Modern legislation is structurally reactive — it intervenes after harm materialises. This paper argues that the appropriate response to anticipatory risks (algorithmic discrimination, demographic aging, climate feedback loops, pandemic spillovers) is not faster reaction but earlier reasoning. We propose Statistically Informed Anticipatory Legislation (SIAL): a framework in which quantitative foresight tools — probabilistic modelling, demographic projection, Bayesian updating — are embedded into the legislative process before harms become irreversible. The paper develops the theoretical foundations, examines latent anticipatory reasoning within existing legal doctrine, and illustrates the framework through applied case studies.