Anyone who has ever devoured a triple-chocolate brownie after an intense workout knows how tempting it can be to indulge after behaving virtuously. A new study suggests, however, that we often apply this thought process to inappropriate scenarios, giving ourselves license to act in unhealthy or antisocial ways.

Researchers in Taiwan gave a sugar pill to 74 smokers, misleading half of them to think it was a vitamin C supplement. All the participants then took an unrelated survey and were told they could smoke if they desired. Those who believed they had taken a vitamin smoked twice as many cigarettes as those who knew they had taken a placebo. According to study co-author Wen-Bin Chiou of National Sun Yat-Sen University, the participants may have felt, consciously or unconsciously, that the healthy activity entitled them to partake, a concept known as the licensing effect.

His study, published in the journal Addiction, is the first to examine the health ramifications of the licensing effect, but others have shown its influence on moral behavior. In 2009 a study found that reminding people of their humanitarian attributes reduced their charitable giving. Last year another experiment showed that when individuals buy ecofriendly products, they are more likely to cheat and steal.

"Sometimes after we behave in line with our goals or standards, it's as if our action has earned ourselves some moral credit," says psychologist Nina Mazar of the University of Toronto, an author of the green products study. "This credit can then subsequently be used to engage in self-indulgent or selfish behaviors without feeling bad about it."

You may be able to avoid the pitfall simply by remembering that the feeling of having "earned it" leads down a path of iniquity.

This article was published in print as "License to Sin."