Does Restricting an Observer’s Arm Movements Degrade Their Judgments of Others’ Reaching Abilities?
Authors: Garcia Nicholas A. , Jones Keith S. , Widlus Benjamin P. 
Published: November 2019

Summary Observers can perceive others’ action capabilities. These actions include observers’ abilities to perceive the maximum height that an actor can sit, step across a gap, climb in a bipedal manner, or reach an object (Stoffregen et al., 1999; Mark, 2007; Ramenzoni et al., 2008a, 2008b). While observers’ abilities to perceive others’ action capabilities have been widely studied, researchers debate the information to which observers attend when making such judgments. Some have argued observers attend to actor-environment relations when perceiving others’ action capabilities (e.g., Stoffregen et al., 1999; Mark, 2007; Ramenzoni et al., 2008a). From this perspective, observers attend to relations between relevant characteristics of the actor’s body (e.g., leg length) and their environment (e.g., step height) to perceive actors’ action capabilities (e.g., stair-climbing ability). This perspective has empirical support. For example, observers differentiated short and tall actors’ maximum sitting heights but only when the actors and sitting apparatus were presented in the same scale (Stoffregen et al., 1999). Others have argued observers attend to observer-environment relations when perceiving others’ capabilities (e.g., Knoblich & Sebanz, 2006; Ramenzoni et al., 2008b; Gallese & Sinigaglia, 2011). From this perspective, observers perceive their own action capabilities (Step 1), which serve as a model for the actor’s action capabilities and then adjust that model (Step 2) to account for observer-actor differences (Knoblich & Sebanz, 2006; Gallese & Sinigaglia, 2011). This perspective also has empirical support. For example, observers wearing ankle weights underestimated actors’ maximum jump-to-reach heights (Ramenzoni et al., 2008b). The present study further investigated whether observers attend to observer-environment relations when perceiving others’ maximum reach capabilities. Participants ( n = 34) made judgments about a confederate’s maximum reach capability while participants’ arms were held either freely by their sides (Unrestricted Condition) or placed behind their back (Restricted Condition). Widlus and Jones (2017) demonstrated that such arm restriction led to more erroneous judgments about one’s own reaching capabilities. To make judgments, participants directed the confederate to the farthest point from a hanging object that would still afford the confederate the ability to reach the object. If observers attend to observer-environment relations when judging the confederate’s maximum reach capability, then 1) judgment error would be greater in the Restricted condition than in the Unrestricted condition, 2) judgments would align with observers’ capabilities better than with the confederate’s, and 3) judgment error would positively correlate with the degree of dissimilarity between observers’ and the confederate’s action capabilities. The experiment used a within-subjects design. The independent variable was observer arm exploration, which consisted of two levels: unrestricted and restricted arm exploration. The dependent variable was the participant’s judgment of the farthest distance the confederate could reach. This was operationalized as the distance between the confederate’s clavicle and the to-be-reached object, once participants had directed the confederate to the position where they believed the confederate could just reach the object. Those judgments served as the basis for several measures. The present study’s results suggested arm restriction did not increase judgment error. Second, judgments did not align with observers’ capabilities better than with the confederate’s. Third, judgment error did not positively correlate with the degree of dissimilarity between observers’ and the confederate’s action capabilities. Collectively, these outcomes provide consistent evidence that observers did not base their judgments of the confederate’s reaching capabilities on observer-environment relations. Instead, these results are consistent with previous studies that support the possibility that observers based their judgments on actor-environment relations (Stoffregen et al., 1999; Ramenzoni et al., 2008a). Understanding how observers judge others’ action capabilities allows us to better predict errors that may occur in operational settings, e.g., whether a firefighter will inaccurately judge whether their partner can accomplish a given task. Human factors professionals can then develop solutions to mitigate such errors, e.g., equipment redesign to better reveal actor-environment relations.