A society’s culture can lock in beliefs and practices that inevitably produce persistent poverty and tyranny. But a society’s culture can also provide a foundation for maximizing general prosperity and freedom to produce mass flourishing. This book explains why culture—not genes, geography, institutions, or policies—is therefore what ultimately explains the differential success of societies. In short, when certain kinds of moral beliefs are culturally transmitted, a society can overcome the most fundamental obstacle to societal success: rational self-interest undermining the common good. General prosperity requires large-group cooperation, and the most effective large-group cooperation requires having a high-trust society. This book explains why the larger a society is, the more difficult it is to sustain a high-trust society. At the same time, the larger societies become, the more likely rational self-interest and tribalism will undermine crucial but highly trust-dependent institutions like democratic voting and a free press. This book shows how culture uniquely addresses this problem by aligning individual interests with the common good when specific kinds of moral beliefs are strongly held by most people. Culture also matters instrumentally because childhood instruction, a hallmark of culture, helps overcome the irrationality of adult individuals choosing to have moral beliefs that they know will limit their ability to promote their own welfare at the expense of the common good in the future. The analysis has surprising implications for the family, religion, government, and the stability of Western free market democracies.