In a major step beyond conventional rational-choice accounts of policy decision-making, this book demonstrates that bounded--not full--rationality drives the spread of innovations across countries. When seeking solutions to domestic problems, decision-makers often consider foreign models, sometimes promoted by development institutions like the World Bank. But, as Kurt Weyland argues, policymakers apply inferential shortcuts at the risk of distortions and biases. Through an in-depth analysis of pension and health reform in Bolivia, Brazil, Costa Rica, El Salvador, and Peru, Weyland demonstrates that decision-makers are captivated by neat, bold, cognitively available models. And rather than thoroughly assessing the costs and benefits of external models, they draw excessively firm conclusions from limited data and overextrapolate from spurts of success or failure. Indications of initial success can thus trigger an upsurge of policy diffusion.