As I noted in A Tale of Two Religion Scholars, Dr. Michael Blume’s research (which you can find at Homo religious) shows that religious groups out-reproduce their secular counterparts. The data are solid and correspond to the commandments of most religions: “Be fruitful and multiply.”
Given that religious people make more babies than secular people, Blume contends that religiosity evolved because it confers reproductive fitness benefits on believers. This is how he recently described his hypothesis and conclusion:
Personally, I assumed – as did Charles Darwin – that evolutionary theory was perfectly able to explain religiosity and religions and that the trait would turn out to be somewhat adaptive. I would have been perfectly content with having found a slight reproductive advantage of religious people when compared to their secular neighbours.
But what I found (and keep finding) during these last years was not only a strong and actually widening demographic gap – but the complete lack of a single case of a secular population, community or movement that would just manage to retain replacement level for a century. What I found…is that secularism is followed by inevitable, demographic decline.
The problem with comparing religious and secular fertility rates, and using those to make evolutionary arguments, is that no humans in the deep past were secular. If the archaeological record is correct and ethnohistoric-ethnographic analogies hold, then it is safe to assume that all Paleolithic human groups practiced some form of shamanism. Because none of these groups were secular, none of them gained a reproductive advantage over another because they believed in the shamanic supernatural.
Hunter-gatherer group sizes are determined first and foremost by resource availability and the carrying capacity of local environments. Foraging groups must constantly concern themselves with the perils of overpopulation. Local ecology explains the limitations that all hunting and gathering groups placed on group size, which they limited in various ways, including longer lactation periods (which prevents ovulation and delays birth intervals), abortion, and infanticide.
Atheism, agnosticism, and secularism are all recent developments in human history — inklings of these ideas are apparent during the Renaissance and small numbers of people begin proclaiming them during the Enlightenment. Larger numbers of people did not begin rejecting religion until after the Darwin-Marx-Nietzsche-Freud assault on supernaturalism, regardless of form.
Even then, the number of truly secular or non-religious people in the world remains tiny — around 1% or less of all people living today. As the sociologist Rodney Stark has shown in numerous studies over the past few decades, the number of truly “secular” people in the world is vastly overestimated (and is usually a product of non-revelatory, superficial survey questions such as “How often do you attend church?” or “How often do you pray?”).
As John Hawks pointed out in this post, “whenever you’re talking about a hypothesis involving ideological causation, there’s a tremendous potential for confirmation bias.” Blume frankly admits he is religious and his non-null hypothesis was that religion could be explained as an evolved adaptation. His research seemingly confirms this, even though the data he uses seems wholly inapplicable to Paleolithic hunter-gatherers.