Given a warning, we can shield ourselves from subliminal messages

Ad-man James Vicary generated excitement and discomfort in equal measure back in the 50s when he boasted about the success of his "subliminal adverts". Presenting the words "Drink Coke" or "Eat Popcorn" on-screen, mid-movie, too fast to be consciously detected, had the effect of increasing the purchase of refreshments by cinema-goers, or so he claimed.

It turns out this was a hoax, but more recent research has confirmed that people can be influenced by subliminal messages. From the Latin "below the threshold", these messages make certain mental concepts more accessible and, under the right conditions, can influence people's behaviour. Most important seems to be that the message is goal-relevant. So a subliminal message for a drink brand is more likely to influence you if you're thirsty.

The idea that our decisions can be influenced by advertising messages we're not aware of raises obvious ethical concerns. Relevant here is whether we can shield ourselves from subliminal messages if we're given a warning about them – a question explored in a new study.

Thijs Verwijmeren and his colleagues exposed 173 students to the name of a drink brand (7-up or Nestea) for just 17ms, far too quick to be consciously detected; or if they were in the control group, no brand was shown. Crucially, for those students shown a subliminal brand name, half had been given a prior warning that they were going to be exposed to a subliminal message and that such messages can affect behaviour. Finally, all the students made a hypothetical choice between two drinks – 7-up or Nestea.

The warning worked. Thirsty participants exposed to a subliminal drink brand were more likely to choose that brand, unless they received the warning message. This suggests we are able to shield ourselves from subliminal advertising, but how?

The addition of a secondary memory task to the procedure made no difference. Warned students were still able to shield themselves regardless of whether they had to remember a long or short number at the same time. This suggests the shielding process doesn't require cognitive resources.

A second study with dozens more students varied the timing of the warning so that it came either before or after the subliminal brand names. In fact the warning was just as effective regardless of when it was shown. This suggests the warning doesn't alter the initial processing of the subliminal message, but somehow alters the downstream effects of the message. Specifically, Verwijmeren and his colleagues think that being warned triggers a more cautious mindset, which stops us relying on the accessibility of thoughts when making a decision. Perhaps we also become less trusting of concepts that enter our thoughts, realising that they are not always self-generated.

These intriguing results suggest that we may be relatively invulnerable to subliminal advertising messages in other situations where we're cautious – perhaps when making an expensive or important purchase. However, we need more research in a real-life setting to know the limits of our subliminal defences. As the researchers acknowledged, the link in this study between the warning and the decision was obvious.  Real life is more complex and it's not clear how far a basic warning would generalise, nor how long it would last.

Further reading

Thijs Verwijmeren, Johan C. Karremans, Stefan F. Bernritter, Wolfgang Stroebe, & Daniël H.J. Wigboldus (2013). Warning: You are being primed! The effect of a warning on the impact of subliminal ads. Journal of Experimental Social Psychology DOI: 10.1016/j.jesp.2013.06.010