It seems that while slogans are read as being deliberately persuasive, logos slip under our advertising radar and trigger a series of brain-friendly associations built up by the company.
…brand names and logos, argue Laran and colleagues, are different from other commercial messages in that they’re not necessarily perceived as inherently persuasive—despite the fact that they’re often designed with great care, we may normally take them to be primarily referential, much as any proper name might be. Slogans (or, as they say in the industry, taglines) are transparently persuasive according to the authors. Perhaps people react to these latter messages in knee-jerk reverse-psychology manner by blocking and even countering the typical brand associations.
Laran et al. found that when they had people look at cost-conscious brand names like Walmart in an alleged memory study and then later take part in an imaginary shopping task, they were able to replicate the implicit priming effect: people were willing to spend quite a bit less than if they’d seen luxury-brand logos. But when subjects saw slogans (e.g. Save money. Live better.) instead of the brand names, there was a reverse priming effect: now, the luxury-brand slogans triggered more penny-pinching behavior than the economy-brand slogans.
You can read the full study online as a pdf but Language Log has some fantastic coverage on the unconscious psychology of advertising.
Link to LanguageLog ‘Not so gullible after all’.