We investigated changes in the neural processing of social information as possible link between social exclusion and aggression. Participants played a virtual ball game with two putative game partners, during which half of the 34 participants were excluded. Then, participants played the Taylor Aggression Paradigm (TAP) against the same partners. An empathy paradigm followed, in which participants watched pictures of neutral and emotional social scenes, while undergoing functional magnetic resonance imaging (fMRI). Excluded participants showed stronger neural reactivity to emotional compared to neutral pictures than included participants in regions associated with cognitive mentalizing and the mirror neuron system (bilateral superior, middle and inferior temporal gyrus, bilateral precuneus, right precentral gyrus). Reactivity of left inferior temporal gyrus and right precentral gyrus was positively correlated with aggressive behavior in the TAP. Our results support previous behavioral findings which suggest changes in social information processing as mediator between exclusion and aggression.
1.1. Social exclusion and aggressive behavior
Forming and maintaining social bonds is considered a basic human need (Baumeister & Leary, 1995). Given the antisocial nature of aggressive behavior, it is somewhat counterintuitive that social exclusion, or interpersonal rejection, has been shown to elicit aggressive behavior in a variety of experimental settings (review in Leary, Twenge, & Quinlivan, 2006). Yet, ample evidence from longitudinal studies in children, for instance, indicates a vicious cycle of poor social skills or deficits in social information processing leading to peer rejection, which in turn increases antisocial behavior and negatively affects social information processing skills, making future peer rejection more likely (Lansford et al., 2010 and Miller-Johnson et al., 2002). Evidencing such long-lasting effects, social exclusion during childhood was found to be associated with aggressive behavior in early adulthood (Rabiner, Coie, Miller-Johnson, Boykin, & Lochman, 2005).
Besides the long-term impact of social exclusion on aggression, numerous studies demonstrated short-term effects on antagonistic behavior. In an experimental setting involving both children and adults, Moor et al. (2012) found that in a dictator game (Cason & Mui, 1998) following exclusion in an online ball-tossing game, participants made more unfair offers toward those who excluded them than toward new partners. In a similar vein, Twenge, Baumeister, Tice, and Stucke (2001) found increased aggressive behavior in the Taylor Aggression Paradigm (TAP; Taylor, 1967) after participants were rejected by their peers. In contrast to the findings of Moor et al. (2012), aggression in this study was increased even toward new game partners, who had not been involved in the previous rejection.
In conclusion, our findings suggest that social exclusion leads to changes in the neural processing of socio-emotional stimuli, as previously shown on a behavioral level (DeWall, Twenge, et al., 2009). Exclusion led to increased recruitment of areas associated with perspective taking and action goal inference in response to negative social stimuli. Reactivity of brain areas associated with these cognitive ToM processes, in turn, was positively related to proactive aggressive behavior during the subsequent competitive reaction time task.
