When people behave selfishly, they have a reliable ally to keep their self-image well-polished — their own memory.

When asked to recall how generous they were in the past, selfish people tend to remember being more benevolent than they actually were, according to a series of experiments by Yale psychologists and economists at University of Zurich published April 29 in the journal Nature Communications.

“When people behave in ways that fall short of their personal standards, one way they maintain their moral self-image is by misremembering their ethical lapses,” said Yale’s Molly Crockett, assistant professor of psychology and senior author of the study.

Psychologists have long been interested in how people balance their self-interest with their desire to be viewed as moral. To justify self-serving behaviors to themselves and others, people engage in a process called motivated reasoning — for example, when leaving a stingy tip, customers might convince themselves that their server didn’t deserve any more.

But a team of researchers led by Crockett and Ryan Carlson, a Ph.D. student at Yale and first author of the study, wanted to explore whether people’s memories of their behaviors help them preserve their moral self-image, perhaps even negating the need to employ motivated reasoning.

Instead of convincing themselves their server didn’t deserve a better tip, for example, a customer might misremember tipping more generously than they actually did.

In their first lab experiment, conducted at the University of Zurich with economists Michel Maréchal and Ernst Fehr, the researchers presented subjects with a pot of money and asked them to decide how much to keep and how much to give to anonymous strangers. After answering some intervening survey questions, participants then were asked to recall how much they had given to the anonymous strangers. Crucially, participants received bonus money if they recalled their decisions accurately.

Even with a financial incentive, stingier subjects tended to recall giving more money than they actually did.

In another pair of experiments conducted in the lab and online, the researchers asked subjects what they thought was a fair distribution of money before asking them to divide the pot. The researchers found that only those subjects who had given less than what they personally deemed fair recalled being more generous than they actually were.

A final pair of online studies showed that subjects only misremembered their stinginess when they felt personally responsible for their decisions. When participants were explicitly instructed by the experimenters to give lower amounts, and so felt no responsibility for their actions, they remembered their giving behavior accurately.

“Most people strive to behave ethically, but people sometimes fail to uphold their ideals,” Carlson said. “In such cases, the desire to preserve a moral self-image can be a powerful force and not only motivate us to rationalize our unethical actions, but also ‘revise’ such actions in our memory.”

Crockett cautioned that because the experiments were conducted in Switzerland and the USA, it is not yet clear whether the results will generalize across different cultures.

She also stressed that this tendency for faulty recall only applied to the selfish. The majority of people behaved generously toward their anonymous strangers, and remembered their behavior accurately.