of army propaganda. It was found that the difference in opinions of those who had observed the army propaganda movie and those who did not watch the movie were greater nine weeks after viewing it than five days. The difference in delayed persuasion is (which Hovland et al. termed) the sleeper effect, where there was a significant increase of persuasion in the experimental group. The first efforts to justify the effect were consistent with the understanding of persuasion processes at that time. Hovland and his colleagues introduced a program of research to study how recall of the message and the source persuaded the sleeper effect. They first hypothesized that message receivers forget the noncredible communicator as time goes by, and therefore the initial message rejection diminishes. Nevertheless, they later propositioned that message receivers may not entirely forget the cue, yet the association between the representations of the discounting cue and the message content may fade over time and produce a sleeper effect. These two formulations vary in that (a) forgetting suggests that the traces of the cue disappear or become unavailable in memory over time, while (b) dissociation suggests that cue remains available in memory but is simply less easily retrieved (less accessible) in relation to the topic of communication. Hypotheses on how the sleeper effect occurs Because the sleeper effect has been considered to be counter-intuitive, researchers since the early 1950s have attempted to explain how and why it occurs. Forgetting and dissociation According to the forgetting hypothesis, a discounting cue associated with a message initially decreases acceptance of the message. As time goes by, one may observe a delayed increase of persuasion if the recipient forgets the cue but recalls the merits of the message (Hovland et al., 1949). To test this hypothesis, Hovland and his colleagues (Hovland & Weiss, 1951; Kelman & Hovland, 1953; Weiss, 1953) initiated a series of experiments in which participants received messages attributed to either trustworthy or untrustworthy sources and then completed measures of opinions as well as of recall of the message content and the source. Overall, messages with credible sources produced greater initial persuasion than messages delivered by non credible sources. Hovland, Lumsdaine, and Sheffield (1949) first discovered the effect by a well-known study that demonstrated the delayed impact of a World War II propaganda movie on American soldiers. With a subset of conditions that caused participants to question the credibility