According to the Pew U.S. Religious Landscape Survey the replacement rate of the Church of Jesus Christ of Latter-day Saints has dropped to 80%. That means that for every five members who leave Mormonism behind four new converts join the Church.
In 2001, the CUNY Religious Identification Survey had found that Mormons, as well as Jehavah’s Witnesses, recruited large numbers of converts but lost members at the same rates. If the Pew Forum’s findings are correct then the LDS Church is no longer replacing its losses in the United States.
The Pew Forum surveyed 35,000 respondents, which is a magnificent asset for religion researchers because even relatively small religions will be represented with sufficiently large sub-samples yielding reasonable margins of error. The U.S. Religious Landscape Survey found that 1.7% of adult Americans are Mormon, which means that the sample contains 595 Mormons.
A random stratified sample of 512 respondents would yield a margin of error of +/-4%.
In this sample, the Pew researchers found that about 162 respondents had left Mormonism but only 132 had joined, which yields a deficit of 33 people (~5.5%). I am not sure but in the absence of information to the contrary, I am assuming that joining and leaving Mormonism applies to the life time of the respondents. Even so, that figure amounts to a substantial deficit.
Just for fun, lets consider the best and worst case scenarios in light of the samples margin of error. In the best case, which assumes the lowest possible number of departers and the highest possible number of converts, the Mormon replacement rate might exceed 1.1. That would mean that eleven members join for every ten that leave.
On the other hand, the worst case scenario would depress the replacement rate to less than 58% where less than six people would convert for every ten departures.
However, the mean departure rate of 80% is most likely the correct estimate. If that number were correct, and that is what the evidence says, it would indicate a substantial demographic challenge for LDS leaders and the Mormon community.
PS: This is a self-identification survey. Respondents will reveal their views of themselves, which means that there will be any number of people who will say that they are no longer Mormons but have not mailed a resignation letter to Salt Lake.