Abstract Over the past two decades, Nigeria has elevated social investment to a central pillar of its development strategy, committing substantial public resources to poverty reduction, human capital formation and social protection. Despite the scale and visibility of initiatives such as the National Social Investment Programme (NSIP), poverty incidence, inequality and vulnerability remain persistently high. This paper interrogates the paradox of expansive social investment with limited developmental returns. Drawing on capability theory, institutional economics, political economy perspectives and law-and-development analysis, the study argues that Nigeria’s social investment outcomes are constrained not only by weak policy design but also by gaps in legal accountability, institutional coordination, statutory enforcement, rights-based participation, transparent governance and policy learning. Using a qualitative, desk-based analytical approach grounded in official data, legal materials and global development scholarship, the paper demonstrates that scale without legally supported systems produces visibility rather than impact. The paper concludes that for social investment to become transformative rather than performative, Nigeria must embed social programmes within strong institutions, evidence-based design, enforceable accountability mechanisms, rights-sensitive implementation and long-term policy coherence.