It was a stunt that launched a thousand conspiracy theories. Market researcher James Vicary claimed in 1957 that he could get movie-goers to "drink Coca-Cola" and "eat popcorn" by flashing those messages on the screen for such a short time that viewers were unaware of it. People were outraged, and the practice was banned in the UK, Australia and the US.

Vicary later admitted that his study was fabricated, and scientists through the years who have tried to replicate it have largely failed. But now researchers have shown that if the conditions are right, subliminal advertising to promote a brand can be made to work.

Johan Karremans at the University of Nijmegen in the Netherlands and his colleagues wanted to see if they could subliminally induce volunteers to favour a particular brand of drink, Lipton Ice. For comparison, they chose a brand of mineral water called Spa Rood, as it was deemed to be as well known as Lipton Ice and equally thirst-quenching.

The researchers asked 61 volunteers to perform a nonsense task - counting how many times a string of capital Bs was infiltrated by a lower-case b as they flashed up on a screen. The B strings appeared for 300 milliseconds each, and before them, a string of Xs always appeared, flanking a 23-millisecond subliminal message. For the experimental group, the message was "Lipton Ice". Controls saw "Nipeic Tol".

When the volunteers had completed this task, they were asked to choose between Lipton Ice and Spa Rood by clicking one of two keys - though they were told this was part of a separate study. They were also asked how likely they would be to order either of these drinks if they were sitting on a terrace, and to rate how thirsty they were. Volunteers who rated themselves as thirsty were more likely to choose Lipton Ice, but only if they had received the subliminal message.

In a second study the researchers made half of their 105 volunteers thirsty by giving them a very salty candy before the task. As predicted, among the thirsty, subliminal messaging had an impact. Eighty per cent of thirsty volunteers who had been exposed to the Lipton Ice message chose that product, compared to only 20 per cent of the controls.

The thirstier volunteers rated themselves to be, the more likely they were to choose Lipton Ice. Those who were not thirsty were only slightly more likely to pick the iced tea. "Priming only works when the prime is goal-relevant," says Karremans. The researchers are now planning to study just how long-lasting these effects are.

Meanwhile, advertisers have found alternative means of pushing their products.

Earlier this month, the Archives of Pediatrics and Adolescent Medicine published a report showing that for each additional hour per day that a child watched television an average of one additional request was made for an advertised product. The effect of the commercials on children lasted up to 20 weeks.

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