of 52 canonical JSEs (reflecting the seminal study) revealed a larger JSE of gD =.72. Finally, moderation by study characteristics suggested that JSEs at least partly reflect social representations of one’s co-actor. Importantly, our findings lend support to the continued use of the JSE as a measure of self-other integration.

Grasping others’ intentions from their actions is essential for learning, as it enhances the ability to identify collaborative acts and anticipate others’ actions, facilitating effective coordination toward shared goals. From a young age, children seem to recognize when others are working together based on their interactions and use this understanding to inform their own learning. Although much of early learning occurs in joint contexts, little attention has been devoted to understanding how children learn by participating in joint actions and by observing others acting together. Using a puzzle box paradigm, we tested 3–6-year-old children’s imitation of an inefficient performance following individual and joint demonstrations in which the inefficient performance did or did not involve bimanual or joint coordination. This allowed us to test whether the tendency to overimitate extends to joint actions and how action coordination modulates imitative behavior. We found that overimitation extends to joint actions, as indicated by similar rates of inefficient copying following individual and joint action demonstrations. Furthermore, our results suggest that action coordination did not play a significant role in modulating children’s tendency to overimitate. Taken together, the results of the study advance our understanding of how learning occurs in social interactions.

For two decades, the joint Simon effect (JSE) has been a cornerstone of joint action research. Initially presented as evidence that humans spontaneously represent others’ actions in a functionally equivalent way to their own, the JSE now serves as an interdisciplinary measure of self-other integration across research on social cognition, human-robot interaction, and animal cognition. However, whether the JSE is better explained by non-social mechanisms remains an open question. The present meta-analytic review sought to provide an overview of the JSE literature and to evaluate its suitability as a measure of self-other integration. Our meta-analysis of 274 JSEs from 80 studies (7042 participants across 186 samples) revealed a significant overall effect of gD = .60. Analysis of 52 canonical JSEs (reflecting the seminal study) revealed a larger JSE of gD =.72. Finally, moderation by study characteristics suggested that JSEs at least partly reflect social representations of one’s co-actor. Importantly, our findings lend support to the continued use of the JSE as a measure of self-other integration.

Research on joint action planning has demonstrated that, when acting with others, individuals will choose more effortful actions if it improves co-efficiency by reducing the overall effort exerted by the group. However, because these studies use actions for which time and effort costs are confounded, it is unclear which costs participants sought to minimise and what processes underlie decision-making about individual contributions to joint actions. Across three experiments, we tested (1) whether dyads aim to minimise effort vs. time costs (Experiments 1–2) and (2) whether individuals choose actions based on a rational, deliberative process or a heuristic (Experiment 3). Data from a joint object-dragging computer task revealed that participants chose to drag objects to the closer of two goals even when it required more effort from the dyad (Experiments 1–2). Participants preferred closer goals when time and effort costs were equal (Experiment 3a), but only when the objects’ locations were salient (Experiment 3b). Together, these results suggest that: (1) individuals minimise time, rather than effort, when sharing object-moving tasks, and (2) they do so – at least in part - by using distance as a heuristic. Our findings also replicate and extend earlier work showing that people prefer acting together versus alone, even when this is less efficient.

Successful interactions require not only representing others’ mental states but also flexibly updating them, whenever one’s original inferences may no longer hold. Such situations arise, for instance, when a partner’s behavior is incongruent with one’s expectations. Although these situations are rather common, the question whether people update others’ mental states spontaneously upon encountering unexpected behaviors and whether they use the updated mental states in novel contexts, has been largely unexplored. We addressed these issues in two experiments. In each experiment participants first performed an anticipatory looking task, reacting to a virtual ‘partner’, who categorized pictures based on their ambiguous or non-ambiguous color. Importantly, to perform the task participants did not have to track their partner’s perspective. Following a correct categorization phase, the ‘partner’ started to systematically miscategorize one of the ambiguous colors (e.g., as if she would now believe that the greenish blue is green). We measured how participants’ anticipatory looking preceding the partner’s categorization changed across trials. Afterward, we asked whether participants implicitly transferred their knowledge about the partner’s updated perspective to a new task. Finally, they performed an explicit perspective-taking task, to test whether they selectively updated the partner’s perspective, but not their own. Results revealed that correct anticipations started to emerge only after a few miscategorizations, indicating the spontaneous updating of the other’s perspective regarding the miscategorized color. Signatures of updating emerged somewhat earlier when the partner made similarity judgments (Experiment 2), highlighting the subjective nature of her decisions, compared to when following an explicit color-categorization rule (Experiment 1). In the explicit perspective-taking task of both experiments, roughly half of the participants could categorize items according to the partner’s (spontaneously updated) perspective and also used their partner’s updated perspective in the implicit transfer task to some degree, while they were the ones who displayed more pronounced anticipatory patterns as well. Such data provides strong evidence that the observed changes in anticipatory looking reflect spontaneous and flexible mental state updating. In addition, the findings also point to a high individual variability both in the updating of attributed mental states and the use of the updated mental state content.

The terminology used in discussions on mental state attribution is extensive and lacks consistency. In the current paper, experts from various disciplines collaborate to introduce a shared set of concepts and make recommendations regarding future use.

The ability to take each other’s visuospatial perspective has been linked to people’s capacity to perceive another’s action possibilities and to predict their actions. Research has also shown that visuospatial perspective taking is supported by one’s own mental own-body transformation. However, how these two processes of action perception and visuospatial perspective taking might interact remains largely unknown. By introducing seven angular disparities between participants and the model in the stimuli pictures across “Action” and “No Action” conditions, we investigated whether the observation of a goal-directed action facilitates perspective taking and whether this facilitation depends on the level of mental own-body transformation required to take perspective. The results showed that action observation facilitated performance independently of the level of mental-own body transformation. The processes behind this facilitation could involve anatomical mapping that is independent of the congruency between the participants’ and the model’s perspectives. Further, we replicated previous research findings, showing that participants were more accurate and faster when taking the perspective of a person compared to an inanimate object (a chair). The strongest facilitation effects were seen at the highest angular disparities between participants and the model in the stimuli pictures. Together, these findings enhance our knowledge of the mechanisms behind visuospatial perspective taking.

Benefiting from a cooperative interaction requires people to estimate how cooperatively other members of a group will act so that they can calibrate their own behavior accordingly. We investigated whether the synchrony of a group's actions influences observers' estimates of cooperation. Participants (recruited through Prolific) watched animations of actors deciding how much to donate in a public-goods game and using a mouse to drag donations to a public pot. Participants then estimated how much was in the pot in total (as an index of how cooperative they thought the group members were). Experiment 1 (N = 136 adults) manipulated the synchrony between players' decision-making time, and Experiment 2 (N = 136 adults) manipulated the synchrony between players' decision-implementing movements. For both experiments, estimates of how much was in the pot were higher for synchronous than asynchronous groups, demonstrating that the temporal dynamics of an interaction contain signals of a group's level of cooperativity.

Prudently choosing who to interact with and who to avoid is an important ability to ensure that we benefit from a cooperative interaction. While the role of others' preferences, attributes, and values in partner choice have been established (Rossetti, Hilbe & Hauser, 2022), much less is known about whether the manner in which a potential partner plans and implements a decision provides helpful cues for partner choice. We used a partner choice paradigm in which participants chose who to interact with in the Prisoners' Dilemma. Before choosing a cooperation partner, participants were presented with information about the potential partners' decision-related actions in another round of the Prisoners' Dilemma. They received either information about the potential partners' planning during decision making (i.e., decision-time; Experiment 1) or action execution during decision implementation (i.e., movement directness; Experiment 2). Across both games, participants preferred to interact with those who planned actions quickly or executed actions with direct and smooth movements, indicating that they were cooperating confidently and without deliberation. This demonstrates that action cues present in either the planning or implementation of economic decisions influence partner choice. We discuss implications of this finding for human decision-making and perception-action coupling in action understanding.

Contextual cues have been shown to inform our understanding and predictions of others’ actions. In this study, we tested whether observers’ predictions about unfolding actions depend upon the social context in which they occur. Across five experiments, we showed participants videos of an actor walking toward a piece of furniture either with (joint context) or without (solo context) a partner standing by it. We found greater predictive bias, indicative of stronger action expectations when videos contained a second actor (Experiment 1), even when the solo condition had a perceptually-matched control object in place of the actor (Experiment 2). Critically, belief manipulations about the actions the walking actor would perform suppressed the difference between social context conditions when the manipulation specified an action possible in both contexts (Experiment 5) but not when the action was one that would be difficult without a partner (Experiment 4). Interestingly, the social context effect persisted when the belief manipulation specified an unlikely action given the depicted scene (Experiment 3). These findings provide novel evidence that kinematically-identical actions can elicit different predictions depending on the social context in which they occur.