Human adults prefer to cooperate even when it is costly.

Joint actions are cooperative activities where humans coordinate their actions to achieve individual and shared goals. While the motivation to engage in joint action is clear when a goal cannot be achieved by individuals alone, we asked whether humans are motivated to act together even when acting together is not necessary and implies incurring additional costs compared to individual goal achievement. Using a utility-based empirical approach, we investigated the extent of humans' preference for joint action over individual action, when the instrumental costs of performing joint actions outweigh the benefits. The results of five experiments showed that human adults have a stable preference for joint action, even if individual action is more effective to achieve a certain goal. We propose that such preferences can be understood as ascribing additional reward value to performing actions together.

Affective compatibility with the self modulates the self-prioritisation effect.

The "self" shapes the way in which we process the world around us. It makes sense then, that self-related information is reliably prioritised over non self-related information in cognition. How might other factors such as self-compatibility shape the way self-relevant information is prioritised? The present work asks whether affective consistency between the self and arbitrarily self-associated stimuli influences the degree to which self-prioritisation can be observed. To this end, participants were asked to associate themselves with either a positive or a negative concept and to then indicate if a given stimulus (Experiment 1: Emotional faces; Experiment 2: Luminance cues) and an identity label matched. If affective consistency is key to self-prioritisation, negative constructs should dampen self-prioritisation and positive constructs should boost self-prioritisation because the self is universally construed as positive. Indeed, the results of the two experiments indicate that participants who made the negative association had more difficulty confirming whether the stimulus and the label matched than those who made the positive association. The implications of this finding are discussed in terms of "self" theories that span various levels of information processing. The data reveal that self-referential information processing goes beyond a default elevation of priority to the self.

Motor constraints influence cultural evolution of rhythm.

While widely acknowledged in the cultural evolution literature, ecological factors-aspects of the physical environment that affect the way in which cultural productions evolve-have not been investigated experimentally. Here, we present an experimental investigation of this type of factor by using a transmission chain (iterated learning) experiment. We predicted that differences in the distance between identical tools (drums) and in the order in which they are to be used would cause the evolution of different rhythms. The evidence confirms our predictions and thus provides a proof of concept that ecological factors-here a motor constraint-can influence cultural productions and that their effects can be experimentally isolated and measured. One noteworthy finding is that ecological factors can on their own lead to more complex rhythms.

Avoiding Accidents at the Champagne Reception.

Using a lifting and balancing task, we contrasted two alternative views of planning joint actions: one postulating that joint action involves distinct predictions for self and other, the other postulating that joint action involves coordinated plans between the coactors and reuse of bimanual models. We compared compensatory movements required to keep a tray balanced when 2 participants lifted glasses from each other's trays at the same time (simultaneous joint action) and when they took turns lifting (sequential joint action). Compared with sequential joint action, simultaneous joint action made it easier to keep the tray balanced. Thus, in keeping with the view that bimanual models are reused for joint action, predicting the timing of their own lifting action helped participants compensate for another person's lifting action. These results raise the possibility that simultaneous joint actions do not necessarily require distinguishing between one's own and the coactor's contributions to the action plan and may afford an agent-neutral stance.

Attention allocation and task representation during joint action planning.

We investigated whether people take into account an interaction partner's attentional focus and whether they represent in advance their partner's part of the task when planning to engage in a synchronous joint action. The experiment involved two participants planning and performing joint actions (i.e., synchronously lifting and clinking glasses), unimanual individual actions (i.e., lifting and moving a glass as if clinking with another person), and bimanual individual actions. EEG was recorded from one of the participants. We employed a choice reaction paradigm where a visual cue indicated the type of action to be planned, followed 1.5 sec later by a visual go stimulus, prompting the participants to act. We studied attention allocation processes by examining two lateralized EEG components, namely the anterior directing attention negativity and the late directing attention positivity. Action planning processes were examined using the late contingent negative variation and the movement-related potential. The results show that early stages of joint action planning involve dividing attention between locations in space relevant for one's own part of the joint action and locations relevant for one's partner's part of the joint action. At later stages of joint action planning, participants represented in advance their partner's upcoming action in addition to their own action, although not at an effector-specific level. Our study provides electrophysiological evidence supporting the operation of attention sharing processes and predictive self/other action representation during the planning phase of a synchronous joint task.

The dynamics of search, impasse, and representational change provide a coherent explanation of difficulty in the nine-dot problem.

The nine-dot problem is often used to demonstrate and explain mental impasse, creativity, and out of the box thinking. The present study investigated the interplay of a restricted initial search space, the likelihood of invoking a representational change, and the subsequent constraining of an unrestricted search space. In three experimental conditions, participants worked on different versions of the nine-dot problem that hinted at removing particular sources of difficulty from the standard problem. The hints were incremental such that the first suggested a possible route for a solution attempt; the second additionally indicated the dot at which lines meet on the solution path; and the final condition also provided non-dot locations that appear in the solution path. The results showed that in the experimental conditions, representational change is encountered more quickly and problems are solved more often than for the control group. We propose a cognitive model that focuses on general problem-solving heuristics and representational change to explain problem difficulty.

Me or we? Action-outcome learning in synchronous joint action.

Goal-directed behaviour requires mental representations that encode instrumental relationships between actions and their outcomes. The present study investigated how people acquire representations of joint actions where co-actors perform synchronized action contributions to produce joint outcomes in the environment. Adapting an experimental procedure to assess individual action-outcome learning, we tested whether co-acting individuals link jointly produced action outcomes to individual-level features of their own action contributions or to group-level features of their joint action instead. In a learning phase, pairs of participants produced musical chords by synchronizing individual key press responses. In a subsequent test phase, the previously produced chords were presented as imperative stimuli requiring forced-choice responses by both pair members. Stimulus-response mappings were systematically manipulated to be either compatible or incompatible with the individual and joint action-outcome mappings of the preceding learning phase. Only joint but not individual compatibility was found to modulate participants' performance in the test phase. Yet, opposite to predictions of associative accounts of action-outcome learning, jointly incompatible mappings between learning and test phase resulted in better performance. We discuss a possible explanation of this finding, proposing that pairs' group-level learning experience modulated how participants encoded ambiguous task instructions in the test phase. Our findings inform current debates about mechanistic explanations of action-outcome learning effects and provide novel evidence that joint action is supported by dedicated mental representations encoding own and others' actions on a group level.

Communicative modulations of early action components support the prediction of distal goals.

The successful unfolding of many social interactions relies on our capacity to predict other people's action goals, whether these are proximal (i.e., immediate) or distal (i.e., upcoming). The present set of studies asks whether observers can predict the distal goal of two-step action sequences when presented with communicative modulations of the first movement component of the sequence. We conducted three online experiments in which we presented participants with animations of a box moving to a first target location before moving onwards to a final, either near or far, target location. The second movement component and the target locations were occluded. After observing the first movement, participants were asked to select the most likely final target location, i.e., the distal goal of the sequence. Experiment 1 showed that participants relied on the velocity modulations of the first movement to infer the distal goal. The results of Experiment 2 indicated that such predictions of distal goals are possible even when the second movement in the sequence does not contain any velocity information, thus suggesting that the information present in the first movement plays the major role in the process of linking movements to their distal goals. However, Experiment 3 showed that under some circumstances the second movement can also contribute to how observers predict a distal goal. We discuss these results in terms of the underlying simulation processes that enable observers to predict a distal goal from the observation of proximal communicative modulations.

Better Together: 14-Month-Old Infants Expect Agents to Cooperate.

Humans engage in cooperative activities from early on and the breadth of human cooperation is unparalleled. Human preference for cooperation might reflect cognitive and motivational mechanisms that drive engagement in cooperative activities. Here we investigate early indices of humans' cooperative abilities and test whether 14-month-old infants expect agents to prefer cooperative over individual goal achievement. Three groups of infants saw videos of agents facing a choice between two actions that led to identical rewards but differed in the individual costs. Our results show that, in line with prior research, infants expect agents to make instrumentally rational choices and prefer the less costly of two individual action alternatives. In contrast, when one of the action alternatives is cooperative, infants expect agents to choose cooperation over individual action, even though the cooperative action demands more effort from each agent to achieve the same outcome. Finally, we do not find evidence that infants expect agents to choose the less costly alternative when both options entail cooperative action. Combined, these results indicate an ontogenetically early expectation of cooperation, and raise interesting implications and questions regarding the nature of infants' representations of cooperative actions and their utility.

Chimpanzees demonstrate a behavioural signature of human joint action.

The strength of human society can largely be attributed to the tendency to work together to achieve outcomes that are not possible alone. Effective social coordination benefits from mentally representing a partner's actions. Specifically, humans optimize social coordination by forming internal action models adapted to joint rather than individual task demands. To what extent do humans share the cognitive mechanisms that support optimal human coordination and collaboration with other species? An ecologically inspired joint handover-to-retrieve task was systematically manipulated across several experiments to assess whether joint action planning in chimpanzees reflects similar patterns to humans. Chimpanzees' chosen handover locations shifted towards the location of the experimenter's free or unobstructed hand, suggesting they represent the constraints of the joint task even though their individual half of the task was unobstructed. These findings indicate that chimpanzees and humans may share common cognitive mechanisms or predispositions that support joint action.

Monitoring individual and joint action outcomes in duet music performance.

We investigated whether people monitor the outcomes of their own and their partners' individual actions as well as the outcome of their combined actions when performing joint actions together. Pairs of pianists memorized both parts of a piano duet. Each pianist then performed one part while their partner performed the other; EEG was recorded from both. Auditory outcomes (pitches) associated with keystrokes produced by the pianists were occasionally altered in a way that either did or did not affect the joint auditory outcome (i.e., the harmony of a chord produced by the two pianists' combined pitches). Altered auditory outcomes elicited a feedback-related negativity whether they occurred in the pianist's own part or the partner's part, and whether they affected individual or joint action outcomes. Altered auditory outcomes also elicited a P300 whose amplitude was larger when the alteration affected the joint outcome compared with individual outcomes and when the alteration affected the pianist's own part compared with the partner's part. Thus, musicians engaged in joint actions monitor their own and their partner's actions as well as their combined action outcomes, while at the same time maintaining a distinction between their own and others' actions and between individual and joint outcomes.

Videos posted on the internet provide evidence for joint rushing in naturalistic social interactions.

When people engage in rhythmic joint actions, they unintentionally increase their tempo. However, this phenomenon of joint rushing has so far been investigated only under very specific and somewhat artificial conditions. Therefore, it remains unclear whether joint rushing generalizes to other instances of rhythmic joint action. In this study our aim was to investigate whether joint rushing can also be observed in a wider range of naturalistic rhythmic social interactions. To achieve this, we retrieved videos of a wide range of rhythmic interactions from an online video-sharing platform. The data suggest that joint rushing indeed can also be observed in more naturalistic social interactions. Furthermore, we provide evidence that group size matters for how tempo unfolds in social interactions with larger groups showing a stronger tempo increase than smaller groups. Comparing the data from naturalistic interactions with data collected in a lab study further showed that unintended tempo changes in social interactions are reduced in naturalistic interactions compared to interactions in a lab context. It is an open question which factors led to this reduction. One possibility is that humans might have come up with strategies to reduce the effects of joint rushing.

The role of reciprocity in dynamic interpersonal coordination of physiological rhythms.

A central question in social cognition research is how people coordinate their bodily rhythms, and how important reciprocity of interaction is for interpersonal coordination. Previous research has primarily focused on interpersonal action coordination, which has been shown to be facilitated by mechanisms of prediction and mutual adaption. Recent research is beginning to show that people also coordinate their physiological rhythms (i.e., respiration, heart rhythms) when they engage in natural forms of social interaction, such as conversation, choir singing, and rituals. However, the mechanisms underlying interpersonal physiological synchronization remain obscure, and could provide insight into the dynamic mechanisms that underlie continuous and regulatory, rather than instrumental, joint actions. Using real-time biofeedback, we investigated whether people synchronize their respiration rhythms by forming a joint dynamical system through reciprocity of interaction, or by producing more predictable respiration rhythms. Our results show that people are more in-phase synchronized but less phase-locked when interacting bidirectionally versus unidirectionally (online), but there is no difference in synchronization during reciprocal interaction and when adapting unidirectionally (offline) to recordings of respiration signals that emerged during the reciprocal interaction. Moreover, the strength of synchronization is driven by the predictability of the respiration rhythms that emerge in the bidirectional interaction - specifically, the slowing of breathing rhythms and stability of breathing frequencies - rather than the online mutual adaptation itself. These results suggest that coordination is facilitated by the emergence of predictable breathing patterns, rather than reciprocity itself.

Understanding others' distal goals from proximal communicative actions.

Many social interactions require individuals to coordinate their actions and to inform each other about their goals. Often these goals concern an immediate (i.e., proximal) action, as when people give each other a brief handshake, but they sometimes also refer to a future (i.e. distal) action, as when football players perform a passing sequence. The present study investigates whether observers can derive information about such distal goals by relying on kinematic modulations of an actor's instrumental actions. In Experiment 1 participants were presented with animations of a box being moved at different velocities towards an apparent endpoint. The distal goal, however, was for the object to be moved past this endpoint, to one of two occluded target locations. Participants then selected the location which they considered the likely distal goal of the action. As predicted, participants were able to detect differences in movement velocity and, based on these differences, systematically mapped the movements to the two distal goal locations. Adding a distal goal led to more variation in the way participants mapped the observed movements onto different target locations. The results of Experiments 2 and 3 indicated that this cannot be explained by difficulties in perceptual discrimination. Rather, the increased variability likely reflects differences in interpreting the underlying connection between proximal communicative actions and distal goals. The present findings extend previous research on sensorimotor communication by demonstrating that communicative action modulations are not restricted to predicting proximal goals but can also be used to infer more distal goals.

Joint rushing alters internal timekeeping in non-musicians and musicians.

Recent studies have shown that people engaging in joint rhythmic activity unintentionally increase their tempo. The same tempo increase does not occur when the same rhythmic activity is performed alone. This phenomenon is known as joint rushing. In two experiments, we investigated whether joint rushing is caused by correction mechanisms that facilitate sensorimotor synchronization. Because such correction mechanisms require perceptual input, joint rushing should discontinue when auditory feedback in a joint rhythmic activity is interrupted. This prediction was clearly supported in two experiments, one with musicians and one with non-musicians. Surprisingly, there was no indication that the amount of joint rushing differed between musicians and non-musicians. Furthermore, neither musicians nor non-musicians were able to return to the initially instructed tempo after feedback had been interrupted. This result indicates that joint rushing has a lasting effect on an internal timekeeper. An important question for future research is whether joint rushing is only a dysfunctional side effect of the way sensorimotor synchronization works or whether it has a function in enabling precise temporal coordination between different individuals.

Expert pianists make specific exaggerations for teaching.

Experts modulate their performance of actions for teaching purposes, performing slower and exaggerated movements when demonstrating novel actions to novices. The present study asked whether such modulations also occur during teaching performance of a music instrument, where subtle movement modulations are crucial for achieving artistic expression. While exaggerating performances of goal-directed actions outside of artistic contexts may be straightforward, it is an open question whether and how exaggeration for the purpose of teaching operates for actions that are expressive even when performed outside of a teaching context. Pianists were asked to demonstrate to students the techniques required for implementing notated expressions, compared to performing the piece without didactic intentions. Expressions in the piece concerned either articulation (i.e., legato and staccato) or dynamics (i.e., forte and piano). The pianists played either with the goal to perform the piece to an audience or with the goal to teach the respective techniques to novices. When intending to teach articulation, skilled pianists produced more exaggerated staccato. When intending to teach dynamics, they created a larger contrast between forte and piano. We found consistent results across a simple musical scale (Experiment 1) and a more naturalistic piece of music (Experiment 2). These findings show that teaching-specific action modulations generalise to expressive actions and suggest that action modulations serve to highlight the most relevant aspects of the actions to be learnt.

Inter-individual coordination in walking chimpanzees.

Humans, like many other animals, live in groups and coordinate actions with others in social settings.1 Such interpersonal coordination may emerge unconsciously and when the goal is not the coordination of movements, as when falling into the same rhythm when walking together.2 Although one of our closest living relatives, the chimpanzee (Pan troglodytes), shows the ability to succeed in complex joint action tasks where coordination is the goal,3 little is known about simpler forms of joint action. Here, we examine whether chimpanzees spontaneously synchronize their actions with conspecifics while walking together. We collected data on individual walking behavior of two groups of chimpanzees under semi-natural conditions. In addition, we assessed social relationships to investigate potential effects on the strength of coordination. When walking with a conspecific, individuals walked faster than when alone. The relative phase was symmetrically distributed around 0° with the highest frequencies around 0, indicating a tendency to coordinate actions. Further, coordination was stronger when walking with a partner compared with two individuals walking independently. Although the inter-limb entrainment was more pronounced between individuals of similar age as a proxy for height, it was not affected by the kinship or bonding status of the walkers or the behaviors they engaged in immediately after the walk. We conclude that chimpanzees adapt their individual behavior to temporally coordinate actions with others, which might provide a basis for engaging in other more complex forms of joint action. This spontaneous form of inter-individual coordination, often called entrainment, is thus shared with humans.

Giving a helping hand: effects of joint attention on mental rotation of body parts.

Research on joint attention has addressed both the effects of gaze following and the ability to share representations. It is largely unknown, however, whether sharing attention also affects the perceptual processing of jointly attended objects. This study tested whether attending to stimuli with another person from opposite perspectives induces a tendency to adopt an allocentric rather than an egocentric reference frame. Pairs of participants performed a handedness task while individually or jointly attending to rotated hand stimuli from opposite sides. Results revealed a significant flattening of the performance rotation curve when participants attended jointly (experiment 1). The effect of joint attention was robust to manipulations of social interaction (cooperation versus competition, experiment 2), but was modulated by the extent to which an allocentric reference frame was primed (experiment 3). Thus, attending to objects together from opposite perspectives makes people adopt an allocentric rather than the default egocentric reference frame.

The joint flanker effect: sharing tasks with real and imagined co-actors.

The Eriksen flanker task (Eriksen and Eriksen in Percept Psychophys 16:143-149, 1974) was distributed among pairs of participants to investigate whether individuals take into account a co-actor's S-R mapping even when coordination is not required. Participants responded to target letters (Experiment 1) or colors (Experiment 2) surrounded by distractors. When performing their part of the task next to another person performing the complementary part of the task, participants responded more slowly to stimuli containing flankers that were potential targets for their co-actor (incompatible trials), compared to stimuli containing identical, compatible, or neutral flankers. This joint Flanker effect also occurred when participants merely believed to be performing the task with a co-actor (Experiment 3). Furthermore, Experiment 4 demonstrated that people form shared task representations only when they perceive their co-actor as intentionally controlling her actions. These findings substantiate and generalize earlier results on shared task representations and advance our understanding of the basic mechanisms subserving joint action.

Making oneself predictable: reduced temporal variability facilitates joint action coordination.

Performing joint actions often requires precise temporal coordination of individual actions. The present study investigated how people coordinate their actions at discrete points in time when continuous or rhythmic information about others' actions is not available. In particular, we tested the hypothesis that making oneself predictable is used as a coordination strategy. Pairs of participants were instructed to coordinate key presses in a two-choice reaction time task, either responding in synchrony (Experiments 1 and 2) or in close temporal succession (Experiment 3). Across all experiments, we found that coactors reduced the variability of their actions in the joint context compared with the same task performed individually. Correlation analyses indicated that the less variable the actions were, the better was interpersonal coordination. The relation between reduced variability and improved coordination performance was not observed when pairs of participants performed independent tasks next to each other without intending to coordinate. These findings support the claim that reducing variability is used as a coordination strategy to achieve predictability. Identifying coordination strategies contributes to the understanding of the mechanisms involved in real-time coordination.