show program for a message about domestic violence, but the nature of the talk show, which may be sympathetic to the message but whose primary purpose is attracting audiences, will affect the final shape of the message. A health communication campaign that depends on “free” exposure is forced to use channels over which it has less control.

How is each channel perceived by the audience? A continuing issue in the literature compares mass media versus interpersonal channels with regard to their role at various stages in the diffusion of a new idea (Rogers, 1995). People often rely more heavily on mass media channels when they are first learning about an idea, but use interpersonal channels as they move toward making a decision (Rogers, 1995). Depending on the nature of the health behavior change and its context, either mass media or interpersonal communication channels can play an important role at either stage (Chaffee, 1982; Hornik, 1989; Schooler et al., 1998; Hornik, 1997). Audience individuals may not view messages from various channels as equally credible. Some communication channels and sources may be perceived by certain individuals as credible when they address health behavior change, while other channels (such as salespeople or Web sites on the Internet) may engender skepticism. These perceptions of communication channels may vary at different stages in the diffusion process.

When multiple channels are present, how do they interact in their effects? There are three possibilities. Channels may be additive—an exposure achieved through one channel is the same as exposure through any other channel. Alternatively, channels may positively interact or reinforce one another, such that the effects of exposure through any one channel increase when other channels are present. For example, the effect of an antismoking television message may be greater when a complementary school-based antismoking program reaches some audience individuals. Finally, channels may interact negatively, if additional exposures through one channel add nothing to the already available message exposures provided by another channel. Here, channels may substitute for one another. In addition, messages coming from different chan-

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