There's an old paradox from the humorist Will Rogers that uses the migration of people during 1930's Great Depression as a backdrop. Rogers said "When the Okies left Oklahoma and moved to California, they raised the average intelligence level in both states."
It certainly sounds paradoxical, but if you think about it a while, you realize that it's possible if the bottom-most people in Oklahoma were smarter on average than the people in California. They leave, Oklahoma's average IQ goes up and so does California's when they arrive.
It's a neat puzzle, and in medicine they've actually named a statistical characteristic of cancer studies the `Will Rogers Phenomenon' after this paradox.
Closer to home, for us, I think the Will Rogers Paradox applies to workplaces. I've noticed that they stratify. If you pick two companies, A and B in a geographical region, they are either rather close to each other in terms of developer skill level, or they are wildly different: the top developers at A would just barely being able to keep up at B, and if developers from B moved to A, they'd be bored senseless and they'd bolt back to B.
I don't have any hard evidence for this, but it does seem like you don't have as many places in the middle. Pick two organizations at random in an area and it's likely that they are either on par or wildly different.
Developers self-select and we end up with stratification of skill across the industry.
Thoughts?
