The sleeper effect is a term used to describe a particular process of delayed persuasion in psychology. In theory, this effect occurs when someone initially ignores a persuasive message because it doesn’t seem to be credible, and then gradually starts to believe the message. This is the opposite of the normal way persuasion works because typically, people tend to become less convinced by messages as time passes, and may actually need a lot of reinforcement to maintain their changed opinions. Most experts think the effect happens because people may gradually lose the sense of connectedness between the message they received and the circumstances of the message which made them initially distrust it.
Generally, the sleeper effect is most prominent with propaganda, advertisements, or other situations where the creator of a message is notably difficult to trust. If an individual receives a message with a very strong slant from one of these sources, even though the person may feel a strong emotional tug, he or she will still often feel inclined to dismiss it. Then, over time, the person might start to find that the message seems more valuable than it did initially, and experts believe this is because the emotions of the message are strong enough to outlast the distrust at the initial point when the message was received. If the person is asked about this seeming contradiction, he or she will generally still remember having a sour feeling towards the creator of the message, but the connection between those feelings and the trustworthiness of the message will often be less powerful than it was initially.
Sometimes the sleeper effect doesn’t work, and laboratory tests have shown that it can only work when everything is exactly right. Basically, the message itself has to be so powerful that it can outlast the initial distrust that people feel for the source. Also, it is usually better if people receive the message before they realize the identity of the source. So, for example, if people were to see a well-made propaganda documentary, and then find out at the end that it was created by an unscrupulous individual, the sleeper effect could potentially occur. The strictness of the required circumstances needed to produce the effect has sometimes made certain scientists skeptical, and some of that skepticism still exists.
Generally speaking, the sleeper effect can be more useful in contexts where it is difficult to produce a message without letting the audience know about the source. For example, there are often laws about political advertisements that require disclosure of the identity of the creator. Studies have shown that even though people know it isn't easy to trust the creator of some of these messages, they might still eventually adopt attitudes based on the messages if enough time passes.