Major Depressive Disorder (MDD) is one of the most prevalent health conditions in the world, characterised by persistent low mood and disruption to education, relationships, and employment. Disruption to social functioning is a core feature of MDD, and this dimension of the disorder may offer valuable insight into its aetiology. This thesis aims to extend our understanding of social processing in MDD by testing hypotheses generated from a socio-evolutionary theoretical framework of MDD, with particular emphasis on the Social Risk Hypothesis of Depressed Mood, which conceptualises depressed mood as an adaptive response to elevated risk of social exclusion. The thesis pursues these aims utilising novel protocols and neuroeconomic games to examine social risk-taking and self-discrepancies, and by examining the role of regions of the physical pain network in social function and processing of unexpected social information. The thesis consists of nine chapters; one general methodology chapter (Chapter 3), five chapters detailing novel experimental studies (Chapters 4,5,6,7 and 8), one describing a reanalysis of existing data (Chapter 2), one introductory chapter and one discussion chapter (Chapters 1 and 9 respectively). Across these chapters, the thesis presents neural and behavioural evidence that MDD is associated with reduced social risk-taking, increased sensitivity to an exclusion-relevant context (in-group interactions) and stronger enforcement of social norms. The thesis presents neural evidence of a negative processing bias for self-discrepancies in MDD, linked to activation in the dorsal anterior cingulate cortex and anterior insula, and suggesting a role for perfectionism as a transdiagnostic sensitivity to such discrepancies. Suggestions for future research are discussed, including increased utilisation of neuroeconomic games, particularly in relation to assessing social function as a transdiagnostic marker. Overall, the thesis provides support for socio-evolutionary frameworks of affect, and highlights their unique perspective for understanding affective disorders, with some ‘deficits’ usefully reconceptualised as adaptive mechanisms.