I'd be curious to know if researchers ever performed a comparative study of the brains and behavior of people in small towns versus people in big cities. I say this because I wonder if the small town residents might have a slightly larger pool. Small town folks have a smaller group as a whole to know, and tend to have more time to form associations (you're not sitting in traffic alone for hours at a time, for example, which gives more time for interaction).
It's also something to consider in light of the behavior of trolls on the Internet -- masses and anonymity.
The article is old in 'net time - it has been around since at least October of 2007 - but it was new to me, and I found it an interesting read.