Advertising men once used it to try to increase sales of popcorn, and Formula One teams have been accused of indulging in it to sell cigarettes. But it turns out that subliminal messaging – flashing an image or words on a screen for a fraction of a second – works best if it leaves the viewer in a state of fear.
An experiment by British researchers has found that even though subliminal messages are shown so briefly that the human eye cannot consciously read them, the brain is particularly good at picking up on the emotional meaning of a word if it is negative.
Scientists at University College London believe the results of the study, in which participants correctly identified when a subliminally transmitted word had negative connotations more than seven times out of 10, shows that humans are programmed at a sub-conscious level to respond to any stimulus that contains a potential threat.
Volunteers were shown a series of positive, negative and neutral words, such as "cheerful", "despair" or "box", for as little as 17 milliseconds – far too quick for the mind to perceive conventionally – and asked to decide whether or not each word had an emotional value.
Where the volunteers correctly identified a word as emotional – for example "flower", "peace", "agony" or "murder" – they were more efficient at picking out those with a negative meaning. In one test, where participants were shown each word for just 33 milliseconds, negative words were correctly spotted 77 per cent of the time, as opposed to 59 per cent for positive terms.
Subliminal messaging has long been a controversial area, with psychologists pondering whether the mind can be influenced by imperceptible stimuli. The idea was attractive to advertisers, who famously tried to increase sales of fizzy drinks and popcorn to a 1950s cinema audience by projecting the words "Drink Coca-Cola" and "Hungry? Eat popcorn" on to the screen for a fraction of a second.
The claimed results – that Coke sales rose by 58 per cent and popcorn by 18 per cent – were later found to have been fabricated, and a repeat experiment by scientists found no effect on sales at all.
Professor Nilli Lavie, who led the UCL team, said the latest study had provided the first unambiguous proof that people can process emotional information from subliminal images. "We have demonstrated conclusively that people are much more attuned to negative words," she said. "Clearly, there are evolutionary advantages to responding rapidly to emotional information. We can't wait for our consciousness to kick in if we see someone running towards us with a knife or if we drive under rainy or foggy weather conditions and see a sign warning 'danger'."
Although subliminal advertising is banned in Britain and elsewhere, the insertion of single-frame images has featured in mainstream media from a 1943 Daffy Duck cartoon which flashed the words "Buy Bonds" to the 1980s comedy The Young Ones.
The researchers believe their study, which may point to a sub-conscious "fast link" between primitive parts of the brain and those associated with decision making, has implications for the use of subliminal messaging in marketing. Professor Lavie said: "Negative words may have a more rapid impact. 'Kill your speed' should be more noticeable than 'Slow down'. More controversially, highlighting a competitor's negative qualities may work on a subliminal level much more effectively than shouting about your own selling points."