As an outsider to the American tax system: is this a strong selection that could impact results? Here in Germany for example filing is optional for a lot of people (not just legally, but also in terms of expected returns) which would have quite unpredictable results on a study like this.
Excludes recent immigrants or orphans then, both of which groups could be particularly acute case-studies of upward mobility.
Big picture though, walkability and public transport definitely imply at least the potential for interaction and empathy, and cars the opposite.
The Highway and Automobile culture are symbols of totalitarian cultures which deny people more sustainable and equitable alternatives for mobility and transport. - Vandana Shiva, February 19, 2004.
Has anyone on HN paid US taxes living abroad?
You will have to pay some tax if you work in a country whose tax rates (for your bracket) are lower than US federal tax rates. That can be the case for Americans working in places like Hong Kong or the Bahamas.