The obvious thing to read, just in case you haven't already, is Jared Diamond's Guns, Germs, and Steel. An excellent (and relatively brief) companion essay about "optimal fragmentation" may be found here. I concur that there are several dimensions to consider when diagnosing the West's relative success in this area.

Yes, I've read GG&S, although not recently. Perhaps I should go back to it. I found GG&S to be quite brilliant in promoting the "geographic" hypothesis. Thanks for pointing me to the "optimal fragmentation" article.

In general, I think that just as small populations are factories of genetic change, through microevolution, so are they factories of cultural change. But small populations may suffer from "provinciality", or be dominated by "big neighbors".

Personally, I think that globalization and all it implies is diminishing the flexibility and evolutionary potential that multiple competing groups can provide. But, new groups, perhaps socially defined, may form.

In classical times, multiple competing small populations, isolated but not too isolated, were indeed the incubators of such cultural innovation as took place -- Greece being the perfect example; eventually most of Europe functioned this way.

Thanks to effectively instantaneous communication and under-1-day travel times between almost any two cities on Earth, any such isolation today must, as you allude, take a different form. The situation is even more extreme in the US, where trade barriers between states are forbidden by the Federal Constitution and the population is notoriously mobile (warning: 779k *.pdf) -- almost 8 million people move to a different state in any given year (I've lived in 5 states myself).

In spite of all the highly visible commonalities, to say nothing of the monolithic impression obtained (and robotlike behavior noticed) by foreign visitors, the US contains a vast number of subcultures. Most compete with one another only over the time of their participants -- I think everyone can be said to belong to at least several. Lacking a physical frontier (until space settlement becomes economical), development is "intensive" rather than "extensive." Another good book which discusses how to maintain an innovative polity is Virginia Postrel's The Future and Its Enemies.

>> In spite of all the highly visible commonalities, to say nothing of the monolithic impression obtained (and robotlike behavior noticed) by foreign visitors, the US contains a vast number of subcultures.

True. I think that any group of people that have to live together end up evolving a common culture. That is what I call the "cultural common denominator". I think that as the cultural elements involved become more diverse, the common denominator becomes increasingly generic - to accommodate the divergent tendencies.

Foreigners tend to notice the shallowness of the common denominator. Variation around the common denominator occurs because (a) the immigrants' backgrounds are retained, (b) they recombine and play against each other, (c) the US is too big of a country to be able to sustain a truly homogenous culture.