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The overarching aim of the work reported in this thesis is to build on the current research investigating the psychological and social processes in mental health disorders; in particular the socio-cognitive affective processing in depression across the lifespan. Depression risk is prevalent amongst adolescents, and there has been considerable research interest focusing on building a clear profile of the mechanisms of depression based on adult models. Here, I address these mechanisms and also integrate evolutionary principles to help better understand why depression may have evolved as an adaptive process involving elevated sensitivity to socially threatening information, negative self-referential processing biases, and behaviours associated with mitigating low social-rank status. Chapter 1 introduces the reader to depression and current theoretical frameworks relevant to social processing. Depression risk is then discussed, with a particular focus on adolescence as a sensitive period for both social development and depression vulnerability. Chapter 2 investigates how sensitivity to affectively-laden social information can impact the cognitive systems needed for everyday functioning, with a focus on cognitive control. Chapter 3 focuses on negative biases in the construction of the social self in adolescents at risk versus lower risk of developing depression. Chapter 4 takes a data-driven approach to investigate the relational social values that are important for social inclusivity. Then, further analysis looks at how these social values change as a function of age and mood. Chapter 5 investigates how hierarchical social information might influence memory, using a fully mature cognitive system (i.e. non-depressed adults). Finally, Chapter 6 integrates the evidence from Chapters 2-5 and provides a general discussion about the social processes in adolescents at risk of developing depression.

Affective experiences colour much of human experience, shaping how we feel about, respond to, and regulate daily life. While emotion and mood are distinct though related affective phenomena, many studies use these terms interchangeably and draw conclusions on the latter based on findings that may be more pertinent to the former. Key theoretical differences delineate emotion versus mood, with importance placed in maladaptive experiences of long-term mood rather than short-term emotion in mood disorders such as depression. Unpacking differences in these affective dynamics is vital to approaching improvements in mental health and well-being. In addition, much of waking life is spent mentally wandering, and furthering our understanding of mentation and mind wandering in mental health is of importance in conjunction with affect. Where the mind may go to at rest free from distraction may possess important insights into the nature of the mental landscape and mental well-being. This thesis investigates differential aspects of emotion, mood, and mind wandering in diverse clinical populations with the goal of elucidating these experiences in relation to mental health. This includes investigations through a series of studies on: (i) the underlying structure of emotion and mood representations in adolescents, (ii) intraday emotions dynamics using clinical diagnostic and data-driven assessment of person-specific models of temporal emotion, (iii) interrelationships of emotion and mood over time and summary metrics of group-level complexity for both affect types, (iv) naturalistic mood regulation strategy use and outcomes, (v) a theoretical framework for comprehensive mind wandering study, and finally, (vi) naturalistic mind wandering, related affect, and a sensory deprived assessment of mind dimensions using novel methodologies. Altogether these findings provide evidence for the significance in studying emotion, mood, and mind wandering with the aim of providing a foundation for clarifying affective experience and multidimensional aspects of thought content in mental health.

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.

Persistent depression is a debilitating health condition with a poor prognosis even in the context of current gold-standard pharmacological and psychological interventions. A better understanding of the mechanisms contributing to its maintenance is needed to facilitate the development of more targeted psychological interventions. Bayesian predictive processing models of depression propose that negative emotional and physiological outcomes arise in depressive illness as a result of disturbed interoceptive precision estimation in depressed individuals; however, evidence from the clinical and cognitive neuroscience literatures suggests the hypothesis that sensory precision is attenuated in persistent depression across sensory modalities in general. A series of studies was designed to index sensory precision across somatic and auditory modalities and to identify the level at which any disruption manifested in persistently-depressed participants relative to controls. Study 1 (Chapter 3) measured baseline signal discriminability under conditions of focused attention. Study 2 (Chapter 4) measured the impact of failures of voluntary attention on signal discriminability. Study 3 (Chapter 5) used a simulation approach to model sensory precision in the first two studies and to identify mechanisms which could successfully predict the data. Studies 4 (Chapter 6) and 5 (Chapter 7) measured attentional capture by task-irrelevant and predictive sensory cues respectively. Study 6 (Chapter 8) partially replicated Studies 4 and 5 and used the resulting data to estimate the group-level sensory precision and salience parameters of a predictive processing model of precision optimization. The results suggest that sensory precision is attenuated in persistent depression across sensory modalities, and that this attenuation results from disturbances of voluntary and involuntary attention rather than baseline perceptual sensitivity. Under conditions of voluntary attention, reduced sensory precision may result from efforts at resource conservation; and under conditions of involuntary attentional capture, it may be related to a loss of target discriminability and salience. Conversely, the bottom-up salience of somatic stimuli was uniquely enhanced among depressed participants and was predicted by high anxiety and by low interoceptive sensibility. These findings open up new avenues for investigation of the mechanisms underlying persistent forms of depression, and have direct implications for clinical practice with respect to psychological intervention.

Individuals who experience repeated interpersonal trauma often present with Posttraumatic Stress Disorder (PTSD) with more complex features than those exposed to single-incident traumas. However, there is contention in the literature regarding whether PTSD and Complex PTSD can be conceptualised as different disorders, and there is currently no consensus regarding whether tailoring current evidence-based interventions for PTSD for complex features will improve treatment outcomes. This thesis is structured as a series of five stand-alone research papers, with each addressing one of the overarching themes associated with PTSD following repeated traumas (outlined in chapter one); The first study (chapter 2) explores the structure of autobiographical memory, by examining the organisation of past autobiographical knowledge. The second study (chapter 3) explores self-identity by examining the structure of the self-concept. The third study (chapter 4) explores the prevalence of pseudohallucinations in a sample of adult survivors of repeated physical and sexual trauma. The fourth study (chapter 5) explores the relationship between emodiversity and clinical manifestations of PTSD. The fifth study (chapter six) outlines the development and preliminary evaluation of a group intervention for individuals following repeated interpersonal trauma. Finally, in the General Discussion (chapter seven) the findings from all five research papers are considered in light of current theories of PTSD. I critically evaluate whether the extant theories are adequate in their conceptualisations of more complex presentations of the disorder and whether the current treatments available are adequate in effectively treating more complex presentations of PTSD. This thesis contributes to the conceptualisation of CPTSD through the identification of particular symptoms in a client group who have experienced repeated interpersonal trauma, but only with more research in this area can we further refine our understanding and of and develop efficacious treatments for these more complex presentations of CPTSD. Limitations and future directions are considered.