Digging deeper: Exploring mechanisms underlying subgroup variation in mental health

Mental health is not equally distributed across the population, and despite extensive research, these differences persist. The overarching aim of this dissertation was to gain a deeper understanding of the underlying mechanisms that contribute to subgroup differences in mental health.
This thesis used the potential outcomes framework, the g-formula, marginal structural models and genetically informed designs with the aim to add knowledge about the impact of determinants that are potential targets for interventions (i.e. health behavior, obesity, or socioeconomic factors), and determinants that are not intended as potential intervention targets but might substantially explain subgroup differences in mental health and ill-health (i.e. genetics). Two key mechanisms were examined: differential exposure and differential impact. Differential exposure indicates that the variation in mental health is partly driven by the unequal distribution of intermediary determinants across subgroups, whereas differential impact indicates that subgroup variations are driven by a stronger effect of intermediary determinants on mental health among certain subgroups.
This dissertation found that differential exposure to modifiable determinants partially explains subgroup differences in mental health, though its contribution depends on the type of determinants that were examined, the underlying hypothetical intervention, and the intersection with other subgroups. The contributions of differential impact are less clear and may be time dependent and harder to quantify as it may be a consequence of differential exposure. The methodological considerations underscore the need to balance simplicity and complexity in our models, evaluate the trade-off between conservative (but well-defined) or radical (but vague) interventions, distinguish between modifiable and non-modifiable determinants, and clarify the advantages and disadvantages of subjective and objective measures of mental health. In conclusion, tackling subgroup variation in mental health requires not only targeting vulnerable individuals but also transforming the broader socioeconomic and political structures that shape the subgroup variation.