Just about the time I was wrapping up Right-Wing Critics, I started doing some consulting work for one of the big Protestant denominations that is suffering a period of long-term decline. (I will share more details of this project at another time; I am actually expanding this project into a book that I hope to finish in the coming months.) Christianity in general is in trouble in the U.S., as the ranks of the religiously unaffiliated have swelled, but some denominations have performed better than others.
Many commentators have offered explanations for why some churches are in free-fall, whereas others are reasonably healthy or even growing. These explanations usually flatter the theological or political preferences of the person making the argument -- the church needs to be more/less liberal, conservative, tolerant, hardline, ecumenical, etc.
I argue that fertility is one of the main drivers of denominational growth and decline. The days of growing your church through mass conversion are clearly behind us. If you want to make new Methodists, Baptists, Mormons, or whatever, you really need to make your own from scratch, rather than try to poach them from the ranks of some other religion or convert them from secularism. That is, you need to give birth to them. I am not the first person to make this argument. Mary Eberstadt made a similar argument in her book on the subject, for example. But I think I have found some interesting and compelling new evidence on this front.
For example, the image below shows the linear and fairly strong relationship between the largest Protestant denominations' percentage change (in terms of total adherents) between 2007 and 2014, and the percentage of women affiliated with that denomination between the ages of 30 and 55 who have three or more children living in their home. There is obviously a fair amount of variance, but the relationship is umistakable. The lesson: if you want your denomination to survive, large families are a necessary (but, as I will show another time, not sufficient) precondition.