http://religiondispatches.org/mormon-numbers-not-adding-up/
Official LDS Church
statistics for 2011 count 6,144,582 Mormons in the United States in 2011, comprising about 2% of the nation’s population. Church statistics also show a 30% membership increase between 1990 and 2008—a rate double general US population growth.
But recent studies tell a different story—different because whereas LDS Church records count anyone who has ever been baptized, demographers and pollsters count only those who currently identify themselves as Mormon. Those are the parameters for the landmark Trinity College American Religious Identification Survey: a two-decade project that has produced the largest and most accurate database of self-reported religious identification ever compiled, with 100,000 randomly sampled participants. According to Rick Phillips and Ryan Cragun, the authors of a
study of Mormons based on ARIS data, self-identified adult Mormons make up not 2% but rather 1.4% of the adult US population—that’s about 4.4 million LDS adults.