Doing a good deed can lead some people to more kind acts while spurring others to backslide. But how people respond depends on their moral outlook, according to a new study.

People who believe the ends justify the means are likelier to offset good deeds with bad ones and vice versa. By contrast, those who believe right and wrong are defined by principle, not outcome, tend to be more consistent, even if they're behaving unethically.

The findings were published Feb. 27 in the journal Psychological Science.

Some studies show that people maintain a kind of moral equilibrium, meaning that giving money to charity may lead them to skimp on the tip at dinner, whereas partying too much may inspire a volunteer day at the soup kitchen.

But other studies found just the opposite: Behaving ethically leads people to more good deeds later, said study co-author, Gert Cornelissen, a psychologist at the University Pompeu Fabra in Spain.

To sort out this conflicting picture, Cornelissen and his colleagues asked 84 undergraduates what they would do in a hypothetical dilemma where a runaway trolley is on a collision course with five people, and the only way to save them is to flip a switch, reroute the trolley and kill one person. 

People who would flip the switch were considered to have outcome-based morality, where the end results (saving four lives), not the actions (causing one person's death), matter most. Those in the opposite group were assumed to base their morality on rules, such as "deliberate killing is always wrong."

Half of the participants were then asked to remember a time they behaved ethically, while the other group remembered past unethical behavior. They then asked participants to share a pot of money with partners.  

Those who had an ends-justify-the-means mindset were likelier to be stingier with others if they were reminded of their past good deeds and more generous if they recalled past unethical behavior. By contrast, those who tended towards rules-based morality showed the opposite trend, suggesting that past good deeds or bad deeds were prompting similar behavior later on.

In another experiment, students showed the same trends in their likeliness to cheat on a self-graded quiz. Consistent with that trend, remembering past  bad deeds made people with rule-based morality more likely to cheat.

For people who are keeping a mental balance sheet of their good and bad deeds, one bad act can be an offset in their minds with a nice one, Cornelissen said.

But for those with rule-based morality, that bad deed can cause a slippery slope, Cornelissen said.

"When people are thinking in terms of rules, they think once a rule is broken, the harm is done, so it's very difficult to undo that, the stain remains," Cornelissen told LiveScience. "The more efficient way for people in that case to feel is to convince themselves that whatever wrong they did is not that bad."

Once that's the case, it's easier for them to behave unethically in the future, he said.

Of course in real life, most people have a messier moral approach, mixing outcome-based morality with firm principles in different areas of their lives, he said.

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