Governance, structure, and access: a Bayesian hierarchical analysis of water, sanitation, and hygiene in South Asia

BACKGROUND: Access to safe water, sanitation, and hygiene (WASH) remains inadequate for large populations in South Asia, constrained by persistent structural inequalities, institutional weaknesses, and uneven development. This study investigates the complex interplay of structural, demographic, environmental, and institutional factors shaping WASH outcomes across eight South Asian countries over the period 2000-2024. Employing a Bayesian hierarchical linear model-robustly validated through alternative specifications-we provide a comprehensive probabilistic assessment of WASH outcomes. A key methodological contribution is the construction of a composite governance quality index using an autoencoder neural network based on six governance dimensions: voice and accountability, regulatory quality, political stability, control of corruption, government effectiveness, and rule of law. RESULTS: Urbanisation improves sanitation access and reduces open defecation but strains water access, highlighting its dualistic nature. Health expenditures consistently enhance WASH outcomes. Economic capacity, sectoral vulnerability, precipitation shocks, and governance exhibit no direct impact. Education significantly improves water access and reduces open defecation. Population density exerts small but positive effects, suggesting economies of scale in service delivery. Digitalisation also emerges as a key driver of WASH outcomes, reflecting the growing role of information technology in service delivery and accountability mechanisms. A counterintuitive positive association between inequality and WASH outcomes suggests Kuznets-style transitional dynamics, where benefits initially accrue to more affluent groups. However, these relationships shift notably when governance is modelled as a moderator. Governance quality acts as a key moderating mechanism, amplifying the benefits of growth, urbanisation, and health spending while reshaping the influence of education and inequality. CONCLUSIONS: Governance does not operate primarily as a direct driver of WASH outcomes. Instead, it acts as a critical enabling mechanism that shapes how structural factors translate into equitable access. Strengthening governance quality amplifies the effectiveness of sectoral investments and urbanisation policies-a finding with important implications for WASH policy and institutional reform across South Asia.