Religion plays a much more important role in "Civilization IV" than in the previous installments of the franchise. Impacting many of the game's key mechanics like government civics and diplomatic relations, the game's new religious system, according to Firaxis Games producer Barry Caudill, was added to increase gameplay depth over the entire game. The game features seven religions (Buddhism, Christianity, Confucianism, Hinduism, Islam, Judaism, and Taoism) that are founded by the first civilization to research a certain technology which varies per religion. Religions can then be spread actively through the production of missionary units or passively through means such as trade routes. Religions may be spread to domestic and foreign cities and there is no limit to the number of religions that can be present in any single city. The player may only choose one state religion at a time and all seven offer identical advantages (e.g. cities with the state religion receive bonuses in happiness, and potentially in production, science, gold and even military unit experience points).