What do the counties in the first list have in common? Their economies were all heavily driven by information technology in the late 90’s. This is true for the rest of the list of winners as well. Harris, Tex. (home to Houston and Enron); Middlesex, Mass. (home to Harvard and M.I.T.); Fairfield, Conn.; Alameda, Calif.; and Westchester, N.Y., were also among the top 10 income gainers in this period.

The authors point out that half the 80 American companies in the CNET Tech Index are in those top 10 counties. Furthermore, when income inequality decreased after 2000, the income drop in the high-tech counties contributed most to the decline.

New York, interestingly enough, showed large increases in per capita income both during the Internet boom and the Internet winter that followed.

The implication is that the income gains of the 1990’s associated with the technology bubble not only accrued to a relatively small number of people but also occurred in a relatively small number of geographic areas.

To drive this point home, the authors asked what would have happened to the index if just 4 of the 3,100 counties in the United States exhibited average income growth in the technology boom years. The four are Santa Clara, San Mateo, San Francisco (all associated with Silicon Valley) and King County, Wash. (home of Microsoft).

Note the remarkable difference in the Theil index computed with the adjusted growth rates for these four counties: If the per capita income in just these four counties had grown at the same rate as the average in the United States, income inequality across counties would have changed little in the late 1990’s. In other words, only four counties drove most of the change across the 3,100 counties.

The findings by Mr. Galbraith and Mr. Hale offer something to all sides in the debate. Perhaps options grants and initial public stock offerings had a lot to do with income inequality. Changing compensation patterns for technology-related skills could also be significant.

But the biggest point that I take away is a simple one: there’s no substitute for being in the right place at the right time.