Justification explicitness and persuasive effect: a meta-analytic review of the effects of varying support articulation in persuasive messages.

• Author: O'Keefe, Daniel J.

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. Meta-analytic reviews instead commonly focus on the size of the effect obtained in each study, with these then being combined to give an observed average effect (with an associated confidence interval). In this review, the effect of central interest is the persuasive outcome associated with variation in support explicitness.

Any careful empirical examination of this matter will quickly encounter a potential obstacle, namely, the lack of a well-worked-out principled conceptualization of alternative ways in which argumentative support might vary in explicitness. However, the interest of this article is in mining the extant research literature for what evidence it can provide. That is, this analysis is driven less by some master conceptualization of all the possible ways in which support might vary in explicitness than by what sorts of potentially-relevant message variations have been considered in the persuasion effects literature.

That literature contains studies of three distinctive message variations that represent variations in support explicitness. One is variation in information-source citation, that is, whether the advocate explicitly identifies the source(s) of information and opinion that are offered in the message. A second may be characterized as variation in the completeness of arguments, that is, whether the advocate explicitly spells out the underlying bases of message claims (provides explicit articulation of the premises, supporting information, and the like). A third is variation in quantitative specificity, that is, variation in the specificity of quantitative information given ("75%" versus "most," for instance). Plainly, advocates who spell out the premises of their supporting arguments, identify the sources of their information, and provide specific quantitative information offer more explicit argumentative support than do advocates who leave their supporting premises and information implicit, omit mention of their sources of information, or offer relatively non-specific quantitative information. That is, each of these message variations instantiates variation in argumentative explicitness. Hence the purpose of the present investigation may be more carefully formulated as that of reviewing extant research on the persuasive effects of these three specific variations in support explicitness.

A number of studies relevant to this question are ones commonly characterized as studies of the effects of "evidence" in persuasive messages (e.g., McCroskey, 1969; Reinard, 1988). The question of interest in these studies is what difference it makes to persuasive effectiveness if the advocate provides evidence supporting the message's claims. As Kellermann (1980) has pointed out, however, the concept of evidence invoked in this research is not carefully formulated; correspondingly, evidence research has seen a large number of different experimental realizations of evidence variations (see Kellermann, 1980, pp. 163-164). Kellermann has argued quite pointedly for the importance of more careful conceptualization of the relevant message properties.

Despite such observations, discussions of research on evidence commonly lump distinctive experimental manipulations under a generalized "evidence" heading, without consistently attending closely to the specific message manipulations employed in the research (e.g., McCroskey, 1969; Reinard, 1994; Reynolds & Burgton, 1983). For example, both Harte's (1972) manipulation and Anderson's (1958) manipulation have been labelled manipulations of evidence, although Harte's (1972) study varied both information-source citation and argument completeness, whereas Anderson's (1958) varied only information-source citation.

The present review thus has a somewhat sharper focus than those in discussions of evidence, by virtue of being based on the identification of three distinctive message variations examined in persuasion-effects research that reflect variations in the explicitness of supporting argumentation. This more careful specification of message properties has also made it possible to locate relevant research not commonly mentioned in discussions of evidence. Moreover, the present focus on specific message properties permits one to distinguish cases in which only one relevant property varies from cases that simultaneously vary more than one such property. As just noted, empirical investigations of the persuasive effects of the message variations of interest have sometimes manipulated several of these features simultaneously (e.g., Harte, 1972; McCroskey, 1966). Studies of such joint manipulations are of distinctive interest, precisely because they shed light on the question of the effects of combining support-explicitness variations, and hence are included in the present review.

METHOD

Identification of Relevant Investigations

Literature search. Relevant research reports were located through personal knowledge of the literature, examination of previous reviews and textbooks, and inspection of reference lists in previously-located reports. Additionally, searches were made through databases and document-retrieval services using such terms as "documentation," "evidence," and "support" in conjunction with "persuasion" and "persuasive" as search bases; these searches covered material through at least January 1998 in PsycINFO, ERIC (Educational Resources Information Center), Current Contents, ABI/Inform, and Dissertation Abstracts Online.

Inclusion criteria. Studies selected had to satisfy two criteria. First, the study had to compare two messages varying in the articulation of the message's support for its overall conclusion; specifically, included studies varied information-source citation, argument completeness, or quantitative specificity. Second, the investigation had to...