They found that high-income households (those making $100,000 or more a year) bought their toilet paper on sale 39 percent of the time, whereas low-income households (those making $20,000 or less a year) only did so 28 percent of the time. High-income households were also more likely to buy more rolls of toilet paper at a time, which meant not only that they were saving money on each roll, but that they didn’t have to make as many trips to the store. “Low income households,” Orhun and Palazzolo write, “are less likely to utilize these strategies even though they have greater incentives to do so.”

These differences produce a striking pattern: Orhun and Palazzolo calculate that because low-income shoppers don’t take full advantage of sales and buying in bulk, they end up paying about 6 percent more per sheet of toilet paper than high-income households. At the same time, lower-income households seem to be compensating for this premium by buying cheaper brands—a trend working in the other direction, saving them about 9 percent per sheet, compared to high-income households. “Therefore,” Orhun and Palazzolo write, “about two-thirds of the savings low income households accrue through brand choice is forfeited by their relative inability to utilize intertemporal money-saving strategies.” (Whether products with fancier brand names are truly better, and whether it’s a loss to miss out on them, is another matter.)

High-income households might be buying more rolls of toilet paper at a time because they’re more likely to have the cars to transport them and then the space to store them at home. But even after controlling for those possibilities, Orhun and Palazzolo find that what matters is how much cash a given household has on hand when a deal presents itself.

These disparities—paying 6 percent more when not buying in bulk or on sale—may seem minuscule, but they can matter on the scale of a household. Considering every sheet of toilet paper, every can of soda, every garbage bag, every bottle of vegetable oil, the premium adds up. (Plus, the researchers note that because their data set was less than perfect, they consider 6 percent a lower bound for the toilet-paper premium.)

On top of that, Orhun and Palazzolo’s data suggests that poorer and richer consumers aren’t just buying products in different quantities—they’re sometimes doing so at different stores entirely. At corner stores, the price per sheet of toilet paper (or paper towel, or tissue, and so on) is much higher than at warehouse stores such as Costco and Sam’s Club—stores where the average customer tends to be somewhat well-off.

There is something of a debate among economists about how and where lower-income consumers choose to spend their money. One on hand, there are academics who have documented the ways in which an environment of poverty—all of the uncertainty and stress that comes from not having enough money—makes people worse decision makers. The argument isn’t that they’re inherently less sharp, but that they become, as a result of their circumstances, more prone to making irrational, present-biased choices. On the other side, there is a body of evidence supporting the idea that those without much money are simply making the best possible decisions they can make, given their crummy circumstances.