Petty and Cacioppo [1986a, 1986b] state that there are two �routes� to persuasion: central and peripheral. The central route to persuasion consists of thoughtful consideration of the arguments [ideas, content] of the message. When a receiver is doing central processing, he or she is being an active participant in the process of persuasion. Central processing has two prerequisites: It can only occur when the receiver has both the motivation and the ability to think about the message and its topic. If the listener doesn�t care about the topic of the persuasive message, he or she will almost certainly lack the motivation to do central processing. On the other hand, if the listener is distracted or has trouble understanding the message, he or she will lack the ability to do central processing.
The peripheral route to persuasion occurs when the listener decides whether to agree with the message based on other cues besides the strength of the arguments or ideas in the message. For example, a listener may decide to agree with a message because the source appears to be an expert, or is attractive. The peripheral route also occurs when a listener is persuaded because he or she notices that a message has many arguments -- but lacks the ability or motivation to think about them individually. In other words, peripheral cues, like source expertise [credibility] or many arguments in one message, are a short-cut. I don�t want to or can�t think carefully about the ideas in this persuasive message, but it is a fair gamble to go ahead agree with the message if the source appears to be knowledgeable or if there are many arguments in support of the message. This route occurs when the auditor is unable or unwilling to engage in much thought on the message. Receivers engaged in peripheral processing are more passive than those doing central processing.
Why does it matter which �route� an audience member takes when hearing or watching or reading a persuasive message? A key prediction of the ELM is that attitudes which are changed through the central route to persuasion will have different effects from attitudes changed via the peripheral route. Petty and Cacioppo explain that �Attitude changes that result mostly from processing issue-relevant arguments [central route] will show greater temporal persistence, greater prediction of behavior, and greater resistance to counter persuasion than attitude changes that result mostly from peripheral cues� [Petty & Cacioppo, 1986a, p. 21]. It should be obvious that these are important outcomes: Surely in most cases, persuaders would very much want to know how to make attitude change last longer, have a greater influence on behavior, and be more resistant to change. However, even though central processing has advantages, receivers do not always oblige us by having the motivation and ability to think about the message. We need to understand both of these processes of persuasion because both of them occur in receivers.