Prejudiced interactions: implicit racial bias reduces predictive simulation during joint action with an out-group avatar

Abstract

During social interactions people automatically apply stereotypes in order to rapidly categorize others. Racial differences are among the most powerful cues that drive these categorizations and modulate our emotional and cognitive reactivity to others. We investigated whether implicit racial bias may also shape hand kinematics during the execution of realistic joint actions with virtual in- and out-group partners. Caucasian participants were required to perform synchronous imitative or complementary reach-to-grasp movements with avatars that had different skin color (white and black) but showed identical action kinematics. Results demonstrate that stronger visuo-motor interference (indexed here as hand kinematics differences between complementary and imitative actions) emerged: i) when participants were required to predict the partner's action goal in order to on-line adapt their own movements accordingly; ii) during interactions with the in-group partner, indicating the partner's racial membership modulates interactive behaviors. Importantly, the in-group/out-group effect positively correlated with the implicit racial bias of each participant. Thus visuo-motor interference during joint action, likely reflecting predictive embodied simulation of the partner's movements, is affected by cultural inter-individual differences.

Figure 1. The figure illustrates (a) the steps followed to create the clips of the avatar's movements; kinematics of the virtual partners were recorded using a Vicon MX optical tracking system (Vicon Motion Systems, Oxford, UK) and the final processed trajectories were animated using commercial software (Autodesk, Motion Builder); (b) the experimental set-up; (c) the position of the markers on participants' thumb and index finger of the right hand placed on the table in the starting position with thumb and index finger gently opposed; and (d) a schematic representation of Action-types (Complementary/Imitative).

Figure 2. The graph illustrates the shape of grip aperture stereotypical patterns (taken from single tracks of one participant) and shows that in the JA condition correction trials (THICK LINES) showed a similar shape compared to no-correction (DASHED LINES) trials until the closure phase, and then they diverged, thus highlighting that participants on-line adapted to the avatar's movement.

Grip aperture data have been normalized on movement time so that the abscissa reports percentages of movement. On the left panel, the kinematic profile of grip aperture in corrections from power to precision grip (THICK LINE): it shows that the participant firstly opened the hand as to perform a power grip during the opening phase, e.g. their grip aperture was similar to a power grip (see DASHED LINE in this panel), and then reduced the aperture so as to match the kinematics of a precision grip during the closure phase (i.e. the final grip aperture is equal to the one of a precision grip, see dashed line in the right panel). Conversely, on the right panel, the figure illustrates grip aperture data in corrections from precision to power grip (THICK LINE): it shows that participants firstly opened their hand as to perform a precision grip, e.g. their grip aperture was similar to a precision grip (see DASHED LINE in this panel), and they then corrected their movement until it matched the aperture of the power grip (i.e. the final grip aperture is equal to the one of a power grip, see dashed line in the left panel).

Figure 3. On the left: the analyses of visuo-motor interference on MaxAp data (interference index obtained dividing Complementary/Imitative trails).

The graph shows the Partner x Interaction-type x Grip-type significant interaction (F(1,13) = 8.8, p = .011, partial η² = .4) indicating that visuo-motor interference effects emerged only in Precision grips during JA interactions with the In-group. Error bars indicate s.e.m. (***) p < .001. On the right: the highly significant positive correlation (r = .67, p = .012) indicates that the difference between visuo-motor interference (Complementary/Imitative MaxAp, mm/mm) emerged with the In-group as compared to with the Out-group partner [(In-group visuo-motor interference) – (Out-group visuo-motor interference)] was higher in the participants with stronger bias.