The positive and negative emotion functions related to loneliness: A systematic review of behavioural and neuroimaging studies

Loneliness is associated with high prevalences of major psychiatric illnesses such as major depression. However, the underlying emotional mechanisms of loneliness remained unclear. We hypothesized that loneliness originates from both decreases in positive emotional processing and increase of negative emotion processing. To test this, we conducted a systematic review of 29 previous studies (total participant n=19560, mean age=37.16 years, female proportion=59.7%), including 18 studies which included questionnaire measures of emotions only, and 11 studies which examined the brain correlates of emotions. The main findings were that loneliness was negatively correlated with general positive emotions and positively correlated with general negative emotions. Furthermore, limited evidence indicates loneliness exhibited negative and positive correlations with the brain positive (e.g., the striatum) and negative (e.g., insula) emotion systems respectively, but the sign of correlation was not entirely consistent. Additionally, loneliness was associated with the structure and function of the brain emotion regulation systems, particularly the prefrontal cortex, but the direction of this relationship remained ambiguous. We concluded that the existing evidence supported a bivalence model of loneliness, but several critical gaps existed which could be addressed by future studies which include adolescent and middle-aged samples, employ both questionnaire and task measures of emotions, distinguish between general emotion and social emotion as well as between positive and negative emotion regulation, and adopt a longitudinal design which allows ascertaining the causal relationships between loneliness and emotion dysfunction. Our findings provide new insights into the underlying emotion mechanisms of loneliness that can inform interventions on lonely individuals.