Although public health bodies frequently post signs and advertisements warning people not to binge drink, smoke or take drugs, our unconscious mind often fails to notice the negative part of the message.
It means that ubiquitous "no smoking" or "don't drink and drive" signs which we notice in pubs and restaurants without paying full attention may in fact encourage rather than deter us.
Researchers from Oxford University and American colleagues found that smokers showed more interest in cigarettes after being shown a series of images which contained "no smoking" signs in the background.
The results suggest that campaigns against smoking and other unhealthy behaviour could ironically "trigger the very behaviours they are meant to discourage".
Businesses in England are obliged to place "no smoking" signs in smoke-free premises and vehicles as part of smoke-free legislation introduced in 2007.
Previous studies have shown that cues in the environment can "prime" the brain for particular behaviour, with one paper showing that students walked more slowly and performed worse in memory tests after hearing words related to the elderly.
But in a paper to be published in the Journal of Applied Social Psychology, Brian Earp and colleagues explained that we tend to disregard negations, such as the word "don't" in the phrase "don't drink and drive", when we are not paying full attention.
They wrote: "For unconscious or automatic processing, "not good" is often taken as "good", and "not bad" as "bad". Negation of concepts does not occur automatically but requires cognitive effort."
"Our model suggests that public health campaigns which explicitly discourage the consumption of unhealthy substances could in fact trigger that very consumption."
The researchers tested their theory by asking a group of 32 volunteers, all of whom were smokers, to look at a series of photographs, some of which featured "no smoking" signs in the background.
Their motivation to smoke was then estimated by a test in which the participants were shown several images of objects including cigarettes, and told to "approach" or "avoid" them as quickly as possible by pushing or pulling a lever.
Their findings showed that volunteers who had seen photographs containing "no smoking" signs were quicker to approach smoking-related images than neutral ones.
In contrast, a control group who initially saw a neutral set of photographs without "no smoking" signs showed no preference for smoking-related images.
There was also no difference between participants who spotted the "no smoking" signs in the photographs and those who saw the same pictures but did not consciously notice the signs.
"This finding indicates that the ironic effect of incidental exposure to no-smoking signs can occur regardless of whether the smoker consciously notices the signs, and that conscious awareness may not be sufficient to override the automatic motivation to attain the negated item," the researchers wrote.
Brian Earp, who led the study, said "no smoking" signs have an important legal role, but added that the same effects may apply to public health campaigns which urge people to "just say no".
He said: "You might not notice you are exposed to the stimuluis, or you might be aware of the stimulus but then forget about it and not be aware of how it is affecting you, and then its influence on you is unconscious."
The Department for Health spends about £50 million a year on public health advertising including campaigns on smoking, healthy living, alcohol and cancer awareness.
Martin Dockrell, director of research and policy for the Action on Smoking and Health (ASH) charity, said: “Many things can trigger a smoker’s desire to smoke – seeing somebody else smoking, spotting the logo of their favourite brand, having a cup of tea and most of all, nicotine withdrawal.
"Whatever else they do, “no smoking” signs tell people when they are not permitted to smoke”.