Justification Explicitness and Persuasive Effect: A Meta-Analytic Review of the Effects of Varying Support Articulation in Persuasive Messages.

Argumentative explicitness is one commonly-recognized normative good in the conduct of advocates. That is, it is normatively desirable that arguers articulate their viewpoints fully and specifically: "Evasion, concealment, and artful dodging... are and should be excluded from an ideal model of critical discussion" (van Eemeren, Grootendorst, Jackson, & Jacobs, 1993, p. 173). Explicit argumentation is normatively desirable because explicitness opens the advocated view for critical scrutiny. However, an advocate might fear that explicit argumentation would not be instrumentally successful, that is, persuasive. If such fears are justified, then arguers face an unhappy choice between instrumentally-successful and normatively-desirable conduct. The question thus is whether such concerns are in fact warranted, that is, whether argumentative explicitness necessarily damages persuasive success. One facet of this question has been addressed by O'Keefe (1997), who reviewed research concerning the persuasive effects of variations in the explicitness of a message's conclusion (the degree of articulation of the message's overall standpoint or recommendation). His review suggested that better-articulated message conclusions are dependably more persuasive than less-articulated ones. This article concerns the persuasive effects of another aspect of argumentative explicitness, namely, variation in the explicitness of an message's supporting argumentation. Making such supporting argumentation more explicit could plausibly be supposed either to impair or enhance persuasive success. The critical scrutiny such explicitness enables the very property that makes explicitness normatively desirable-might reduce a message's persuasiveness. Expressed most broadly, explicitness enlarges the apparent "disagreement space," in the sense that it makes more obvious just what claims are being advanced (on the idea of disagreement space, see van Eemeren, Grootendorst, Jackson, & Jacobs, 1993, pp. 95-96; Jackson & Jacobs, 1980). Each further specification of an advocate's standpoint invites scrutiny and objection, and thus courts rejection. An advocate might also think that less-explicit argumentation would enjoy greater persuasive success because it actively engages the audience (in an enthymematic fashion). When the advocate does not provide fully-articulated support, message receivers must mentally supply missing argumentative elements; such active participation might lead the audience to be more persuaded than if the advocate had explicitly supplied those elements. On the other hand, support explicitness could enhance persuasive success. Explicitly laying out strong supporting material might make the message more convincing than it would have been otherwise. As another possibility, advocates whose viewpoints are more fully articulated might be perceived as more credible (more trustworthy and more competent), since receivers could reason that an advocate willing to be so explicit about the supporting materials must be especially honest and well-informed; such enhanced credibility then might make for greater persuasive effectiveness. But the question of the relationship between support explicitness and persuasive effectiveness is an empirical one. As will be seen, a number of studies have (implicitly or explicitly) addressed this question, though many of these have never been systematically collected or reviewed. The purpose of the present study is to provide a meta-analytic review of this research, and thus to consider what light is shed by existing research on the general question of the persuasive consequences of variation in explicitness of support. Meta-analytic literature reviews aim at providing systematic quantitative summaries of research studies (Rosenthal, 1991, provides a useful general discussion of meta-analysis). Traditional narrative literature reviews emphasize statistical significance (whether a given study finds a statistically significant effect), but this can be a misleading way of characterizing research findings; whether statistical significance is achieved is a matter of, inter alia, sample size. …