In the past four years, the amount of money administered by South Dakota trust companies like these has tripled to $121 billion, almost all of it from out of state. The families needn’t actually move to South Dakota, or deposit their money at a local bank, or even touch down in the private jet. Little more than renting an address in Sioux Falls is required to take advantage of South Dakota’s tax-friendly trust laws.

States like South Dakota are “creating laws that are conducive to a massive exploitation of a federal tax loophole,” said Edward McCaffery, a professor at the University of Southern California’s Gould School of Law. “We have a tax haven in our midst.”

South Dakota’s sudden popularity illustrates how, at a time of rising U.S. economic inequality, the wealthiest Americans are embracing ever more creative ways to reduce taxes legally. Executives at South Dakota Trust Co., one of the biggest in the state, estimate that one-quarter of their business comes from special vehicles known as “dynasty trusts,” which are designed to avoid the federal estate tax. Creation of such trusts has surged in recent years as changes in federal law enabled more money to be placed in them.