The trend is small enough to go unnoticed year by year. For a given family, it might mean shelling out just pennies more on a grocery run or back-to-school shopping trip. But such changes compound over time, wedging apart the welfare of struggling households and flourishing ones. Rich families get competitive prices on organic groceries and athleisure and better-and-better electronics; poor families end up paying more for worse-quality alternatives.

Jaravel suggested a mechanism behind the finding: Rising wealth and income inequality mean that richer people have ever more disposable income, creating a market incentive for retailers to cater to the needs of lawyers in Chicago and tech analysts in Boston over child-care workers in the Mississippi Delta and part-time retail workers in California’s Central Valley. More firms catering to well-off consumers means more competition for those consumers means more product innovation and product choice for those consumers, as well as suppressed prices. Fewer firms catering to low-income consumers means less competition for those consumers means less product innovation and product choice for those consumers, and increasing prices.

In an interview, Jaravel offered beer as an example. In the past 20 years, the tastes of higher-income consumers have fueled an explosion in craft brewing, whereas the market at the lower end has remained stagnant in comparison. A small-batch sour IPA is more expensive than a Keystone Light, but fancy beers are cheaper, more plentiful, and more accessible than they were in the early Aughts, whereas beers that come in a 30-pack taste the same and cost as much as they always did. “Of course, luxury products are more expensive,” Jaravel told me. “But inflation is about a different thing. It’s not about the level. It’s about the change.”

Annie Lowrey: California is becoming unlivable

Inflation inequality has not filtered into government calculations, at least not yet. “These patterns are all very, very strong,” Jaravel told me. “But you can’t measure them with the standard data that the Bureau of Labor Statistics has, because the Bureau of Labor Statistics doesn’t keep track of spending patterns within these very detailed product categories.”

But new research from Jaravel, along with Christopher Wimer and Sophie Collyer of the Center on Poverty and Social Policy at Columbia University, suggests that it should. Accounting for differential changes in prices would bump up the 2018 poverty rate by 8 percent—adding 3.2 million people to the ranks of the officially poor, and 836,000 people to the ranks of those in deep poverty. According to standard government measures, the real household income of the bottom quintile fell 1 percent from 2004 to 2018; using the new, inflation-sensitive accounting, it fell more than 7 percent.