Numerous studies reported that people with a high socioeconomic status (SES) tend to be healthier than those with a low SES (House & Williams, 2000). It is argued that low-SES people have relatively poor health partly because of their lack of health knowledge. Even though health knowledge does not always translate into healthy lifestyles and disease-screening behaviors, it is no doubt a necessary condition for diverse healthy practices (Viswanath et al., 2006). To increase low-SES people's health knowledge, various agents made public-health efforts such as disseminating health information through mass-media channels (Rimal, Flora, & Schooler, 1999).
However, the knowledge-gap hypothesis posits that media information does not equally benefit population subgroups and that this inequality may restrict the effectiveness of public health campaigns. Even though mass media constantly infuse health information into society, individuals with low SES have been found to acquire knowledge from the media more slowly than those with high SES (Gaziano, 1997; Tichenor, Donohue, & Olien, 1970). Therefore, as health information enters a society through the media, the preexisting health knowledge gap between the society's haves and have-nots widens.
Recently, the Internet has become a crucial health-information source for the general public (Cline & Haynes, 2001; Lee, 2008). However, few studies have tested whether the readily accessible, increasing amount of online information enlarges the preexisting knowledge gap among different SES segments of the population. Because inequalities in information and communication technologies (ICT) use merit our attention insofar as they actually bring about differences in terms of one's life chances, health, and social participation (DiMaggio, Hargittai, Celeste, & Shafer, 2004; Selwyn, 2004a), the lack of empirical research on the effects of Internet use is somewhat surprising. Moreover, several researchers have expressed concern that, because of the heterogeneous information supply and the unequal access and usage that characterize the Internet, knowledge-acquisition inequalities between SES groups might be more evident on the Internet than in other mass media (Bonfadelli, 2002).
Therefore, the current research examines whether people with high education levels gain more general health knowledge from the Internet than people with low education levels, even if the frequency of the two groups' Internet use is the same. (1) Because older adults are more vulnerable to diverse health threats than are younger adults (Adams, Stubbs, & Woods, 2005), the current research focuses on adults between the ages of 40 and 70. It is even more important that, as a response to Gaziano's (1997) call for "more focus on processes contributing to gaps" (p. 253), the current research tries to reveal the mechanism of the knowledge gap in the case of the Internet by taking into account the relationship that individuals have developed with the Internet (Bucy & Newhagen, 2004; Jung, Qiu, & Kim, 2001; Selwyn, 2004a). To this end, the current research develops the construct of Internet engagement and examines whether the extent to which one engages with the Internet moderates the association between Internet use for health information and health knowledge. The Internet engagement is fully elaborated below; however, it can briefly be described as a composite of psychological comfort, heterogeneity and frequency of use, length of use, and other elements.
Health-Knowledge Gap on the Internet
Before proposing the "health-knowledge gap" hypothesis with regard to the Internet, it should be noted that there are three ways to empirically test the knowledge-gap phenomena. First, as Tichenor et al. (1970) suggested, one can conduct a longitudinal analysis of changes in the relationship between SES and knowledge over time. Second, Tichenor et al. also provided an alternative approach using a cross-sectional dataset. Without a longitudinal dataset, more than two issues--each having a different level of media publicity--could be employed and be compared in terms of the relationship between education and knowledge across the issues.
Third, a group of scholars proposed another method to deal with cross-sectional data and a single issue (e.g., Eveland & Scheufele, 2000; Kwak, 1999). They examined whether the association between education and knowledge is different depending on the levels of media use. That is, they measured individuals' media use as an "individual-level counterpart to varying media publicity" (Gaziano, 1997, p. 242). As Eveland and Scheufele stated, this approach has some advantages in that it "provides a more precise estimate of actual exposure to information than does the more macro measure of media publicity" (p. 219). Moreover, this approach is very useful in examining whether knowledge gaps occur online, insofar as information on the Internet does not automatically lead to individuals' exposure to that information because "the Internet requires more active engagement of users than television or newspaper, and incidental exposure to information is less likely to occur online" (Shim, 2008, p. 452).
Based on these considerations, the current research adopts the third individual-level method. Thus, the following hypothesis is derived:
[H.sub.1]: There will be interactive effects of Internet use for health information and education on general health knowledge, such that the association between Internet use and health knowledge is stronger for people with high levels of education than for people with low levels of education.
Mechanisms of the Health-Knowledge Gap
Beyond demonstrating the existence of knowledge gaps, several scholars (e.g., Grabe, Lang, Zhou, & Bolls, 2000) have wrestled with the underlying reasons for the knowledge gap ever since Tichenor et al. (1970) provided several preliminary explanations. These scholars found several factors that mediate the effects of education on knowledge acquisition. The factors can be categorized into the following five groups.
First, communication skills and information-processing abilities differ among SES groups. Compared to low-SES people, high-SES people are likely to have better reading skills and comprehension abilities, which are necessary to understand media content. Also, previous studies showed that people with high levels of communication skills are more likely than people with low levels of communication skills to process media information in a more elaborate way, which facilitates knowledge acquisition from media (e.g., Eveland, 2002; Grabe et al., 2000).
A differing amount of prior knowledge regarding the issue at hand is a second reason for the knowledge gap. It has been found that people with high levels of education tend to have prior knowledge of media-presented topics, which helps them process and comprehend the relevant media information (e.g., Hsu & Price, 1993).
Third, people with high SES tend to know more people who are knowledgeable about media-covered topics than do people with low SES; thus, people in the former category are able to discuss and make sense of media information more quickly than people in the latter category. This finding has been supported by the "differential gains" studies in political communication (e.g., Scheufele, 2002), which showed that interpersonal political discussion amplifies the effects of public-affairs media use on political knowledge.
Fourth, the selective exposure/attention explanation assumes that low-SES people are more likely to use media for entertainment than are high-SES people, who are more likely to consume media for information gains than are low-SES people. Because people in low-SES groups use media for information acquisition less frequently than do their high-SES counterparts, the members of the former group are less likely to acquire knowledge from their media use than the members of the latter group (e.g., McLeod & Perse, 1994).
Fifth, a group of scholars focused on individuals' motivation, such as issue interest and issue involvement, as a causal link between SES and knowledge acquisition from mass media (e.g., Bonfadelli, 2002; Ettema, Brown, & Luepker, 1983). As Ettema and colleagues stated, "the sort of information disseminated by the mass media ... is not so complicated that it requires highly sophisticated information processing skills. This information is, however, probably of more interest and use to higher SES individuals" (1983, p. 517). (2) These scholars argue that SES relates not so much to communication skills and to information-processing abilities as to differential interest in media information, a difference that eventually causes gaps in knowledge acquisition across SES groups.
To examine the process of the health-knowledge gap with regard to the Internet, the current research refines the first explanation (i.e., communication skills and information-processing abilities). Notably, there are several ways to extend the previous discussions on this topic. On the one hand, one can elaborate on the types of skills required for effective Internet use (e.g., van Dijk & Hacker, 2003), examine their roles in Internet use (e.g., Eveland, Marton, & Seo, 2004), and explore their antecedents and consequences (e.g., Hargittai, 2004a). Conversely, one can focus on a larger social environment where Internet-use skills are put into use, and thus can elaborate on the structural and psychological antecedents of these skills. This approach "expands the technology-individual relationship into the context of a broader social structure" (Jung et al., 2001, p. 513), which "allows more thorough appreciation of different relationships people may have with media" (Loges & Jung, 2001, p. 539). The current research adopts the latter approach, assuming that Internet-use skills or online skills could be regarded as one sub-dimension or as one outcome of Internet engagement. Therefore, rather than...