What every skeptic should know about subliminal persuasion

Classic research by cognitive and social psychologists suggests that subliminally presented stimuli can be perceived and can influence individuals' low-level cognitions. More recent investigations suggest that such stimuli can also affect individuals' high-level cognitive processes, including attitudes, preferences, judgments, and even their behavior.

The report of my death was an exaggeration.

- Mark Twain, in a note to the New York Journal, June 1, 1897

Readers of the SKEPTICAL INQUIRER are well acquainted with instances of mismatch between popular belief and scientific evidence. Despite an utter lack of scientific support, for example, many individuals place a great deal of belief in such topics as astrology (Carlson 1985; Dean 1987), facilitated communication (Dillon 1993; Mulick, Jacobson, and Kobe 1993), homeopathy (Barrett 1.987), alien abductions (Carlsburg 1995; Randles 1993; Turner 1994) and even Elvis sightings (Moody 1987). Issues such as these are "slam dunks" for skeptics: There can be little reconciling such beliefs with evidence that simply does not exist.

In other cases, though, where there is some scientific support on which to pin one's belief, there may still be more belief than is warranted. Graphologists, for example, who use samples of individuals' handwriting to determine enduring aspects of their personalities, consistently claim greater predictive validity than can be supported empirically (Nevo 1986; Scanlon and Mauro 1992). Some might argue the same for ESP, for which some evidence might actually exist (Bern and Honorton 1994; but see Hyman 1994). It is in domains such as these that the skeptic's role is more subtle, but just as important. One key aspect of this role is to determine what the available scientific evidence does and does not support. With this in mind, our purpose here is to explore the psychological research on subliminal persuasion, an area in which popular belief may again outstrip available evidence.

Subliminal persuasion refers to the use of subliminally presented stimuli, or messages presented to individuals beneath their level of conscious awareness, that are intended to influence their attitudes, choices, or actions. Not surprisingly, reports that unscrupulous marketers were using this technique to influence consumer behavior have historically prompted alarm (Cousins 1957; Key 1980). Yet, as many writers have suggested, such panic is probably unwarranted: There is simply no good evidence to support the conclusion that subliminal messages implanted in advertisements can exert an influence over whether one drinks Coke or Pepsi, endorses a particular viewpoint, or votes for candidate X over candidate Y (Moore 1988; Pratkanis and Greenwald 1988; Trappey 1996; Vokey and Read 1985).

Or is there? We will explore why the notion of subliminal persuasion might not be as far-fetched as some have supposed. Our point of departure, in particular, is an article appearing in the SKEPTICAL INQUIRER in 1992 by Anthony Pratkanis (see also Moore 1992). In his article, Pratkanis traced the historical roots of the belief in the powers of the unconscious, nicely debunked James Vicary's famous "Eat Popcorn/Drink Coke" hoax, and described the compelling results of some of his own research on the ineffectiveness of subliminal self-help audio tapes.

Still, for all its strengths, we believe the Pratkanis article, and others like it, may have left readers with an incomplete picture of the state of the art regarding subliminal presentation of stimuli. Accordingly, we endeavor to acquaint readers of the SKEPTICAL INQUIRER with the varied (and thriving) use of subliminally presented stimuli in cognitive and social psychological research. Specifically, we review evidence suggesting that cognition can occur without conscious awareness, and that this unconscious cognition can be affected by subliminal stimuli, thereby influencing individuals' judgments, attitudes, and even their behavior. Indeed, this recent evidence suggesting that subliminal stimuli can influence behavior gives us pause in contemplating the possible effectiveness of subliminal persuasion in advertising.

Clarifying Ambiguities

The exact meaning of "subliminal" has been a source of controversy and confusion for decades. A common definition, however, is that a stimulus is subliminal (that is, below threshold) if it cannot be verbally identified (e.g., Cheesman and Merikle 1986; Fowler 1986, Greenwald and Draine 1997). The threshold used in this definition is that of conscious awareness, sometimes called a subjective threshold (Cheesman and Merikle 1986). This definition, of course, allows for the possibility that an individual perceives that some material was presented, but requires that its exact: nature be unidentifiable. Nearly all of the studies we review use this definition, while the remaining adhere to a more conservative one: that individuals be unable to report even the presence of the stimulus.

Furthermore, there is a critical distinction to be made between subliminal perception and subliminal persuasion. Subliminal perception refers simply to the perception of stimuli that are below the threshold of conscious awareness.(1) Subliminal persuasion, on the other hand, requires that the subliminally presented stimulus have some effect, not simply on an individual's judgments, but on his or her attitudes or behavior. As others have noted, subliminal perception need not imply subliminal persuasion (e.g., Moore 1988).