This study hypothesizes a link between societal secularism and fertility. Using country-level data from multiple sources (N=181) and multilevel data from 55 countries in the World Values Survey (N=78,639), I document a strong negative relationship between societal secularism and both country-level fertility rates and individual-level fertility behavior. Secularism, even in small amounts, is associated with population stagnation or even decline, whereas highly religious countries have higher fertility rates that promote population growth. This country-level pattern is driven by more than aggregate lower fertility of individual nonreligious people. In fact, secularism is more closely linked to religious than nonreligious people's fertility behavior and appears to be a function of different cultural values related to gender and reproduction in more secular societies. Beyond its importance for the religious composition of the world population, the societal-level association between secularism and fertility is relevant to key fertility theories and may help account, in part, for below-replacement fertility. Rather than religious people becoming a smaller proportion of the world population, demographers have projected that because the nonreligious have fewer children they will make up a smaller proportion of the world population over time (Hackett et al. 2015;Kaufmann 2010;Skirbekk, Kaufmann, and Goujon 2010;Stonawski et al. 2015). This literature improves on secularization theory by highlighting how individual-level secularity limits fertility behavior and thus constrains secularization, but has thus far neglected potential impacts of societal-level secularity despite secularism being more than just an individual attribute. Culture is transmitted, and religious beliefs and preferences acquired, both vertically (i.e., within families) and 1 "Religious" and "nonreligious" can be defined in many ways. Past research has often conceptualized secularism as the inverse of absence of religion, and work on religion and fertility in particularly has frequently focused on binary measures that categorize people as religious or not. In order to consider varieties of secularism, I examine multiple religion/non-religion measures, some of which are binary and some gradational, and aggregations of these items.Secularism and Fertility Worldwide 2 horizontally (i.e., peer-to-peer within the wider culture) Verdier 2000, 2011).Research on religion and fertility has highlighted how the lower fertility of secular individuals limits vertical transmission of secularism, has thus far largely neglected potential demographic limits to horizontal transmission of secularism. The key unanswered question is whether a country-level relationship exists between secularism and fertility net of individual-level religion, which would further constrain secularization-due to fewer children being socialized in more secular contexts-and could contribute to below-replacement fertility.To examine the country-level relationship between secularism and f...