The basic stats are simple. The religious have more children than the non-religious. Although more people convert from religion into non-religion, conversion probably won't be enough to tip the balance. As a result, the religious will make up an increasingly large proportion of the population as the century progresses.
Why should this be a problem? After all, most religious people are inoffensive, liberal types who are good neighbours. They surely don't pose a threat to liberal society.
But the problem comes when you dig into the details - especially when you separate out the devoutly religious (fundamentalists, born again Christians, cultists etc) from the mainstream religious.
Because the major fertility fault line is not between the religious and atheists, but between the mainstream religious (and atheists) and the fanatics.
In modern America fertility rates among the mainstream religious, although a little higher than among the non-religious, are pretty low. Kaufmann shows that, regardless of faith, fertility rates among the religious have declined throughout the 20th century, tracking (although always slightly higher) those of the non-religious.
Today, the fertility rate for liberal protestants in the USA stands at 1.84 children per woman (p90), while that of moderate protestants is 2.01 and conservative protestants has fallen to only 2.13. Mormon fertility rates, although consistently higher, have on average tracked those of the US population at large.
And it's not just Christians that are seeing fertility rates drop. In 1981, Muslims in Austria had a fertility rate of 3.09 (p172). Twenty years later, that had dropped to 2.34 (still well above the fertility rate of the natives, however).
In other words, this is good old-fashioned conservatism. Liberals have lead the way, with female emancipation causing the 'Second Demographic Transition' to small family sizes. Conservatives have come on board more slowly, but young conservatives are adopting the values of the liberals of a generation ago.
But these average statistics conceal an ugly reality. Because while fertility rates among the 'normal' religious are dropping, those among the hardliners are staying high or even increasing.
The most dramatic examples given by include the ultra-orthodox Jews, ultra conservative cults like the Amish (who have increased from 5,000 to 250,000 in the past century), the 'Quiverful' fundamentalists, and pentecostals in Finland. In all cases, these groups are characterised by isolationism and high birth rates. Their new recruits are born, not made.
There seems to be two reasons why this happens. Firstly, these ultra-conservatives have remained highly patriarchal.
'Normal' conservative Christianity places a high value on conversions. While this increases their numbers, it also means that their members and values are influenced by those in the wider world.
Not so the patriarchal cults. Cut off from influence from the outside world, women remain relegated to their traditional roles. With restricted access to education, and little opportunity for independence or escape, they become, in practice, children factories.
The statistics demonstrate the power of these cults in enforcing traditional values and preventing female emancipation. The Mennonites, who are Anabaptists like the Amish but who speak English at home and accept intermarriage and modern technology, have average fertility rates. And while the most conservative old-Amish retain 95% of their children, the slightly more liberal new order Amish retain only 57% (p36).
But there is more to the story than female subjugation. Because these cults actively and consciously promote fertility as a way to increase their power.
In other words, they recognize that conversion has failed as a strategy to promulgate religion. So the fundamentalists have retreated from conversion, turn in upon themselves, and intend to achieve victory through remorseless demographics.
In Israel and Palestine, both orthodox Jews and religious Muslims have astonishingly high birth rates, at least in part as a consequence of waging war 'by other means'. Throughout the Islamic world, those who have the most extreme beliefs are also the most likely to endorse the desirability of large families.
And, back in the USA, the leaders of the half-crazed "Quiverful" cult fantasise about their future armies of "mini-me's". Geoffrey Botkin, one of the architects of the movement, even:
...produced a spreadsheet which predicts that he will be the patriarch of 186,000 male descendants within two centuries. At the birth of his latest addition, Anna Sofia, Botkin passed his hand over the abdomen of the sleeping newborn, praying for her to be the 'future mother of tens of millions'. (p95)
These religious cults are a lethal combination of female subjugation and male power fantasies. And that's what makes them dangerous to all liberals - both religious and non-religious.
So what should be the appropriate liberal response? Well, Kaufmann does not say. Although he holds out hope that moderate religion, with its feel-good fuzzies, may somehow triumph over the hardliners, he recognizes that action is needed:
It will be a century or more before the world completes its demographic transition. There is still too much smoke in the air for us to pick out the peaks and valleys of the emerging social order. This much seems certain: without an ideology to inspire social cohesion, fundamentalism cannot be stopped. The religious shall inherit the earth.
I, for one, see hope for a liberal, secular future, despite these grim statistics.
We can start by recognizing that some liberal values hold within them the seeds of their own destruction. For the cults to survive, they need to isolate their children from external influence. They do this in a number of ways, especially by physical isolation - home schooling, restrict access to alternative world views, separate schools, and even (in the case of the Mormons) separate universities.
This can only happen when children are regarded as the property of their parents, rather than as individuals with rights of their own to an open, diverse education and interaction with the wider world. Liberals need to weigh carefully these two rights to ensure that liberal values do not empower most those who seek to destroy them..
What's more, these cults thrive upon fear of outsiders. Both Muslim and Christian fundamentalists play upon fears that their culture will be overwhelmed by secularists. Atheists need to think hard about how they engage with the religious, since ratcheting up the level of conflict serves to paradoxically increase the power of the religious patriarchs.
Furthermore, part of the reason birth rates have fallen with female emancipation is that, all too often, women are forced to choice between an independent career and a family. Signs are that this is already changing.
Men are devoting more time to child care than ever before. In Europe, fertility rates are actually highest in the least religious countries. If liberals can create a society in which women can couple a secure, independent existence with children, then fertility rates may yet rebound.
And lastly, I wonder about the future of liberal fertility. It's true that liberal fertility rates are continuing to fall. But liberal values, like fundamentalist ones, are inherited as well as communicated.
Many liberals do have children. They have found a solution that balances liberal values, modern society, and family. And since they are still in the vast majority, it is their values (and genes), that will make up the overwhelming majority of future generations.
Maybe, just maybe, it will be these liberals that shall inherit the earth!
This article by Tom Rees was first published on Epiphenom. It is licensed under Creative Commons.