The rapid aging of populations has prompted the introduction of social pension programs aimed at preserving the welfare of the elderly. However, adverse socioeconomic behaviors may dampen the intended policy effects. Using a fuzzy regression discontinuity design, this study examines the impact of social pension receipt on expenditure patterns and material hardship among older adults aged 80 year or older in Vietnam. We find that social pension increases the risk of material hardship in rural areas and reduces non-food expenditures in urban areas. To explain these findings, we explore two potential pathways: economic behavioral mechanisms and social behavioral mechanisms. The results suggest that behavioral responses may offset the intended welfare benefits of social pensions, underscoring the need to account for such adjustments in the design of aging-related policies in developing countries.