This economic explanation is partly true, but it falls short of the full picture. Many poor people have large families despite having access to birth control and movie tickets; it turns out they generally want larger families. Sure, they are more economically dependent on their children, but on the other hand, rich people can afford more children. Wealthy people in America today tend to have one child or none, but wealthy families in the past tended to have three or more children. The real difference is not merely in the level of income—it is that in the past children were valued as gifts from God, and traditional cul- tures still view them that way.

Muslim countries, with their oil revenues, are by no means the poorest in the world and yet they have among the highest birth rates. Practicing Catholics, orthodox Jews, Mormons, and evangelical Protestants are by no means the poorest groups in America, and yet they have large families. Clearly religious factors are at work here. The declining birth rates in the West as a whole are, in considerable part, due to secularization. The religious motive for childbearing has been greatly attenuated, and children are now viewed by many people as instruments of self-gratification. The old biblical principle was “Be fruitful and multiply.” The new one is “Have as many children as will enhance your lifestyle.”

The economic forecasters of the disappearance of religion have proven themselves to be false prophets. Not only is religion thriving, but it is thriving because it helps people to adapt and survive in the world. In his book Darwin’s Cathedral, evolutionary biologist David Sloan Wilson argues that religion provides something that secular society doesn’t: a vision of transcendent purpose. Consequently, religious people develop a zest for life that is, in a sense, unnatural. They exhibit a hopefulness about the future that may exceed what is warranted by how the world is going. And they forge principles of morality and charity that simply make them more cohesive, adaptive, and successful than groups whose members lack this binding and elevating force.

My conclusion is that it is not religion but atheism that requires a Darwinian explanation. Atheism is a bit like homosexuality: one is not sure where it fits into a doctrine of natural selection. Why would nature select people who mate with others of the same sex, a process with no reproductive advantage at all? It seems equally perplexing why nature would breed a group of people who see no higher purpose to life or the universe. Here is where the biological expertise of Dawkins, Pinker, and Wilson could prove illuminating. Maybe they can turn their Darwinian lens on themselves and help us understand how atheism, like the human tailbone and the panda’s thumb, somehow survived as an evolutionary leftover of our primitive past.