Ownership Status Influences the Degree of Joint Facilitatory Behavior

Abstract

When engaging in joint activities, humans tend to sacrifice some of their own sensorimotor comfort and efficiency to facilitate a partner’s performance. In the two experiments reported here, we investigated whether ownership—a socioculturally based nonphysical feature ascribed to objects—influenced facilitatory motor behavior in joint action. Participants passed mugs that differed in ownership status across a table to a partner. We found that participants oriented handles less toward their partners when passing their own mugs than when passing mugs owned by their partners (Experiment 1) and mugs owned by the experimenter (Experiment 2). These findings indicate that individuals plan and execute actions that assist their partners but do so to a smaller degree if it is the individuals’ own property that the partners intend to manipulate. We discuss these findings in terms of underlying variables associated with ownership and conclude that a self-other distinction can be found in the human sensorimotor system.

Keywords

ownership joint action beginning-state comfort action prediction response selection shared task representation self-relevance

People have sophisticated and flexible internal models of action that allow effective action planning when they act alone (Wolpert & Flanagan, 2001). Individuals can also incorporate or adapt internal models to account for other agents (Flanagan & Johansson, 2003; Welsh, Wong, & Chandrasekharan, 2013). This extension or adaptation of individual internal models to other people allows humans to predict action outcomes on the basis of dynamically unfolding observable events performed by other people around them. These internal models of action can also be influenced by shared task representations and goals established before action execution (Sebanz, Bekkering, & Knoblich, 2006). For example, when passing an object to a partner, knowing the partner’s intention allows a person to activate the appropriate internal model to predict the manner in which the partner will use the object. Predictions made on the basis of this model in turn lead the person to spontaneously orient that object to a more comfortable position for the partner (Ray & Welsh, 2011). Known as the beginning-state-comfort effect, this facilitatory behavior can be seen with a range of objects, including hammers and calculators (Gonzalez, Studenka, Glazebrook, & Lyons, 2011).

Actionable objects, however, are not defined solely by their physical features. They are also defined by nonphysical (i.e., conceptual) features such as aesthetic qualities, value, and ownership status. In the case of ownership, self-owned objects enjoy elevated importance with regard to a number of psychological variables, including attention (Turk et al., 2011) and memory (Cunningham, Turk, MacDonald, & Macrae, 2008). Ownership has also been shown to influence physical interactions with objects when individuals act alone: (a) Actors reach lower peak accelerations when lifting someone else’s objects than when they lift their own objects, and (b) actors’ movements are consistent with a general reticence to interact with other people’s objects (Constable, Kritikos, & Bayliss, 2011; Constable, Kritikos, Lipp, & Bayliss, 2014). These effects reflect important biases in attachment, preference, and perhaps learned motor programs (for a discussion, see Constable et al., 2014); such biases alter the manner in which people plan and produce actions toward objects. The present study was conducted to determine whether the conceptual feature of object ownership influences action planning and execution in dyadic interactions.

Given that ownership is socially based, it stands to reason that the ownership status of an object may shape action planning in social (i.e., joint) interactions. This reasoning is predicated on previous work showing that social factors related to the partner influence the emergence of joint-action effects. For example, joint-action effects that are based on the corepresentation of action are stronger when the relationship between two coacting agents is positive than when the relationship is negative (Hommel, Colzato, & van den Wildenberg, 2009). In such studies of joint action (Sebanz, Knoblich, & Prinz, 2003; Welsh et al., 2005), however, the social environment is incidental to the task because participants perform independent tasks beside another individual. These scenarios may not reflect what occurs in more dynamic and interactive task environments that afford a level of real-world applicability and cooperation rooted in predictions made on the basis of internal models of actions. In the current experiments, we investigated how the attribution of ownership status to an object could influence the everyday act of passing that object between two people. In Experiment 1, we explored whether an action goal modulated the effect of ownership on joint-action performance, and in Experiment 2, we examined whether the status of the relationship between the participant and the owner of the object moderated the effect of ownership in joint action.

Experiment 1

Each participant was given a mug to keep before the testing session. One to 2 weeks after receiving mugs, pairs of participants completed a task in which one member of the pair passed a mug (their own or their partner’s) across a table to the other member of the pair. Their partner then either acted on the mug (action condition) or remained stationary (no-action condition). We recorded the angle of the mug’s handle relative to the partner’s hand to determine (a) how passers would orient the mug on the basis of their partner’s task, and (b) whether the ownership status of the object modulated any joint-action effect.

Given previous findings associated with beginning-state comfort (Ray & Welsh, 2011), we expected that an overall facilitatory behavior would emerge—the handle would be rotated further toward the partner’s optimal grasping angle in the action condition compared with the no-action condition. Moreover, we predicted that if ownership modulates all types of actions, then an ownership bias would be present in both conditions. However, if the context of the action interacts with ownership, the effect of ownership status will be present only in the action condition, in which the receiver actually interacts with the object.

Method

Participants

Thirty-eight right-handed participants (18–25 years old, mean age = 21.40 years, SD = 2.26; 13 men) were recruited in pairs from the University of Toronto academic community. On the basis of pilot research, we set a target of 20 pairs; however, 1 pair dropped out of the study before the testing session. All pairs of participants were friends. All participants provided written informed consent before testing and received $10 Canadian (Can) and a mug to keep (worth approximately Can$1.50). The procedures were approved by the Health Sciences Research Ethics Board of the University of Toronto and complied with the ethical standards outlined in the Declaration of Helsinki (World Medical Association, 2013).

Stimuli and apparatus

Owned objects

Each person in a pair received a different mug to ensure that participants could distinguish between their own mug and their partner’s mug. All mugs had the same shape and had a simple design: black lines that formed a stem and a schematic of a maple leaf, and a second red maple leaf. The mugs differed only in background color (white, green, dark brown, black, or light brown).

Motion-capture system

The location of each participant’s hand and the mug was recorded using an eight-camera Qualisys Oqus-1 motion-capture system (Qualisys AB, Gothenburg, Sweden). Four infrared reflective markers were attached to each participant’s right hand (at the tips of the index finger and thumb) and wrist (at the radius and ulna). A marker was placed on the rim of the mug at the point at which the handle attached; three additional markers were placed on the rim at 90°, 180°, and 270° relative to the first marker.

Procedure and design

Participants were tested in pairs and visited the laboratory twice. During the first meeting, all participants were given a mug and asked to take it home or to work and to use it every day. They were specifically instructed that no one else was to use this mug (to avoid the possibility of diluting the self-object association or the feeling of psychological ownership). Participants returned to the laboratory after 6 to 14 days (M = 9.35 days, SD = 1.95) for testing.

During the second session, the paired participants sat at a table, across from each other, with their right hands resting on the table at a marked location, approximately 10 cm from the edge of the table. Participants were instructed to rest their hands on the table with thumb and forefinger gently opposing. The experimenter, who was located at the end of the table, between the two participants, placed a mug in front of the participant who was acting as the passer on a given trial. The mug was always empty and was oriented with the handle parallel to the nearest table edge and pointing right. When the experimenter said “Go,” the passer picked up the mug and placed it in front of the receiver. The passer was told to pass the mug in a natural manner using the right hand but were given no additional instructions on where or how to pass it.

There were two conditions. In the action condition, the receiver picked up the mug using the handle, lifted it, and then placed it back down on the table. In the no-action condition, the receiver remained stationary after the passer placed the mug on the table. Before each block of trials began, the participants were informed whether a given block would require action or no action. The participant who acted as the passer was varied randomly on a trial-by-trial basis, and participants became aware of the role they would be playing when the experimenter placed the mug in front of them. Each pair completed two blocks of 120 trials. The action and no-action conditions were blocked, and the order was counterbalanced between participant pairs. Assignment to the passer role and ownership condition (whose mug was used) were fully randomized within each block.

Data processing

The data were processed in MATLAB (The MathWorks, Natick, MA) to derive the critical measure of difference in hand-object angle. The measure was chosen on the basis of a pilot experiment that used a similar paradigm and had promising results (n = 39; between-participants design). The hand-object angle represents the orientation of the mug handle relative to the receiver’s hand. The angle was formed by the intersection of two lines: (a) the line between the marker on the index finger and the marker on the radius and (b) the line between two markers on the rim, one at the mug handle and the other 180° around the rim (Fig. 1, inset). This measure can be characterized as a participant-centered measure that incorporates the partner’s hand orientation. Essentially, the measure represents how much farther the mug would need to be rotated to be perfectly matched with the receiver’s hand angle regardless of the direction. Note that absolute values were computed. Therefore, the same value would result whether the mug needed to be rotated a further 60° in a counterclockwise or clockwise direction to match the receiver’s hand. A smaller angle indicated a greater match between the orientation of the hand and mug (possibly indicating greater facilitatory behavior). Trials on which data recording was poor or incomplete (9%) were removed before analysis (for the data submitted to inferential statistics, please see the Supplemental Data in the Supplemental Material available online).

Fig. 1.

Results for Experiment 1: average difference in angle between the passed mug and the receiver’s hand. The inset shows how this angle was measured. The vertex of the angle was the point at which the handle attached to the mug. One side of the angle was the line extending from the marker on the radius of the wrist through the marker on the index finger and continuing to the vertex (red lines). The other side of the angle was the line extending from the marker on the far side of the mug’s rim through the vertex (dashed and dotted lines). The thick black arcs show results for the action condition, and the thick gray arcs show results for the no-action condition. Results are shown separately for the passer’s mug and the receiver’s mug.

Results

Figure 1 presents the mean difference in hand angle in each condition. A 2 (action condition: action, no action) × 2 (ownership condition: participant, partner) repeated measures analysis of variance (ANOVA) on the difference in hand angle yielded two important findings. First, there was a statistically significant main effect of action condition, F(1, 35) = 31.13, MSE = 5,823, p < .001, η_p² = .47. This means that when the participants placed the mugs in front of their partners, the angle was smaller in the action condition (121°) than in the no-action condition (192°). This finding indicates that the mug handle was rotated closer to the receiver’s hand when action was expected than when action was not expected—a replication of the beginning-state-comfort effect (Ray & Welsh, 2011).

The second finding, which was more theoretically relevant and novel, was that although the main effect of ownership did not reach statistical significance, F(1, 35) = 1.14, MSE = 92, p = .293, η_p² = .03, the interaction between action condition and ownership was statistically significant, F(1, 35) = 4.32, MSE = 65, p = .045, η_p² = .11. Planned comparisons revealed that this interaction emerged because the difference in hand-object angle for the passer’s mug and the receiver’s mug was statistically significant only in the action condition. Specifically, when passers used their own mugs, they oriented them less toward the receivers than when using the receivers’ mugs, t(35) = 2.10, p = .04, d_z = 0.35, 95% confidence interval, or CI, for the difference between conditions = [0.16, 8.82]. In the no-action condition, the difference in hand-object angle between the passer’s mug and the receiver’s mug did not reach significance, t(35) = −0.53, p = .60, d_z = 0.09, 95% CI for the difference between conditions = [−5.20, 3.04]. Thus, ownership modulated the passer’s behavior only when the receiver was to act on the object—when the passer placed the receiver’s mug on the table, the handle was closer to the receiver’s hand angle than when the passer used his or her own mug (for further exploration of the ownership effect relative to individual participants, see the Supplemental Graphs in the Supplemental Material).

Experiment 2

In Experiment 1 our primary finding was that ownership modulated facilitatory behavior in the action condition such that the final position of the mug handle was closer to the receiver’s hand for the receiver’s mug than for the passer’s mug. Do these effects reflect an increase or a decrease of facilitatory behavior dependent on the relative relationship between the owner of the mug and the passer? To this end, we added an experimenter-owned mug to the design to represent an ecologically valid other-owned object that was not owned by the receiver. Our second question concerned length of ownership because previous research has demonstrated a modulatory effect of time on the ownership effect (Shu & Peck, 2011). In Experiment 2, we retained the long-term condition but also had participants complete the task immediately after ownership assignment.

Method

Participants

Forty-six right-handed participants (18–26 years old, mean age = 20.96 years, SD = 2.34; 13 men) were recruited from the University of Toronto academic community. We aimed to recruit 20 pairs, as in Experiment 1, but eliminated and replaced 3 pairs of participants because they indicated that they did not believe that they would be able to keep the mug at the end of the study, which invalidated the manipulation of ownership for these individuals. Each participant received Can$10 per hour and the mug in exchange for his or her time. All participants provided written informed consent before testing. The procedures were approved by the health sciences research ethics board of the University of Toronto and complied with the ethical standards outlined in the Declaration of Helsinki (World Medical Association, 2013).

Owned objects

Each participant was given a mug to keep (worth approximately Can$1.50). The mugs in Experiment 2 were larger than those used in Experiment 1 because they needed to accommodate the components (battery packs, wires, and controller) of a different motion-tracking system. Consequently, the mugs were also heavier than the mugs used in Experiment 1. All mugs in Experiment 2 were the same size; they differed only in color.

Motion-capture system

An Optotrak Certus motion capture system (Northern Digital Inc., Waterloo, ON, Canada) with a combination of wired and wireless active, infrared markers was used for Experiment 2. Two wired markers were attached to the receiver, one on the tip of the index finger and one at the radial styloid process. A battery pack with three active markers attached to a rigid body was used to record the relative location and orientation of the mug. One marker corresponded to the handle, one to the front of the mug, and one to the side of the mug opposite the handle (see Fig. 2, inset). On each trial, we recorded five data points in 3-D Cartesian space representing one hand and one mug.

Fig. 2.

Results for Experiment 2: average difference in angle between the passed mug and the receiver’s hand. The angle was measured as described in Figure 1. Results are shown separately for the participant’s mug, the partner’s mug, and the experimenter’s mug.

Procedure and design

The procedure was similar to that in Experiment 1 except for some slight modifications to the design to account for the different technology used and the specific nature of the research question. First, because we were interested in the effects of the relationship between the mug’s owner and the passer on facilitatory behavior, we included only the action condition in which the receiver lifted the mug after it was passed. Second, because we were interested in assessing the influences of duration of ownership on the behavior, the passing task was completed immediately after the participants acquired the mug and again approximately 2 weeks later (12–18 days, M = 14.05 days, SD = 1.05). During the passing task, one member of each pair acted as the passer for the first 90 trials, and then the other member was the passer for the remaining 90 trials. Within each 90-trial segment, participants passed the experimenter’s mug, their partner’s mug, and their own mug 30 times each. That is, participants’ role and the mug’s ownership were blocked and counterbalanced in Experiment 2, unlike in Experiment 1, in which participants’ role and mug ownership were randomized.

Data processing

The measure used was identical to that in Experiment 1. One participant broke her mug before the second session, and we were therefore unable to collect a complete data set from that pair. As mentioned earlier, three other pairs did not believe they would be able to keep the mug, and so they were not included. A further participant was removed because he was unable to follow the instruction to place the mug in front of his partner. Thus, the final sample included 37 participants. Trials on which data recording was poor, incomplete, or divergent from the task (3%) were removed before analysis (for data submitted to inferential statistics, please see the Supplemental Data).

Results

The 3 (ownership condition: participant, partner, experimenter) × 2 (time: immediate testing, delayed testing) repeated measures ANOVA revealed several important findings. First, the statistically significant main effect of ownership condition, F(2, 72) = 3.81, MSE = 970, p = .027, η_p² = .096, replicated and extended the findings of Experiment 1. Specifically, follow-up comparisons across ownership conditions (collapsed across the time factor) revealed a significant difference in hand angle between the condition in which the experimenter owned the mug (95°) and the condition in which the participant owned the mug (109°), t(36) = 2.47, p = .02, d_z = 0.41, 95% CI for the difference between conditions = [2.55, 25.81]. The differences in hand angle between the condition in which the experimenter owned the mug (95°) and the condition in which the partner owned the mug (101°), t(36) = 1.44, p = .16, d_z = 0.24, 95% CI for the difference between conditions = [−2.56, 14.95], and between the condition in which the partner owned the mug (101°) and the condition in which the participant owned the mug (109°), t(36) = 1.53, p = .14, d_z = 0.25, 95% CI for the difference between conditions = [−2.60, 18.57], did not reach significance (see Fig. 2). Second, neither the main effect of time, F(1, 36) = 0.001, MSE = 3,339, p = .974, η_p² < .001, nor the interaction between ownership condition and time, F(2, 72) = 2.13, MSE = 437, p = .13, η_p² = .06, reached statistical significance. Thus, the amount of time owning the object did not modulate the expression of object ownership in this joint-action task (for further exploration of the ownership effect relative to individual participants, see the Supplemental Graphs).

General Discussion

The present study was conducted to investigate whether ownership, a socially relevant nonphysical feature of objects, can influence joint-action behavior. Our study had two findings: (a) a broad replication of a well-documented effect in the joint action literature using a more continuous and precise measure and (b) evidence that ownership influences action planning and execution in a manner that is isolated to a joint-action task. We focus on the critical effects associated with ownership. In Experiment 1, when the passer moved the receiver’s mug toward the receiver for action, it was rotated farther toward the receiver than when passers moved their own mugs. In Experiment 2, this finding was replicated with the experimenter’s mug and the participant’s own mug. Because the objects were physically identical (aside from color), the difference in rotation is driven by top-down modulation of motor commands associated with ownership status rather than bottom-up factors associated with the low-level perceptual properties of the mugs. Moreover, because this effect does not interact with time, familiarity with the mugs is unlikely to play a role.

Given that the facilitatory joint-action behavior was expressed for both one’s own and someone else’s mugs, and only the magnitude of that behavior was altered, we suggest that such high-level conceptual knowledge and associated motor programs may implicitly leak into intended movements despite having no functional basis in this particular instance. We propose that when the concept of ownership is introduced, established prior motor associations with relevant objects change. Although it is difficult to override a facilitatory consideration in joint action, prior associations may be able to modulate the effect. Perhaps it is more common to use your own (and your friend’s) possessions and therefore optimize your own comfort (for a review of end-state comfort, see Rosenbaum, Chapman, Weigelt, Weiss, & van der Wel, 2012; but see Herbort, 2013). Indeed, prior experience seems to shape response selection when engaging in higher-order planning of sequential action components in both joint and individual situations (individual, Herbort & Butz, 2011; joint and individual, Meyer, van der Wel, & Hunnius, 2013). Furthermore, discordant action consequences can influence kinematic parameters (Pfister, Janczyk, Wirth, Dignath, & Kunde, 2014). Therefore, in the case of the present study, it may be sensible to conclude that underlying cognitive biases, intentions, or prior experience associated with cognitive concepts (i.e., learned associations) are influencing motor programming even though they are discordant with the actual motor intention.

One could also speculate that the effect of ownership may be grounded in embodied mechanisms. For example, it has been reported that it is more natural to make a pushing movement in association with negatively connoted words and a pulling movement in association with positively connoted words (Chen & Bargh, 1999). This effect can then be altered by the target of the verb, which indicates a social modulation (Lugli, Baroni, Gianelli, Borghi, & Nicoletti, 2012). According to this line of reasoning, a participant in the present study may not reach the same level of facilitatory behavior with his or her own object because the movement of passing the mug, in essence, represents an incongruent condition of pushing or giving a positively connoted object away. The learned-association account mentioned in the last paragraph does not necessarily conflict with an embodied account. An embodied account may simply represent a higher degree of abstraction in the mechanisms associated with the effect and may eventually prove to be an extension of a learned associative account whereby ownership tends to be confounded with valence (e.g., the mere ownership effect, Beggan, 1992).

Two additional important insights emerged from Experiment 2. First, facilitatory behavior in joint action is suppressed when using one’s own objects. That is, the ownership effect in this study represented a reduction of the level of facilitatory behavior associated with the owned mug rather than an enhancement of facilitation associated with the other mugs. This conclusion can be made because the strongest joint-action effect was with experimenter’s mug, yet the experimenter was never the partner. Second, it may be necessary to consider both the experimental context (in Experiment 1, the experimenter’s mug was not used) and the mug owners’ relationship to the other owners. In the case of Experiment 2, the self-overlap is higher for the partner’s mug than for the experimenter’s mug because the partner is a friend and thus represents a middle ground. Indeed, the specific numerical pattern of results in Experiment 2 is consistent with data patterns found in literature concerning self-relevance and perception (e.g., Sui, He, & Humphreys, 2012).

In sum, the motor system has developed, both on a collective and individual level, to find elegant solutions to the core problems it faces of efficiently interacting within a social world. The present studies reveal that nonphysical cultural and social features that are culturally and societally based can bias the execution of motor plans in previously unreported ways. Although awareness of the ownership status of an object did not interrupt the overarching goal (facilitation of joint behavior) in these naturalistic passing movements, ownership did shape the dyad’s joint goals in subtle ways.

Footnotes

Acknowledgements

We thank Tyson Beach, Luc Tremblay, Drazen Glisic, Valentin Cranic, and John De Grosbois for assistance with the lab space and equipment and Ada Kritikos and Ottmar V. Lipp for comments on an earlier version of the design.

Action Editor

Marc J. Buehner served as action editor for this article.

Declaration of Conflicting Interests

The authors declared that they had no conflicts of interest with respect to their authorship or the publication of this article.

Funding

This research was supported by the Ontario Ministry of Research and Innovation and by Natural Sciences and Engineering Research Council of Canada Grants 2015-06482 (to T. N. Welsh) and 194537 (to J. Pratt).

Supplemental Material

Additional supporting information can be found at

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Supplementary Material

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