The historical transition to a low fertility regime was central for long-run growth, but what caused it? Existing economic explanations largely focus on the economic incentives to limit fertility. This paper presents new evidence highlighting the importance of cultural forces as a complementary driver of the fertility transition. We leverage a sharp change in fertility in Britain in 1877 and document large synchronized declines in fertility among culturally-British households residing outside of Britain, in Canada, the U.S. and South Africa, relative to their non-British neighbors. We propose a plausible catalyst for the change: the famous Bradlaugh-Besant trial of 1877.