The reason has to do with how these messages employ social norms. There are two main types of social norms--descriptive and injunctive. Descriptive norms inform by way of example. They suggest what is commonly done. Injunctive norms affect behavior by directing us toward what ought to be done--toward what is socially approved or disapproved.

Cialdini has conducted many studies that demonstrate the influence of descriptive norms. For example, people who park in a clean public garage are significantly less likely to litter than if they park in the same garage on a day when it is dirty with litter. In fact, calling people's attention to the litter actually increased the likelihood of their contributing to it.

The problem with the anti-littering campaign then is that its descriptive norm is counterproductive to its injunctive norm. Though the ad aims to convince people they ought not litter, the repeated images of people littering may promote littering by convincing viewers that everybody does it.

Descriptive norms can be useful in encouraging pro-environment behavior if they suggest that pro-environment behavior is the norm. Another study, this one examining what plea best encouraged hotel guests to reuse their towels, so as to conserve energy, provides an example: "Join your fellow citizens in helping to save the environment." This sign was more successful than any of the signs just emphasizing the benefits of protecting the environment.