dwilgar said:
I raised this question early in the "Ask Keith Baker" thread, and am rather surprised at the number of other people who have taken an interest in a somewhat trivial tidbit. I think the numbers are low, but more on the order of a factor of 2, not the type of numbers others are suggesting. I have no idea where someone gets the idea that the 40 people per square mile is some magical minimum number. The USA didn't hit this threshold until 1950. For a good comparison, the USA had a population density of about 5 people/square mile from 1790 to 1820. Based on the conversation in this thread, it is a wonder the USA still exists today. Compare this with Eberron at a population density of about 2 (give or take, as it is hard to get the area all that accurately) and I really don't see a huge problem with the Eberron numbers, especially if you take Keith's advice and double to include children.
(US population data from:
http://www.census.gov/population/www/censusdata/hiscendata.html )
Dwilgar
The US is quite a different case. Firstly, it wasn't feudal, wasn't roughly medieval, had access to much better agricultural technologies (and slaves), and most importantly, had a ready consumer demand that purchased the majority of its excess production (England) as opposed to having to internally balance a supply/demand cycle. It was also a situation of expansion into a previously unclaimed and fairly uncontested land (well, relatively, of course).
And, although I'm not an American historian, I think you'll find that those numbers are (effectively) artificially low because of large territory "purchases" from other countries. Territories that could only in the fanciful dreams of colonial states be claimed as part of the actual country until a much later time.

From 1790 to 1820 the sq. miles of land doubled and total population increased by 2.5 times. Land sq. miles had doubled again by 1870, but population had increased four-fold from 1820. Had there been no territory purchases, density would have rapidly increased, even though the land/population growth feed-back cycle would have been absent.
Ah, found what I was looking for...
http://www.rootsweb.com/~nycoloni/1790intro.html
"[in 1790] The gross area of the United Sates was 827,844 square miles, but the settled area was only 239,935 square miles, or about 29 percent of the total." Which means that real density of the neo-colonial state in 1790 was about 3 times greater or about 15 per sq. mile. England was roughly 70 at the time (about what the USA was in
1990.)
From a more traditional european perspective, I don't think hardly any feudal medieval state had a population density less than around 40 (England). I think Scotland was maybe like 35 or so, but France was more like 100 or so. England was historically a small population country.
And yes, I have nothing better to do.

However, now I know that were I to make a similiar to USA type colonial invasion, 15 per sq. mile. is a respectable amount and that the "claims" on land can exceed 3 times the area actually settled. It wouldn't be perfect, but It would be a good place to start.
joe b.