Some years ago, I stumbled upon a lecture by nobel laureate Friedrich August von Hayek (1899 - 1992), which he had held in 1982 at Schloss Klessheim near Salzburg. Therein, the great economist, evolutionary theorist and social philosopher had pondered his lifetime quest, the complex implications of rationality and creativity in evolutionary theory. And suddenly, he had turned to the evolution of religiosity and religions. Although his great lecture was not really understood by most of his fellows, Hayek would later finish his respective thoughts in the very last chapter of his last book ("Religion and the Guardians of Tradition" in "The Fatal Conceit", 1992). I had found a scientific treasure.
The core idea developped by von Hayek that struck me was his calm observation that non-empirical convictions could be able to surpass rational deliberations in evolutionary terms. If people believed in a God that commanded them to "Be fruitful and multiply" (Genesis 1,28), there was no scientific way to verify this belief or claim. If they believed that children were a "blessing from God" and a "miracle of the Lord", there was (and is) no empirical way to test this religious narrative. But then, such beliefs could result in individual choices as well as communal traditions contributing to more children than any rational deliberation would bring about - and thereby a reproductive advantage of the religious!
These last years, I have worked on that subject together with lots of colleagues - and there is a general finding now beyond any serious doubt. On average, members of established religious traditions "do" have distinctively more children than their secular peers, even if controlled for other variables such as education, income or urbanization. And there are lots of religious traditions such as the Old Order Amish, Hutterites or Mormons who managed to keep up high fertility levels throughout the generations - whereas we didn't find a single case of a secular population retaining replacement level for just a century. You may want to check out a whole directory with available studies from various scientists on religion & demography here on the Web-Resources on Religion & Reproduction page.
And here is a video-clip by a Christian group combining scientific and religious narratives into a shared story espousing the value of (every) life. Please note that I am not posting it because I would like to endorse any specific mythology. I just want to emphasize the evolutionary potential of religiosity with a contemporary example from the Web which has been accessed more than 300.000 times within a year - supporting the observation that Friedrich August von Hayek had formulated three decades ago.