Health Psychology, 2nd Edition

178 MOTIVATION AND BEHAVIOUR

example, people use simple rules or decision-making heuristics to evaluate messages.
These include ‘expertise = accuracy’, that is, she is an expert so what she says must
be right, or ‘consensus = correctness’, that is, if so many people agree they must be
right and ‘length = strength’, that is there are lots of arguments so it must be true.
Sometimes situational constraints force people into peripheral route processing. For
example, the message may be presented quickly amid distractions as is the case in many
television advertisements. In addition, individual differences mean that some people
are more or less likely than others to engage in systematic processing. For example,
Chaiken (1980) identified people who agreed or disagreed with the length = strength
heuristic (using agreement with questionnaire items such as ‘the more reasons a person
has for some point of view the more likely he/she is correct’). These people were
then presented with a message containing 6 arguments in favour of cross-course, end-
of-year examinations for students. However, the message was described to participants
as either containing 10 or 2 arguments (although it always contained the same 6
arguments). The results showed that those who endorsed the length = strength
heuristic were more likely to be persuaded when the message was described as having
10 arguments than were those who did not endorse the heuristic.
Central route (systematic) processing is unlikely among message recipients who do
not understand a message. Figure 8.2 shows results from a study by Wood, Kallgren
and Mueller Preisler (1985). Among message recipients with poor knowledge, attitude
change was almost as likely whether a message contained weak or strong arguments
because the ability to engage in systematic, central route processing was compromised
by lack of knowledge. These recipients relied on peripheral processing and so failed
to differentiate between strong and weak arguments. By contrast, those with good
knowledge clearly differentiated between strong and weak arguments and were only

Extent of attitude change

5

4

3

2

Poor knowledge

Strong

Weak

Good knowledge

FIGURE 8.2Prior knowledge and the effect of argument strength (Wood et al., 1985).

Source: Reprinted from Wood et al.(1985), with permission from Elsevier.