persuaded by the former. These results emphasize the importance of knowledge (and,
therefore, high quality information) in allowing people to evaluate messages about their
health.
When people have time to process messages or make time because they see the
message as personally relevant, they are more likely to engage in systematic processing
so that the content of the message is more important than other characteristics. Figure
8.3 shows how perceiving the message source to be an expert has different effects
depending on whether recipients regard a message as high or low in personal relevance.
The low relevance participants are strongly affected by perceived expertise and are more
persuaded by an expert (rather than inexpert) source whether strong or weak arguments
are used. However, for those who see the message as personally relevant (and so engage
in systematic processing) only the quality of argument determines persuasion. Strong
arguments are persuasive for this group, regardless of source expertise, and even an
expert cannot persuade this group with weak arguments. This does not mean that
source expertise is unimportant but rather that for those with the ability and motivation
to engage in systematic processing poor quality arguments cannot be compensated for
by the impression of expertise. In terms of the ELM this means that persuasion does
occur through boththe central and peripheral routes simultaneously but that one or
other route will be dominant depending on factors such as message relevance.
Individual characteristics also affect whether people are likely to engage mainly in
systematic processing (Briñol and Petty, 2005). For example, some people have a high
‘need for cognition’ (i.e. like thinking about the content of persuasive messages) and
so, in general, are motivated to make time for central route processing (Cacioppo, Petty
and Morris, 1983).
Overall then, research suggests that if you do not have strong arguments then you
are better discouraging systematic processing and relying instead on numerous
arguments, consensus and perceived expertise. Perhaps fortunately, attitude changes
resulting from peripheral route processing are less likely to be stable (that is, long-lived)
and more likely to further change due to counter-persuasion, whereas attitude change
CHANGING MOTIVATION 179Extent of attitude change14
12
10
8
6
4
2
0
Low expertise High expertise16
14
12
10
8
6
4
2
0
Low expertise High expertiseWeak StrongFIGURE 8.3The impact of expertise on persuasion by strong and weak arguments under
conditions of low (left) and high (right) personal relevance.
Source: Reprinted from Petty and Cacioppo (1986), with permission from Elsevier.