I have chosen the word heritage because culture is too broad, but ethnicity is too narrow. There may be unforeseen difficulties with heritage as well. It is not defined sociological or anthropological concept, so far as I know.
I am also aware that there are subtleties I am leaving out, in both this post and the related one here. If the discussion becomes extended we may touch on those.
Violent crime in America has a large regional component. The regions correspond to the colonial immigration patterns from the British Isles 1600 – 1800. These migrations are covered in detail in David Hackett Fisher’s Albion’s Seed, but in oversimplified form are these:
New England, settled by Puritans from East Anglia
Virginia and coastal South: Anglicans, settled by Cavalier aristocracy from SW England
Mid – Atlantic Quakers from the English Midlands
Appalachia, settled by Scots-English Borderers and the Scots-Irish.
For our purposes, it is good to note that these areas were each fountainheads for further American settlement. New Englanders settled much of the north border of the US, joined by Scandinavians. New England has always had very low rates of violent crime, and this persisted in the areas they settled. Norwegians weren’t likely to push the violence rate much higher now, were they?
Virginia’s cultural stamp extended throughout the coastal South. The moderately high rate of violence went with them.
Mid-Atlantic Quakers and the German pietists that joined them early were the jumping off points for the settlement of much of the Midwest. The moderately low rate of violence tended persist into the areas they settled.
The Borderers who settled Appalachia had very high rates of violence, as is common in clan-based societies. Those areas continue to have high rates of violence to this day, on both sides of the Atlantic (see Belfast, Glasgow). They largely settled the American west and southwest, from Tennessee to Arizona.
These divisions were tendencies, not pure, even from the start. Mobility was a mark of the American culture even from the start, and these groups did blend over time, especially as they moved west, and most especially since WWII. There were also dozens of other ethnic groups that moved in to each area, which one would expect would dilute the cultural dominance of the original groups. That dilution did occur, but to a far lesser extent than one might think. What is called founders bias has been remarkably persistent in America.
Some numbers to browse through:
US homicides by region here
US violent crime, breakdown by state
And a master list of same, here
Direct connections to example states New Hampshire, Vermont, Tennessee, North Dakota, and Georgia.
You will notice that murder rates across the top of the country – NH, VT, WI, ND, ID – are still among the very lowest, while TN and GA remain very high. As evidence of the previous post about ethnic groups bumping up against each other driving up the rate of violence, note that Louisiana, Los Angeles, and Washington are highest of all. In the UK, the East Anglian region still has the lowest violent crime rate. In fact, if you take out Ipswich and Waveny, which have had recent influx of immigrants into the cities, the crime rates in Suffolk and the Broadland, Breckland, North and South Norfolk sections are ridiculously low even now. The Strathclyde section of Scotland, which includes Glasgow, still has the highest rates of violence save Manchester and London, both of which had Borderers move in during the Industrial Revolution and still have large percentages of immigrants now.
The direct data for California, plus some numbers on race and small city/big city crime rates provides interesting support for the previous post, about ethnic diversity and upheaval leading to violence.
The California numbers, plus these about immigration in general, give sharp visual evidence of how absorbing fewer immigrants leads to less crime, more immigrants - more crime. (Note about the graph. The quota system was instituted in 1924 and abolished in 1965. Amazing effect, eh?) The nature of the immigrants themselves and where they come from does not seem to be as important as their number and where they move to. Those that move to the Dakotas push the crime rate up some when they arrive, but nowhere near as much as those who move into already-violent Louisiana. And once they've been here awhile, everyone seems to settle down. Caution on this last point. Highly visible differences such as race, dress, or religion, seem to slow down the drop in violence quite a bit.
And when the Europeans start complaining about how violent America is, show them this. The highest-crime cities are not what one would expect.