Final answer:
The sleeper effect might occur because people tend to retain a message but forget its source.
Step-by-step explanation:
The possible explanation for why the sleeper effect might occur is that people tend to retain a message but forget its source. This means that over time, even if the initial source of the message is unreliable or untrustworthy, people may still be persuaded by the message due to forgetting where it came from. For example, if someone hears a persuasive argument from a politician but later forgets who said it, they may still be influenced by the argument.