There is a common perception that public service messaging is not as effective as commercial messaging in influencing people's behavior. According to research (pdf) by Professor Robert Cialdini, even though commercial messages often have greater budgets and production values, there may be an unintended psychological message in many PSAs that undermines their public interest message.
Most PSAs rely on a technique called "injunctive norms" in which a certain type of behavior is shown to be either socially approved or disapproved. This type of message can successfully motivate people because most of us are interested in avoiding the disapproval of others. For instance, a PSA that depicts a teenager positively for abstaining from drug use can be persuasive.
The problem arises when that injunctive message is combined with a "descriptive norm" that portrays the drug use by the teen's peers as commonplace. Just as people are motivated to behave appropriately, they are also motivated to imitate the actions of their peers – even if those actions are viewed disapprovingly by society in general. According to Professor Cialdini's research, the most effective ad in this situation would combine a depiction of most teens abstaining from drug use with disapproval of a single teen's use of drugs. In this way, both descriptive and injunctive norms could work in concert.
This research has broader implications for crime prevention than the content of public service announcements. When communicating with people about crime, stress that most people do not engage in criminal behavior, and that criminal activity is met with deep disapproval by all parts of society.