One is by need: Some people have more than they need, and others need more than they have. Even when liberal leaders describe policies that are beneficial to everyone, they make it clear that the most important beneficiaries are those whose needs are most urgent. Indeed, Warren’s plan was also criticized from the left for insufficiently prioritizing those who need debt relief the most.

Still, there are other ways of judging what’s fair. Conservatives tend to value equity, or proportionality, and they see unfairness when people are asked to contribute more than they should expect to receive in return, or when people receive more than they contribute. Consider a hypothetical comparison of two people who graduated from college five years ago with equal amounts of debt. Jessie successfully implemented a plan to pay off the debt in five years, while Sam still has much to repay. Warren’s plan forgives Sam’s debt, but offers nothing to Jessie, despite her industriousness and self-discipline. To add insult to injury, Jessie must contribute tax dollars to the $640 billion fund necessary to forgive outstanding loans, including Sam’s…

This conservative version of fairness is wired deeply in the human brain, and liberals ignore it at their peril. In the laboratory, psychologists study the roots of economic and political attitudes through exercises like the ultimatum game, in which one player (the allocator) makes an offer to another player (the recipient) about how to split a small pot of money put up by the researchers. The recipient can accept the other player’s offer and take the cash—or reject it, in which case neither player gets anything. Not surprisingly, when the allocator offers a 50-50 split, recipients accept it.