Loneliness is the distressing feeling associated with the perceived absence of satisfying social relationships. Loneliness is increasingly prevalent in modern societies and has detrimental effects on health and happiness. Although situational threats to social relationships can transiently induce the emotion of loneliness, susceptibility to loneliness is a stable trait that varies across individuals [6-8] and is to some extent heritable. However, little is known about the neural processes associated with loneliness (but see [12-14]). Here, we hypothesized that individual differences in loneliness might be reflected in the structure of the brain regions associated with social processes. To test this hypothesis, we used voxel-based morphometry and showed that lonely individuals have less gray matter in the left posterior superior temporal sulcus (pSTS)--an area implicated in basic social perception. As this finding predicted, we further confirmed that loneliness was associated with difficulty in processing social cues. Although other sociopsychological factors such as social network size, anxiety, and empathy independently contributed to loneliness, only basic social perception skills mediated the association between the pSTS volume and loneliness. Taken together, our results suggest that basic social perceptual abilities play an important role in shaping an individual's loneliness.
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