There was a time when farmer’s markets, eco products, recycling, and renewable energy were squarely in the tree hugger’s domain. Then, somewhere along the line, green went mainstream, turning environmental awareness into a socially desirable trait and a mark of morality.

But is eco-friendliness always a boon? When University of Toronto researchers Nina Mazar and Chen-Bo Zhong recently looked into this, they found that under certain circumstances, green products can license us to act immorally.

Through a series of experiments, Mazar and Zhong drew the following distinction between two kinds of exposure to green: When it’s a matter of pure priming (i.e., we are reminded of eco products through words or images), our norms of social responsibility are activated and we become more likely to act ethically afterwards. But if we take the next step and actually purchase the green product (thereby aligning our actions with our moral self-image), we give ourselves the go-ahead to then slack off a little and engage in subsequent dishonest behavior.

So in effect, a green purchase licenses us to say “I’ve done my good deed for the day, and now I can think about my own self-interest.” I gave $20 to a charity, I pledged support to NPR, I did my share — that sort of thing. How moral we choose to be at any given moment depends not only on our stable character traits but also on our recent behavior.

This implies that if you have two important environmental decisions to make on a given day, your early decision may impact the later one. If you choose a mug over a paper cup for your morning coffee, you may later decide it’s okay not to recycle if a bin isn’t handy. This choice could even affect your subsequent moral choices in other areas, since the moral licensing effect is not domain-specific. (Participants in the above-mentioned experiment, for example, were more likely to cheat and steal cash after making green purchases).

All this to say that we need to think carefully about the unintended consequences of all the decisions we make. While we may consider the consequences of questionable decisions (speeding or parking illegally for example), we rarely consider the effects of “good” ones.