As part of our research we created a map of where big gods appeared around the world. In the map below, the size of the circle represents the size of the society: bigger circles represent larger and more complex societies. The numbers in the circle represent the number of thousand years ago we find the first evidence of belief in moralising gods. For example, Emperor Ashoka adopted Buddhism 2,300 years ago after he had already established a large and complex South Asian empire known as the Mauryan Empire.
Although 'big gods' and 'moralising gods' sound vividly descriptive, that these are, despite colloquial appearance, being used as potentially misleading terms of art is seen quite clearly from the inclusion of Buddhism as a 'big god' religion. And the Ashoka example is particularly interesting for highlighting the limitations of this approach. We don't in fact know anything for certain about Ashoka's conversion of his dominions to a Buddhist empire, or about his prior life and beliefs, except what Ashoka tells us in his inscriptions:
(1) He conquered the Kalinga region in the eighth year of his rule.
(2) The violence and devastation of that war was so great (Ashoka claims that hundreds of thousands of men and animals died) that, having annexed Kalinga, he suffered remorse.
(3) As a result of this, he began actively devoting himself and his empire to the dharma or eusebeia (in the Greek translations added to his edicts in previously Greek-ruled areas of his empire), and this is pretty clearly Buddhism in some sense.
(4) This seems to have had several stages, as recorded in the various Edict Rocks of his reign.
We know nothing of Ashoka's earliest views, unless we take highly conflicting legends as evidence. Some legends suggest he was already Buddhist before the Kalinga War; others that he was a Brahman; others that he was a wicked, amoral, and godless man. His conversion may have been a clean break from his past life, or it may have been a case of someone believing loosely and in a worldly fashion converting to a stricter and more intense version of the same religion. According to some legends, he was a devout supporter of Brahmanism in his early years, but was unhappy with the morals of the Brahmin class, and became interested in Buddhism because of the quality of its teachers. According to some legends, by his eighth year, Ashoka had built thousands of Buddhist temples. We just don't really know. We do know, of course, that Buddhism was at least a hundred years old by Ashoka's day, perhaps two hundred years old; Buddhism undeniably pre-exists the Mauryan Empire, and was well-established already in a number of areas that Ashoka came to rule.
So here we have a religion that is definitely highly moralizing, believing in universal moral principles, but in which gods play only a secondary part, definitely pre-existing the major civilization in question, and while we know that Ashoka converted to some strict and devout version of it, we don't know exactly when he actually started affirming his adherence to it. Each clause here indicates a problematic point in the summary.
Looking at the original study, what the study really suggests is that early major empires were not originated as empires specifically favoring religions that held universal moral principles, although most of them eventually favored some such religion at some point; in particular, that, measuring according to common standards used to measure social complexity of ancient civilizations, major expansions of social complexity occurred before they began actively favoring a moralizing religion. Thus the study confirms what other studies have suggested, namely, that there is a connection between the growth of large early empires and the political establishment of religions advocating universal moral principles, but could be used to argue that the latter was used to consolidate and stabilize empires grown large, rather than being a cause of the expansion itself.