The Anatomy of Friendship: Neuroanatomic Homophily of the Social Brain among Classroom Friends

Abstract

Homophily refers to the tendency to like similar others. Here, we ask if homophily extends to brain structure. Specifically: do children who like one another have more similar brain structures? We hypothesized that neuroanatomic similarity tied to friendship is most likely to pertain to brain regions that support social cognition. To test this hypothesis, we analyzed friendship network data from 1186 children in 49 classrooms. Within each classroom, we identified "friendship distance"-mutual friends, friends-of-friends, and more distantly connected or unconnected children. In total, 125 children (mean age = 7.57 years, 65 females) also had good quality neuroanatomic magnetic resonance imaging scans from which we extracted properties of the "social brain." We found that similarity of the social brain varied by friendship distance: mutual friends showed greater similarity in social brain networks compared with friends-of-friends (β = 0.65, t = 2.03, P = 0.045) and even more remotely connected peers (β = 0.77, t = 2.83, P = 0.006); friends-of-friends did not differ from more distantly connected peers (β = -0.13, t = -0.53, P = 0.6). We report that mutual friends have similar "social brain" networks, adding a neuroanatomic dimension to the adage that "birds of a feather flock together."

Keywords: mutual friendship; neuroanatomic homophily; social networks; social neuroscience.

Figure 1

Figures of the brain networks used. Pictured are the (a) social, (b) salience, and (c) executive brain networks. The salience and executive networks were defined by 10 regions and the social network included 12 regions. Brain figures were visualized with the BrainNet Viewer (Xia et al. 2013).

Figure 2

Methods overview. (a) Classroom networks defined by reciprocal friendship. Nominations were used to determine the geodesic distance between children who had neuroanatomic data. Child 1 and Child 2 are mutual friends (geodesic distance = 1). (b) Each child’s social brain is defined by the volumes of 12 regions supporting social cognition and friendship network formation. These 12 volumes provide 12 coordinates for each child’s social brain. Similarity in the social brains between two peers (such as Child 1 and Child 2) was taken as the Mahalanobis distance between the brains. (c) We predicted that the similarity (Mahalanobis distance) between social brains would be associated with geodesic distance. Mutual friends were predicted to have the most similar social brains, followed by friends-of-friends (geodesic distance = 2) and then more distant friends and isolates (geodesic distance = 3+ and infinite).

Figure 3

Associations between friendship distance and similarity in the social, executive, and salience brain networks.