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.
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.
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.
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.
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As indicated in our pre-registration, we aimed to recruit an additional 100 participants. In an effort to expedite data collection, we deviated from our original procedures and recruited some participants over the summer months (May–August), relying solely on on-line classified ads. These participants (N = 24), however, differed, significantly, from those recruited during the academic terms (N = 140); they were older (t (154) = −4.46, p < 0.001), more educated (t (155) = −4.59, p < 0.001), more likely to be employed full-time (χ2 = 22.31, p < 0.001), and less likely to be in school (χ2 = 10.28, p < 0.001), and, perhaps most importantly, performed significantly better on the SWM task (t (154) = −2.24, p = 0.027). Because of the different recruitment procedures and because they differed in non-trivial ways from the main sample, we excluded them from the analyses.
Of note, additional data from another sample of 56 participants who completed the SWM paradigm on two occasions, 12 days apart, indicate that the task is reliable (Meyer & Lieberman, unpublished data). Specifically, the Time 1 and Time 2 correlation for SWM (r(54) = .40) and non-SWM (r(54) = .42) were significant (both p’s < .003), and were not significantly different from one another (p = .91), suggesting that they are similarly reliable.
For the interested reader, we examined the effect of gender on SWM, non-social working memory, and both network sizes. There was no effect of gender on any of these variables (all ps > 0.195).
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We thank Celine Coletta, Costanza Graziani, Emily Ower, Irene Giannis, Jocelyn Ho, Elizabeth Pierce, and the Ahmanson-Lovelace Brain Mapping Center for data collection assistance.
Study 1 was supported by a grant from the Natural Sciences and Engineering Research Council of Canada awarded to JAB and Study 2 was supported by a National Institute of Mental Health Pre-doctoral Ruth L. Kirschstein National Research Service Award awarded to MLM.
Declaration of Conflicting Interests
The authors declare no conflicting interests.
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Krol, S.A., Meyer, M.L., Lieberman, M.D. et al. Social Working Memory Predicts Social Network Size in Humans. Adaptive Human Behavior and Physiology 4, 387–399 (2018). https://doi.org/10.1007/s40750-018-0100-9
- Social working memory
- Social networks
- Social brain hypothesis
- Social bonds
- Individual differences