Every policy relies on explicit or implicit assumptions about how people make choices. Those assumptions typically rest on an idealized model of how people think, rather than an understanding of how everyday thinking actually works. This year’s World Development Report argues that a more realistic account of decision-making and behavior will make development policy more effective. The report emphasizes what it calls 'the three marks of everyday thinking.' In everyday thinking, people use intuition much more than careful analysis. They employ concepts and tools that prior experience in their cultural world has made familiar. And social emotions and social norms motivate much of what they do. These insights together explain the extraordinary persistence of some social practices, and rapid change in others. They also offer new targets for development policy. A richer understanding of why people save, use preventive health care, work hard, learn, and conserve energy provides a basis for innovative and inexpensive interventions. The insights reveal that poverty not only deprives people of resources but is an environment that shapes decision making, a fact that development projects across the board need to recognize. The insights show that the psychological foundations of decision making emerge at a young age and require social support. The report applies insights from modern behavioral and social sciences to development policies for addressing poverty, finance, productivity, health, children, and climate change. It demonstrates that new policy ideas based on a richer view of decision-making can yield high economic returns.