Whatever the reason, nones grew from 8 percent of the U.S. population in 1990 to more than 14 percent in 2001.

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"That makes nones the fastest-growing religious group in the United States, if you think about them as a religious group," said Patricia O'Connell Killen, a professor of religious history at Pacific Lutheran University in Tacoma, Wash. "We're just coming to grips with the reality that this group even exists."