Elaboration likelihood model
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The Elaboration Likelihood Model (ELM) of persuasion  is a dual process theory describing how attitudes are formed and changed, developed by Richard E. Petty and John Cacioppo during the early 1980s. The model examines how an individual's elaboration (thinking deeply) on a persuasive message will affect the message's persuasive success, and proposes two routes of processing--the central route (high elaboration), and peripheral route (low elaboration). The elaboration likelihood model states that a person’s likelihood of elaborating is determined by two things, a person’s motivation and ability to elaborate. ELM resembles the heuristic-systematic model of information processing developed about the same time by Shelly Chaiken.
The Elaboration-Likelihood Model proposes two routes for processing: The central route and the peripheral route. Either route may be used regardless of the message subject or content. Some message subjects or content may be more effective when the central route is used, and some may be more effective when the peripheral route is used. Hypothetically, it's also possible that message features may promote either central or peripheral route processing, if they affect motivation or ability.
The central route is used when a message recipient has greater motivation and/or ability to process the message. When motivation is high, the central route is more likely to lead to long-lasting attitude change, if the message is effective.  Motivation to process the message may be determined by personal interest in the subject of the message, or individual factors like need for cognition. However, if the message recipient already has a strong, negative attitude toward the position advocated by the message, then a boomerang effect (an opposite effect) is likely to occur. That is, they will resist the message, and their attitude may even become more negative. 
The peripheral route is used when the message recipient has little or no interest in the subject and/or has lesser ability to process the message. When the peripheral route is used, message recipients are more likely to rely on general impressions (e.g. "this feels right/good"), early parts of the message rather than later parts, whether there are positive or negative pictures involved, the recipient's mood, etc. If these peripheral influences go completely unnoticed, the message recipient is likely to maintain their previous attitude on the subject of the message. Otherwise, the individual will temporarily change their attitude toward the subject of the message. This attitude change can be long-lasting, although this is less likely to occur as compared to the central route.
Common Misinterpretation of Route Effects
Researchers have sometimes misinterpreted the ELM, thinking that the processing--central or peripheral--routes determine only the type of information that affects message persuasion. For example, that having stronger argument information is only influential when the central route is used, but that information like source attractiveness is only influential when the peripheral route is used. This is incorrect.
In fact, the ELM does not make statements about types of information being related to routes. Rather, the key to the ELM is how any type of information will be used depending on central or peripheral routes, regardless of what that information is. For example, the central route may permit source attractiveness to influence preference for certain language used in the message (e.g. "beautiful") or validate a related product (e.g. cosmetics), while the peripheral route may only lead individuals to associate the "goodness" of source attractiveness with the message. Theoretically, all of these could occur simultaneously. Thus the distinction between central and peripheral routes is not the type of information being processed, but how that information is processed, and ultimately whether processing information in one way or the other will result in different attitudes.
Choice of route
The factors that influence the route an individual will take in a persuasive situation the most are motivation (a desire to process the message; see Petty and Cacioppo, 1979) and ability (the capability for critical evaluation; see Petty, Wells and Brock, 1976). The route taken is determined by the extent of elaboration, and in turn by attitude, motivation, and ability factors.
An attitude is a general evaluation, indicating how a person perceives themselves in relation to their surroundings. Attitudes may be influenced by peripheral cues providing guidance or implications, which cause the audience to draw a conclusion and believe it is their own idea. Motivation includes the relevance of the message and a person's "need for cognition" (their enjoyment of thinking through ideas). Motivation is personal to each person and can vary depending on the topic e.g. Students under 21 who drive would be more motivated to get involved and learn about laws that affect them, more than students who are over 21 because the law will not apply to them, so they are not motivated to think about the issue. Ability includes the availability of cognitive resources (e.g., the presence or absence of time pressures or distractions) and the relevant knowledge needed to examine the arguments. Distractions (for example, a persuader trying to convey a message in a room full of crying babies) can affect the ability to process a message. Many evaluations are based on cognitive intelligence, behavior and guidance. With an understanding of an individual's attitudes, the elaboration may be tailored to the situation. There are two types of elaboration: biased and objective. Elaboration may have positive or negative results, depending on the audience. Individuals with preconceptions about a topic are more difficult to persuade than those who examine the facts.
The advantages of peripheral route thinking is that you can save time, energy, and mental capacity. However this could mean that in the process some of the information may be overlooked and that may lead to detrimental consequences and mistakes, mistakes that could have been prevented. The advantages of central route is that the thinking is opposite, detrimental mistakes can be avoided because an individual looks at all the information, processes it and then makes their decision. However this does use up a lot of time, energy and mental capacity.
In designing a test for the aforementioned model, it is necessary to determine if an argument is viewed as strong or weak. If the argument is not seen as strong, the results of persuasion will be inconsistent. A strong argument is defined by Petty and Cacioppo as "one containing arguments such that when subjects are instructed to think about the message, the thoughts they generate are fundamentally favorable". An argument that is universally viewed as weak will elicit unfavorable results especially if the subject considers it logically (the central route); a strong argument, under similar circumstances, will return favorable results. Test arguments must be rated by ease of understanding, complexity and familiarity. To study either route of the elaboration likelihood model, the arguments must be designed for consistent results.
Predictions and features
The ELM makes several proposals. Attitudes formed under high elaboration (the central route) are stronger than those formed under low elaboration, making this level of persuasion stable and less susceptible to counter-persuasion. Attitudes formed under low elaboration (the peripheral route) are more likely to cause short-term attitude change.
Variables in ELM routes can serve multiple roles in a persuasive setting, depending on other contextual factors. Under high elaboration, a given variable (e.g. expertise) can serve as an argument ("If Einstein agrees with the theory of relativity, then this is a strong reason for me to as well") or a biasing factor ("if an expert agrees with this position it is probably good, so let me see who else agrees with this conclusion", at the expense of contradicting information.
Under low-elaboration conditions, a variable may act as a peripheral cue (for example, the belief that "experts are always right"). While this is similar to the Einstein example above, this is a shortcut which (unlike the Einstein example) does not require thought. Under moderate elaboration, a variable may direct the extent of information processing: "If an expert agrees with this position, I should really listen to what (s)he has to say".
A variable's effect on elaboration may increase (or decrease) persuasion, depending on the strength of the argument. If the argument is strong, enhancing elaboration will enhance persuasion; but if it is weak, thought will undermine persuasion.
Recent adaptations of the ELM  have added an additional role for variables: to affect the extent to which a person trusts their thoughts in response to a message (self-validation role). A person may feel "if an expert presented this information, it is probably correct, and thus I can trust that my reactions to it are informative with respect to my attitude". This role, because of its metacognitive nature, only occurs in high-elaboration conditions.
- Attitude change
- Need for cognition
- Cognitive biases
- Heuristic-Systematic Model
- Extended transportation-imagery model
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