Amygdala size and social networks

People who belong to a larger and/or more complex social network tend to have larger amygdala (Nature Neuroscience: tinyurl.com/28rgcwm). The amygdala is a bilateral structure, located in the medial temporal lobes, that is involved in emotional learning. The researchers, led by Kevin Bickart at Boston University School of Medicine, said their finding was 'consistent with the hypothesis that the primate amygdala evolved, in part, under the pressure of increasingly complex social life'.

Past research has shown that non-human primate species that mix in larger social groups tend to have increased amygdala volume compared with species that mix in smaller social groups. But this investigation of 58 healthy human adults is reportedly the first time that amygdala volume has been shown to be correlated with social network size within a single species.

The main finding was specific. Amygdala volume did not correlate with life satisfaction or perceived social support. Moreover, other subcortical structures, including the hippocampus, were investigated, but their volume did not correlate with social network size or complexity. A whole-brain surface analysis also failed to reveal any equivalent correlations. When a more lenient threshold was used, three further fronto-temporal regions that correlated with network size were identified, two of which have dense connections with the amygdala.

--Christian Jarrett

'Humans are inherently social animals... ' the researchers said. 'A large amygdala might enable us to more effectively identify, learn about and recognise socioemotional cues in conspecifics, allowing us to develop complex strategies to cooperate and compete.'

Not everyone was impressed by the findings. The US-based Neurocritic blogger baulked at the study's failure to find (at conventional significance thresholds) further brain structures that correlated with social network size. 'Are we supposed to believe that only one area of the brain is involved in maintaining social networks? I think not,' he wrote. He also raised the issue of clinical case studies, including those with amygdala damage: 'Anecdotal evidence suggests [these patients] can have close ties with their families and can even become more social after their brain injuries,' he said.