At some level this is obvious; we're so much more squeamish. No more public executions, mass-spectacle bear-baiting or routine Game of Thrones-style interpersonal violence.
At least not in the enlightened west.
Greg Clark's "Genetically Capitalist?", which I cited in passing in an earlier post, has something to say about all this:
"In the Malthusian era on average every woman could have only two surviving offspring. But these two had to be selected by some mechanism from the average of 5 children each woman had in the pre-industrial era.---
"And as long as mothers and fathers varied in their characteristics this survival process favored some types of individuals over others. The Darwinian struggle that has shaped human nature did not end with the Neolithic Revolution, but continued indeed right up to 1800. " (p. 16).
" Genetically modern English males are 50-100% Anglo-Saxon, despite the fact that Anglo-Saxon migrants to England in the fifth century AD are now believed to have constituted no more than 0.5% to 10% of the population.---
"Thus economic orientation had a dynamic of its own in the static Malthusian economy. Middle class values, and economic orientation, were most likely being spread through reproductive advantage across all sections of stable agrarian societies. " (p. 35).
"What were societies like at the dawn of the settled agrarian era with the Neolithic Revolution of c. 8,000 BC?---
"Based on observation of modern forager and shifting cultivation societies we expect that the early agriculturalists were impulsive, violent, innumerate, illiterate, and lazy. Ethnographies of such groups emphasize high rates of time preference, high levels of interpersonal violence, and low work inputs.
"Abstract reasoning abilities were limited. " (p. 53).
Before agriculture and pastoralism, all of hunter-gatherer humanity was like this; it was civilisation that made us genetically-modern (see, for example, the last paragraph here).
The late Henry Harpending summarised and extended Clark's ideas in this instructive post which extended the analysis to the collapse of the Roman Empire and some contemporary events.
It's an interesting idea of Peter Frost's (the Roman Empire link above) that by the fifth century the Pax Romana had altered the genetic distribution of its citizens making them over-welcoming to the Barbarians, militarily incompetent, and .. Christian.
The paper doesn't do the selection-rate maths. It's interesting to consider whether we can get enough data - I have in mind an exercise something like the (in)famous 'Amish Quotient'.
What we really need is whole-genome sequencing of representative samples of Roman Citizens and contemporary Barbarians.
That, plus a decoding map identifying those prosocial alleles underpinning 'human domestication'.
It will come.