I was interested by the recent post and discussion at Henry Harpending's blog
which cited this paper by Peter Frost
which extended Greg Clark's Big Idea to Roman times - I mean the idea that natural selection operating in 'recent' and recorded history shaped human personality and intelligence in ways which changed history.
Greg Clark's book A Farewell to Alms: a brief economic history of the world had a massive impact on me - for example it propelled me into IQ research and discredited mainstream economics in my eyes.
And I think it likely that Frost is correct to suggest that the Roman Civilization did have some kind of pacifying effect on its inhabitants - all all civilizations probably do.
But I think Frost is demonstrably wrong to state (from the Abstract): "By creating a pacified and submissive population, the empire also became conducive to the spread of Christianity—a religion of peace and submission."
This is wrong because Rome was not the capital of Rome at the time of the fall of Rome - the capital of Rome was Constantinople and the Empire was divided into East and West - both of which were Christian but only one half, the Western half, of which fell - the Eastern Empire surviving and indeed thriving for many hundreds more years ('Byzantium').
Rome the City fell in the four hundreds AD, but Rome the Empire certainly did not - and the City of Rome was in the less important half of the Empire.
(This point is well covered in the comments on West Hunter.)
So, according to this matched cohort study (!), Christianity cannot have been a major cause of the fall of Rome
Another difference between the English medieval example of domesticated humans given by Clark, and the putative Roman example, is that English domestication led to world conquest and the Industrial revolution, while the Western Roman Empire example of domestication led to barbarian invasion and economic collapse.
Assuming that domestication of humans comes first, then its consequences follow - a new kind of human is first evolved (in terms of changed average personality and intelligence), then the consequences of that evolution follow.
Since the consequences were so different, the first hypothesis would be that the new kind of human was probably different in the case of Rome and Medieval England - different in intelligence and/ or personality.
The selective pressure towards domestication was, indeed, very different - as different as Ancient Rome and Medieval England.
Having said that, which is critical of details of Frost's idea - I must emphasize that I liked the paper and believe that his broad argument is very likely to be correct: some kind of natural selection surely went on during the Western Roman Empire, and it very likely had historical consequences.
But I do not think that Christianity had the role ascribed by Frost, and I would emphasize the differences from - rather than the similarity to - the process of domestication of Homo sapiens in medieval England.