Irreligion or nonreligion is the absence or rejection of religion, or indifference to it. Irreligion takes many forms, ranging from the casual and unaware to full-fledged philosophies such as secular humanism. Other examples are atheism, agnosticism and antitheism. Social scientists tend to define irreligion as a purely naturalist worldview that excludes a belief in anything supernatural. The broadest and loosest definition, serving as an upper limit, is the lack of religious identification, though many non-identifiers express metaphysical and even religious beliefs. The narrowest and strictest is subscribing to positive atheism.
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According to the Pew Research Center's 2012 global study of 230 countries and territories, 16% of the world's population is not affiliated with any religion. The population of the religiously unaffiliated, sometimes referred to as "nones", grew significantly in recent years, though its future growth is uncertain. Measurement of irreligiosity requires great cultural sensitivity, especially outside the West, where the concepts of "religion" or "the secular" are not always rooted in local culture.