According to Pew's data, the average Mormon can expect to make 3.4 babies in his or her lifetime. Jews, Catholics, and most flavors of Protestantism have fertility rates ranging from 2 to 2.5. At the low end of the baby-making spectrum you've got atheists, with 1.6 kids, and agnostics, who average only 1.3.
On the face of it this flies against the study's main finding: if atheists and agnostics are having so few kids, how are their numbers increasing? The answer is that many of the religiously unaffiliated are not born, but rather made: many Americans are leaving their faiths and not picking up a new one.
"Nearly one-in-five American adults (18%) were raised in a religion and are now unaffiliated, compared with just 4% who have moved in the other direction," Pew's researchers write. "In other words, for every person who has left the unaffiliated and now identifies with a religious group more than four people have joined the ranks of the religious “nones.”
It turns out that "none of the above" is winning the battle for hearts, minds and souls. But this doesn't mean that Americans are renouncing faith completely and becoming die-hard atheists. Indeed, while 22.8 percent of Americans say they are unaffiliated with any faith, only 3.1 percent identify as atheists. And 8.8 percent of Americans say they are unaffiliated, but also say that religion is very important in their lives.
The unaffiliated share of the North American population is projected to grow over the next 40 years, according to an earlier Pew report. But the unaffiliated share of the global population will decline, partially due to those low fertility numbers charted above.
*Full disclosure: I was employed at Pew and worked on the first version of this survey that came out eight years ago.