(Working on a book on persuasion — and how to avoid being manipulated — so I’ll be posting research notes here and there):

Quick summary: The more you know on a subject, the harder you are to persuade. Even a personal with “low bias” (you don’t care much) can become more resistant to persuasion if they are given time and encouraged to consider the matter seriously before they hear the argument.

Also it explains the nature of “peripheral” and “central route” persuasion:

Greg J. Neiymeyer et al., “Changing Personal Beliefs: Effects of Forewarning, Argument Quality, Prior Basis and Personal Exploration,” Journal of Social and Clinical Psychology 10, no. 1 (1991): 1-20.

The basic tenant of ELM [Elaboration Likelihood Model] is that attitudes sometimes changes as a function of thoughtful consideration of relevant arguments and sometimes as a result of inferences based on the association of a position with various positive or negative cues (e.g., counselor credibility). According to the model, if the individual is motivated and able to do so, he or she will actively process and elaborate the persuasive appeal, relating to preexisting cognitive schemas and generating issue-relevant beliefs in support of, or opposed to, the advocated position. According to this orientation, persuasion should occur if the individual’s self-generated “cognitive responses” (i.e., thoughts) are predominately favorable, whereas resistance to persuasion should occur if they are primarily unfavorable (Ciadini, Petty & Cacioppo, 1981). In either event, the persuasion is said to have occurred thorugh the central route since it is through careful and diligent processing of the argument’s perceived merits that the individual forms his or her attitude.

In contrast, when individuals are either unmotiviated or are unable to scrutinize the message, their likelihood of actively processing and elaborating it is correspondingly reduced. In this case they may minimize cognitive effort by appealing instead to environmental cues associated with the message (e.g, the source credibility) to determine their attitudes. This peripheral route to persuasion should be more heaviliy utilized when the individual is distracted from listening to the message or relatively uninterested in its content. The fact that attitude change resulting from peripheral processing is less enduring and less predictive of actual behavioral change than what occurs through central route processing …..

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The more you know about a subject, the harder you are to persuade:

On the other hand, those individuals with relatively few available thoughts in support of their position are mores susceptible to persuasion.

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Researchers have long recognized that “one of the most important variables affecting information processing activity is the extent to which a person has an organized structure of knowledge (schema) concerning an issue” (Petty & Cacioppo, 1986, p. 165). In the case of high bias, individuals have well-articulated networks of preexisting proattitudinal attitudes, and it is precisely under these conditions that we would expect robust counterarguing and effective resistance to persuasion (cf., Ross, Lepper & Hubbard, 1975; Taylor & Fiske, 1984). This finding therefore supports the general conclusion that, “the more issue-relevant knowledge people have, the more they tend to be able to counterargue communications opposing their initial positions and to cognitively bolster (proargue) congruent messages” (Petty & Cacioppo, 1986, p. 165).

P. 15

They discovered that a low-bias person (someone with little concern) became more difficult to persuade when they forewarned about the argument that would be made & were given some time to consider the argument for themselves: which the researchers called “self-exploration”. The researchers were studying what would made someone coming into counseling more willing to change. A step in counseling could be ask the counselee to articulate their existing understanding and reasoning. If change is sought, this might make things more difficult, “The effects of reviewing and articulating self-relevant attitudes may be to render them more resistant to subsequent change.” P. 19