Midjourney: Descendants as the stars of the heaven and as the sand which is on the seashore, in the style of Van Gogh
A recent piece of mine about how many more children US Latter-day Saints are having was recently published by the Deseret News. The TLDR is that we are still having more children than the average American, but here I will take advantage of the added flexibility of blogging to derive some estimates about what that means for Church growth in the US.
How many more children are we having? It’s hard to know for sure given sample size issues, but a rough, reasonable estimate is about twice as many. However, this isn’t as much as one might think given that the US’ fertility rate has tanked and is now solidly below replacement-level (1.64 children per woman).
How does this translate into growth? One way of translating TFR into generation-by-generation growth is by converting it into what’s called the Net Reproductive Rate, which takes sex ratios at birth and mortality rates into account to derive an estimate for how many daughters each woman can be expected to have.
Why daughters and women? Basically, the math is simpler if you assume women reproduce asexually, and with a little intuition you can see that the NRR is equivalent to the proportion by which a population will grow from generation to generation. If the average woman has 1.1 daughters, then the next generation will be 1.1 times as big.
So, if we use a rule-of-thumb that Latter-day Saints have twice as many children, we can simply double the NRR for the US to derive an estimate for how much the Church could be expected to grow from generation to generation given no immigration (in this case, no converts and nobody leaving).
The latest US NRR is .798, which means that the US will shrink about .8 generation-by-generation (once population momentum has worked its way through the system–long story).
If we double that for members that’s about 1.6. That means that the Church will grow by a factor of 1.6 every generation if our current reproductive patterns were frozen in place.
In other words, a little over 1/3 of our children can leave the Church without it shrinking assuming no conversions.
If we add conversion to the mix, we can say that we can have a net outflow (people converting to the Church minus people joining) of 1/3 before the Church starts to shrink.
Of course, I have no idea what our precise, cohort-by-cohort conversion and leaving rates actually are. It can get complicated, since when they leave or join is important for growth.
I am the descendant of somebody who left the Church but whose family stayed in. That is going to have less of an impact on Church growth than somebody who left before they raised a family. Conversely, converts who join after they have influenced their children are going to have less of an impact on Church growth than those who join before they have their children.
So to summarize, the leaving and joining rates are the big unknown, but we can leverage extant data on Latter-day Saint childbearing to get a sense of what the “natural” growth rate of the Church is, and ultimately how much leaving and/or conversions we’d need to see for it to grow or shrink.