Amygdala volume and social network size in humans

nature neuroscience advance online publication B r i e f c o m m u n i c at i o n s For many species, but particularly for primates, living in groups is a major adaptive advantage 1 . But living in a social group also presents its own challenges. To get along while getting ahead, it is necessary to learn who is who, who is friend and who is foe. It might be productive to form an alliance with certain group members in one context, but to outmaneuver them in another. The 'social brain hypothesis' suggests that, evolutionarily, living in larger, more complex social groups selected for larger brain regions with a greater capacity for performing relevant computations 2 . On the basis of its central functional role Comparative neuroanatomical studies in nonhuman primates strongly support a link between amygdala volume and social network size 7 and social behavior 8 . Species characterized by larger social groups have a larger corticobasolateral complex within the amygdala. The corticobasolateral complex conjointly expanded with evolutionarily newer cortex and the lateral geniculate nucleus, particularly the layers of the lateral geniculate nucleus that project to the ventral stream visual system 7 . Taken together, these comparative findings suggest that a larger amygdala provides for the increased processing demands required by a complex social life. In this study we examined whether amygdala volume varies with individual variation in the size and complexity of social groupings within a single primate species, humans. In 58 healthy adults (22 females; mean age M = 52.6, s.d. = 21.2, range = 19-83 years) with confirmed absence of DSM-IV Axis I diagnoses and normal performance on cognitive testing, we examined social network size and complexity with two subscales of the Social Network Index (SNI 9 ). One SNI subscale (Number of People in Social Network) measures the total number of regular contacts that a person maintains, reflecting overall network size. A second subscale (Number of Embedded Networks) measured the number of different groups these contacts belong to, reflecting network complexity. Despite the fact that the two social network variables were strongly correlated within the present sample (r = 0.86, P < 0.001), we opted to consider their separate relation to amygdala and hippocampal volumes. (For more details, see Supplementary Results.) To assess amygdala (and, as a control region, hippocampal) volume, we performed quantitative morphometric analysis of T1-weighted MRI data using an automated segmentation and probabilistic regionof-interest (ROI) labeling technique (FreeSurfer, http://surfer.nmr. mgh.harvard.edu/). For methodological details, see Supplementary Methods. To adjust for differences in head size, amygdala and hippocampal volumes were divided by total intracranial volume, as performed previously Linear regression analyses revealed that individuals with larger and more complex social networks had larger amygdala volumes To assess discriminant validity, we performed a linear regression using right and left hippocampal volumes (corrected for total intracranial volume) as independent variables and social network size and complexity as dependent variables while controlling for age (because hippocampal volume typically diminishes with age). For the whole group, these analyses showed no significant relationship Amygdala volume and social network size in humans We found that amygdala volume correlates with the size and complexity of social networks in adult humans. An exploratory analysis of subcortical structures did not find strong evidence for similar relationships with any other structure, but there were associations between social network variables and cortical thickness in three cortical areas, two of them with amygdala connectivity. These findings indicate that the amygdala is important in social behavior