A number of prior studies have suggested that personalization is more efficacious than non-personalization. However, the existing literature is somewhat ambiguous on whether the test of personalization effects should be based on a message sender's actual personalization process or a message recipient's perception of the message. It is argued in this article that an actual personalization process does not automatically yield more favorable effects because people's perceptions of personalized messages tend to be biased. Through three experiments, it is demonstrated that testing personalization effects based on a message sender's actual personalization process can be problematic and produce misleading results. Specifically, a personalized message can be perceived as non-personalized and a non-personalized message can be perceived as personalized. The key finding is that perceived personalization, instead of actual personalization, is the underlying psychological mechanism of message effectiveness. A message will show superior effects when it is perceived to be personalized by a message recipient, regardless of whether it is actually personalized or not. •Personalized messages are not always more effective than generic messages.•Actual personalization and perceived personalization are two distinct constructs.•A personalized message may be accidentally perceived as non-personalized.•A non-personalized message may be accidentally perceived as personalized.•Perceived personalization is the real driver of favorable personalization effects.