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Globally, those living in less advantaged circumstances have an increased likelihood of having a common mental disorder such as anxiety or depression, and a decreased likelihood that this disorder will be recognised, diagnosed and effectively treated. This is the mental health inequality, which causes significant unfair morbidity. The theory of fundamental causes posits that social circumstances drive health inequalities. Under this framework, social circumstances determine access to ‘resources’ such as money, knowledge, and connection to others. These resources are leveraged to avoid risks that lead to illness, or minimise illness. Such strategies are unavailable to people who have limited access to resources. Fundamental causes is well accepted as an overarching framework, but it is deliberately general as specific mechanisms of action will vary over time, setting and circumstance. Poor understanding about specific mechanisms of action, however, limits our ability to design effective interventions. Detailed theories that are compatible with fundamental causes have been described, but not well synthesised.There is a large observational literature describing mental health inequalities in the UK but it is unknown whether this empirical evidence supports or refutes theoretical mechanisms of action.The overarching aim is to map theories and theory-based evidence to answer two related questions; why, and how, do social circumstances drive UK mental health inequalities?An iterative and flexible strategy of searching literature and expert consultation was applied to locate health inequality theories. Theories were included if they were compatible with fundamental causes and explained why social circumstances drive health inequalities in the developed world. Database searching was mainly used to amass observational studies from the UK that described mental health inequality in prevalence, diagnosis or treatment. A causal inference critique will be applied to studies that tested a theory, or used theory to explain results. Theories and associated support will be mapped in a logic model. If feasible, a simplified, supported, model (or models) will be deduced.Experimental or policy-driven interventions to improve population mental health and reduce inequalities are expensive and time-consuming. The evidence base is consequently sparse. Other than in general terms, e.g. downstream interventions increase inequalities, the use of theory in developing mental health interventions is underutilised. This review will highlight evidence-based plausible mechanisms of action which could be used to inform the design of robust mental health interventions that operate at the maximal point to improve population health while reducing inequalities.