Social Working Memory Predicts Social Network Size in Humans

Objectives: The Social Brain Hypothesis posits a quantitative relationship between primate neocortex size and social network size. However, the precise social-cognitive mechanisms that drive this relationship remain elusive. Social Working Memory (SWM)—the ability to actively maintain and manipulate social information—has been proposed as a potential mechanism, but, to date, has not been linked to network size. Here, we explicitly tested this association. Methods: In Study 1, 125 participants completed a SWM task and reported on their social networks. In Study 2, 25 participants underwent fMRI during the SWM task and reported on their social networks. Results: As predicted, in Study 1, SWM performance was significantly associated with social network size and, specifically, “Sympathy Group” size (i.e., the size of one’s core friend group). In Study 2, we conceptually replicated and extended this effect by showing that neural activity in the dorsal medial prefrontal cortex and medial prefrontal cortex engaged during SWM (vs. non-social working memory) was associated with individual variation in Sympathy Group size. Conclusions: Taken together, these findings provide the first evidence that SWM constrains social network size, and suggest that SWM may be one social cognitive competency that underlies the Social Brain Hypothesis. In addition, whereas prior work investigating the Social Brain Hypothesis has largely focused on correlating brain structure size with social network size, to our knowledge, this is the first functional imaging evidence supporting the Social Brain Hypothesis. © 2018, Springer Nature Switzerland AG.