McMullen County is the sixth least-populous county of the 254 such jurisdictions in Texas. An estimated 820 people live there.
McMullen County also was the richest county in 2015, not only in the Lone Star State, but also in the entire country.
That ranking is per Internal Revenue Service filing data analyzed by the Transactional Records Access Clearinghouse (TRAC).
Energy source = high Texas incomes: Using the tax data from 2015, the most recent year for which the complete information is available, McMullen County's average adjusted gross income (AGI) of $303,717 was tops.
That six-figure average AGI earns this week's By the Numbers honor.
The wealth in the south-central Texas county, about an hour and a half south of San Antonio, is thanks in large part to fracking. McMullen County is smack dab in the middle of the Eagle Ford shale patch.
That energy source and its controversial retrieval technique also helped two other Texas counties climb into the high wealth list.
Glasscock County in West Texas comes in fourth thanks to its average income in 2015 of $181,375. Its boundaries encompass part of the Barnett shale patch.
La Salle County, a neighbor to McMullen amid the Eagle Ford shale source, was 10th based on its average income of $146,991.
In 2005, no Texas county cracked the top 30 of most wealthy in the United States.
"I joke that oil and gas finally made ranching profitable," Thomas Tunstall, research director for the Institute for Economic Development at the University of Texas at San Antonio, told Bloomberg. "A lot of old Texas families live on large ranches in McMullen County, and the older generation went through tough times prior to five years ago."
Other county high and low rankings: So what other counties across the United States join McMullen, Glasscock and La Salle in the top 10? They are:
| County (County Seat) | Average AGI |
| 1. McMullen County (Tilden, Texas) | $303,717 |
| 2. Teton County (Moose, Wyoming) | $248,949 |
| 3. New York County (Manhattan, New York) | $210,233 |
| 4. Glasscock County (Garden City, Texas) | $181,375 |
| 5. Marin County (Point Reyes Station, California) | $158,573 |
| 6. Fairfield County (Danbury, Connecticut) | $158,253 |
| 7. McKenzie County (Watford City, North Dakota) | $154,013 |
| 8. San Mateo County (Menlo Park, California) | $152,911 |
| 9. Pitkin County (Aspen, Colorado) | $147,271 |
| 10. La Salle County (Cotulla, Texas) | $146,991 |
And the 10 counties with the lowest average adjusted gross income in 2015? They are:
| County (County Seat) | Average AGI |
| 1. Greene County (Eutaw, Alabama) | $30,236 |
| 2. Todd County (Rosebud, South Dakota) | $30,141 |
| 3. McCreary County (Pine Knot, Kentucky) | $29,909 |
| 4. Echols County (Haylow, Georgia) | $29,722 |
| 5. Noxubee County (Brooksville, Mississippi) | $29,644 |
| 6. Atkinson County (Axson, Georgia) | $29,451 |
| 7. Buffalo County (Fort Thompson, South Dakota) | $28,911 |
| 8. Hayes County (Hayes Center, Nebraska) | $28,307 |
| 9. Holmes County (Lexington, Mississippi) | $28,147 |
| 10. Clay County (Fort Gaines, Georgia) | $26,649 |
You can see the full U.S. counties average incomes and rankings, or sort by your state only, as well as see other IRS fiscal information you can sort TRAC's tax data page.
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