This paper examines how culture influences the success of fertility-control policies. In the 1970s, many developing countries implemented birth-control measures grounded in the quality-quantity trade-off, yet their outcomes diverged, e.g., Taiwan, Thailand, and South Korea achieved rapid fertility declines, while Pakistan, India, and Brazil did not. We propose that societal conformity—the degree to which individuals adhere to norms such as a government-endorsed ideal family size— determines how effectively policy incentives translate into behavior. Using a unified theoretical framework, we show that higher conformity amplifies the impact of birth-control policies on both reducing fertility and increasing investment in children’s education. Under empirically plausible conditions, this strengthened quality-quantity trade-off not only boosts short-run economic growth but also accelerates the shift from agriculture to manufacturing—measured by manufacturing’s employment share—even when manufacturing is more capital-intensive and benefits from human-capital-driven, labor-saving technologies. Finally, we validate these predictions with cross-country empirical evidence, underscoring the pivotal role of culture in shaping demographic change and economic development.