The ascendancy of Mormonism as a world religion once seemed inevitable. The year was 1984, and sociologist named Rodney Stark made a startling projection: the Church of Jesus Christ of Latter-day Saints would grow to 267 million members by 2080.

That’s the narrative drumbeat to which tens of thousands of young LDS men and women marched off to proselyting missions in Latin America and Asia during the 1980s and 1990s, as LDS Church membership shot up from 4.4 million to 11 million members. Mormons imbued this growth with theological significance as the fulfillment of a prophecy that the Church would one day “fill the earth”—a sense captured in this Church video.

But new data suggests that Mormonism may no longer be (as it is often described) among the fastest-growing faiths in the United States. Instead, American Mormons appear to be settling into the twenty-first century as a maturing minority having an increasingly hard time holding onto younger members.

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.

Phillips and Cragun also place LDS growth rates not at 30% but at 16%—a rate on par with general US population growth. “Despite a large missionary force and a persistent emphasis on growth,” Phillips and Cragun write, “Mormons are actually treading water with respect to their per capita presence in the U.S.” In fact, additional studies by Cragun and Phillips show that retention rates of young people (young men especially) raised Mormon have dropped substantially in the last decade: from 92.6% in the 1970s–2000s to 64.4% from 2000–2010. Rising rates of disaffiliation go a long way towards explaining the gap between LDS Church records and the ARIS population estimates.

Those who do continue to identify as Mormon, according to data released by the Pew Forum in January, form a confident, cohesive core that is deeply invested in LDS institutional life. The Pew Forum found that 77% of self-identified Mormons reported attending church weekly, and 65% reported regular participation in temple worship, a benchmark of highly observant Mormonism. Those are eye-popping numbers that don’t quite match up to what most Mormons experience week-to-week in their congregations. (The problem may be sample bias: the Pew located many of its Mormon respondents through oversampling in core areas of the Mormon culture region, where attendance rates trend higher.) The Mormons surveyed by Pew also indicate high levels of life satisfaction, as well as a sense that Mormons are misunderstood in the U.S.: 46% said Mormons experience discrimination. Insularity is also strong among Pew-sampled LDS people, with 57% reporting that all or most of their friends are also LDS.

Social insularity as well as familial and kinship ties and feelings of religious certainty contribute to the cohesiveness of the self-identified Mormon core. But taken together the Pew and ARIS numbers suggest that while the highly active LDS core is highly self-assured, it may also be shrinking—a fact not immediately evident in Church membership statistics.

The numbers also suggest that cultural or heritage identity sense of Mormonism may be weakening, especially at the margins of the core and among those who disaffiliate. That may be bad news for twenty-first century Mormonism: other stable American minority faiths like Judaism rely on cultural identity to draw individuals back into religious life throughout the life cycle and across changes in belief and practice. Today, after decades of institutional emphasis on orthodox belief and behavior, it may be difficult for some in the highly observant Mormon core to imagine a cultural Mormonism that enfranchises the less observant. But as the 2012 presidential contest brings increased scrutiny and self-awareness of Mormonism as a culture (complete with its own foodways), perhaps the time is right for Mormons to explore how to nourish and strengthen Mormon identity, even if our twenty-first century numbers don’t live up to the projections.