Answer to Question 1
Answer: It depends on how much fear is aroused. If fear is strong enough to motivate people to attend (e.g., films about lung cancer shown to smokers), people will attend and process message contents centrally, so strong arguments will work well. If, however, too much fear is induced, people will grow defensive and will not attend or process the arguments, so few if any messages of any kind will work. It also depends on whether the message contains useful information. Fear-arousing messages can work well when they are accompanied by information that explains to the audience what to do to avoid a fear-arousing outcome (e.g., providing information on how to quit smoking).
Answer to Question 2
Answer: Evidence from strictly controlled laboratory experiments shows that attitudes can indeed be subject to some subliminal influence. For example, researchers found that a subliminal presentation of the name Lipton Ice affected participants' choice of a beverage, if they were already thirsty. Despite the evidence of subliminal effects, we shouldn't be worried about advertisers being able to use these in the real world for two reasons: (1) the effects occur only under highly controlled conditions which are unlikely to be replicated in the real world, and (2) the effects shown are only subtle influences on neutral stimuli; there is no evidence that subliminal messages ever override people's conscious wishes and desires.
A Biologist at Work
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Economic and Social Development
Social Learning in Lobsters