IDAHO CITY, IDAHO — The Wilks brothers grew up in a goat shed, never finished high school and built a billion-dollar fracking business from scratch.

So when the brothers, Dan and Farris, bought a vast stretch of mountain-studded land in southwest Idaho, it was not just an investment, but a sign of their good fortune.

“Through hard work and determination — and they didn’t have a lot of privilege — they’ve reached success,” said Dan Wilks’s son, Justin.

The purchase also placed the Wilkses high on the list of well-heeled landowners who are buying huge parcels of America. In the last decade, private land in the United States has become increasingly concentrated in the hands of a few. Today, just 100 families own about 42 million acres across the country, a 65,000-square-mile expanse, according to the Land Report, a magazine that tracks large purchases. Researchers at the magazine have found that the amount of land owned by those 100 families has jumped 50 percent since 2007.

Much of that land stretches from the Rocky Mountains down into Texas, where, for some, commercial forests and retired ranches have become an […]