So people turn to dimensions of comparison that can be quantified. “Money is a terrific one,” Norton says. “If I need to know if I’m doing better than I was, the easy thing to ask is, Am I making more money? or Does my house have more square feet? or Do I have more houses than I used to?”

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This instinct to measure and compare doesn’t disappear once people have an obscene amount of money. “The problem is, Am I doing better than I was? is only [moving people in] one direction, which is up,” Norton says. And if a family amasses, say, $50 million but upgrades to a neighborhood where everyone has that much money (or more), they feel a lot less rich than if they had stuck to the peer comparisons they were making tens of millions of dollars ago. Hence the ever-shifting goalposts of wealth and satisfaction.

The research Norton has conducted illustrating this phenomenon is dispiriting. In a paper published earlier this year , he and his collaborators asked more than 2,000 people who have a net worth of at least $1 million (including many whose wealth far exceeded that threshold) how happy they were on a scale of one to 10, and then how much more money they would need to get to 10. “All the way up the income-wealth spectrum,” Norton told me, “basically everyone says [they’d need] two or three times as much” to be perfectly happy.

Where did Norton find his rich people? For that particular study, an investment bank connected him with some of its high-net-worth clients. But Norton also told me that he had previously consulted with a pool of Dutch millionaires willing to respond to researchers’ questions, making themselves marginally richer in the process: For one study, Norton and his collaborator paid each respondent about 46 euros for every completed questionnaire. “You can run a survey on regular people for like a dollar,” he says.

Jeffrey Winters, a professor of political science at Northwestern University and the author of Oligarchy , said that in addition to social comparison, really rich people are often motivated to acquire more money by the thrill that comes with multiplying one’s fortune by making investments, buying up businesses, and so forth. “For those of us who make wages and have expenditures that we are trying to meet—a mortgage, pay our health insurance, food, whatever happens to be our kid’s tuition—we link the making of money to our expenses,” he says. Meanwhile, many ultra-wealthy people “use their money to make money,” he says—an exciting, status-enhancing process.

Those two ways of putting money to use—as a way of covering expenses or as a way of building a bigger fortune—come with two different points of diminishing returns. “Say you wanted to have a mega-yacht plus six mansions in six different locations around the world,” Winters says. “You could probably do all of that fairly comfortably with a few hundred million dollars.” It’s different if the goal is to keep accumulating, in which case “there’s no number at which you have enough,” Winters says. He adds, “Every billionaire I’ve spoken to, and I’ve spoken to quite a number of them, is extremely excited by each additional increment of money they make.”