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Subliminal Perception: Facts and Fallacies


Timothy E. Moore

Skeptical Inquirer Volume 16.3, Spring 1992

Can the meaning of a stimulus affect the behavior of observers in some way in the absence of their awareness of the stimulus? In a word, yes. While there is some controversy, there is also respectable scientific evidence that observers’ responses can be shown to be affected by stimuli they claim not to have seen. To a cognitive psychologist this is not particularly earthshaking, but the media and the public have often responded to the notion of subliminal perception with trepidation.

What is subliminal perception? Should we be worried (or perhaps enthused) about covert manipulation of thoughts, attitudes, and behaviors? My reviews (Moore 1982;1988) have dealt primarily with the validity of the more dramatic claims made on behalf of subliminal techniques and devices. Such an appraisal requires a working definition of “subliminal perception.” Then we need to determine whether the conditions under which it occurs and the means by which it is achieved are reflected in the products on the market.

How should “awareness” be defined? One way is simply to ask observers whether or not they are “aware” of a stimulus. If the observer denies any awareness, then the stimulus, is, by definition, below an awareness threshold. Using this approach, unconscious perception consists of demonstrating that observers can be affected by stimuli whose presence they do not report. Another way to define "awareness” involves requiring observers to distinguish between two or more stimuli that are presented successively. With fast exposure durations, observers may be unable to distinguish between stimuli, or between a stimulus’s presence or absence. This method was advocated by Eriksen (1960) and defines consciousness as the observer’s ability to discriminate between two or more alternative stimuli in a forced-choice task. In this context, unconscious perception consists of a demonstration that observers are affected by stimuli whose presence they cannot detect. The approaches are different and involve different sorts of evidence. In the former case the stimuli are not reported; in the latter instance the stimuli cannot be detected.

These two methods of defining consciousness have been referred to as “subjective” and “objective,” respectively, by Merikle and his coworkers (Cheesman and Merikle 1986; Merikle and Cheesman 1986). Higher levels of visibility are typically associated with subjective thresholds. The disadvantage of a subjective definition is that a failure to report a stimulus’s presence may result from response bias (i.e., the observer is ambivalent about the stimulus’s presence and elects to report its absence). As Merikle (1984) has argued, the use of subjective thresholds implies that each participant provides his or her own idiosyncratic definition of “awareness.” Consequently, awareness thresholds could (and would) vary greatly from subject to subject.

Some recent studies (e.g., Cheesman and Merikle 1986) have looked at performance when both subjective and objective thresholds have been assessed. Such studies indicate that subliminal perception is most appropriately viewed as perception in the absence of concurrent phenomenal experience. We sometimes receive information when subjectively we feel that nothing useful has been"seen.” Investigators can establish that perception has occurred in the face of disavowals from participants by forcing them to guess. Respondents may object that they have no basis for making a decision, but by using a forced-choice task we can see that their guesses are more accurate than they would be if they were guessing at random. Clearly, some information is being utilized.

When respondents’ guesses are at chance in a detection task, there is no well-established evidence for perception. Thus, subliminal perception is not perception in the absence of a detectable signal. Rather, it occurs under conditions where subjects can detect a signal on at least some proportion of trials. Subjects may claim to be guessing without realizing that their guesses are better than chance. According to Merikle, the dissociation between these two indicators of perception (signal detection vs. introspective reports) defines the necessary empirical conditions for demonstrating subliminal perception. There is an inconsistency between what observers know and what “they know they know.”

Recent reviews of research findings in subliminal perception have provided very little evidence that stimuli below observers’ subjective thresholds influence motives, attitudes, beliefs, or choices (Moore 1988; 1991b; Pratkanis and Greenwald 1988; Greenwald, in press). In most studies, the stimuli do not consist of directives, commands, or imperatives, and there is no reliable evidence that subliminal stimuli have any pragmatic impact or effects on intentions. Studies that do purport to find such effects are either unreplicated or methodologically flawed in one or more ways. There is very little evidence for any perceptual processing at all (let alone any pragmatic consequences) when perceptual awareness is equated with an objective threshold.

How do the dramatic claims regarding undetectable stimuli stack up against the preceding review? What are these claims and what is their status? I shall confine my comment to claims involving advertising applications and self-help auditory tapes.


Many people believe that most advertisements contain hidden sexual images or words that affect our susceptibility to the ads. This belief is widespread even though there is no evidence for such practices, let alone evidence for such effects. “Embedded” stimuli are difficult to characterize in terms of signal-detection theory Or threshold-determination procedure because most of them remain unidentifiable even when focal attention is directed to them. Nevertheless, the use of the term subliminal is a fait accompli, and belief in such an influence is primarily the consequence of the writings and lectures of just one person-Wilson Bryan Key (1973, 1976; 1980; 1990). Key offers no scientific evidence to support the existence of subliminal images; nor does he provide any empirical documentation of their imputed effects (Creed 1987: see also Vokey and Read).

In a review of Key’s most recent book, John O'Toole, president of the American Association of Advertising Agencies wondered: “Why is there a market for yet another re-run of this troubled man’s paranoid nightmares?” (O'Toole 1989: 26). Part of the reason that Key’s books sell so well may be that they are not what they appear to be. The information is not presented as the subjective fantasies of one person. Instead, it is presented as scientific, empirical fact. Science is respectable. Consequently, if claims are cloaked in scientific jargon, and if propositions are asserted to be scientifically valid, people can be fooled. Key knows this and uses it to his advantage. His intent is to persuade; and if he can do so by misrepresenting scientific data and findings, he is apparently prepared to do so.

Key provided pretrial testimony at the Judas Priest trial in Reno, Nevada, in the summer of 1990. Two teenagers had committed suicide. Their parents sued Judas Priest and CBS Records Inc., alleging that subliminal messages in Judas Priest’s music contributed to the suicides. Key was testifying on behalf of the plaintiffs, and at the trial he responded to a question about scientific methodology by saying: “Science is pretty much what you can get away [with] at any particular point in history and you can get away with a great deal” (Vance/Roberson v. CBS/ Judas Priest, 1990: 60). This unabashed disdain for anything approaching scientific integrity has not endeared him to the scientific community.

Attempting to apply scientific criteria to propositions for which there is no pretense at scientific foundation is a relatively futile exercise. Key’s only interest in science seems to be in the persuasive power of adopting a scientific posture or style. The use of scientific jargon does not necessarily reflect scientific attitudes or methods. Under these circumstances, even to apply the term pseudoscience seems unwarranted.

Extravagant claims notwithstanding, advertising may affect us in subtle and indirect ways. While there is no scientific evidence for the existence of “embedded” figures or words, let alone effects from them, the images and themes contained in advertisements may well influence viewers’ attitudes and values without their awareness. In other words, the viewer may be well aware of the stimulus, but not necessarily aware of the connection between the stimulus and responses or reactions to it. For example, there was a television commercial a few years ago for skin cream in which a mother and daughter were portrayed. The viewer was challenged to distinguish mother from daughter. According to Postman (1988), the unstated message is that in our culture it is desirable that a mother not look older than her daughter. A number of social scientists believe that advertising may play a role in the development of personal identity and social values (Leiss, Klein, and Jhally 1986; Schudson 1984; Wachtel 1983). It is difficult, however, to isolate advertising’s role from the many other social forces at work. Moreover, most research on advertising effects consists of content analyses of the ads themselves. Such studies leave many unanswered questions about the impact of that content on the viewing public.

Subliminal Auditory Self-help Tapes

When claims about covert advertising were raised in September 1957, the New Yorker lamented that “minds had been broken and entered” (Moore 1982). More than three decades later, claims of covert subliminal manipulation persist. Television commercials, magazine ads, and bookstores promote subliminal tapes that promise to induce dramatic improvements in mental and psychological health. These devices are ostensibly capable of producing many desirable effects, including weight loss, breast enlargement, improvement of sexual function, and relief from constipation.

Subliminal tapes represent a change in modality from visual to auditory, and now subliminal stimulation is supposedly being harnessed for a more noble purpose-psychotherapy, clearly a less crass objective than that of covert advertising. However, the scientific grounds for substantiating the utility of today’s self-help tapes is as poor as was the documentation for advertising effects 30 years ago. Proponents seem to have assumed that for obtaining subliminal effects one modality is as good as another. Claims about the utility of subliminal tapes are essentially claims about the subliminal perception of speech-a phenomenon for which there is very little evidence (Moore 1988). The basic problem is that the few studies that purport to have demonstrated effects of subliminal speech used such crude methods for defining subliminality that the findings are quite uninteresting (e.g., Henley 1975; Borgeat et al. 1985).

It is not obvious what the analogue to visual masking is for a speech signal. Masking, in the visual domain, is procedurally defined with relative precision. The mask does not mutilate or change the target stimulus-it simply limits the time available for perceiving the preceding target. In the absence of the mask, the target is easily perceived. In the auditory domain, the target signal is reduced in volume and further attenuated by the superimposition of other supraliminal material. Often the subliminal “message” is accelerated or compressed to such a degree that the message is unintelligible, even when supraliminal. It is an extraordinary claim that an undetectable speech signal engages our nervous system and is perceived-consciously or not. Signal detection is an implicit sine qua non of most theories of speech perception (Massaro 1987). To assert that “subliminal speech” is unconsciously perceived appears to call into question some very fundamental principles of sensory physiology. What is the nature of the signal that arrives at the basilar membrane? If the critical signal is washed out or drowned out by other sounds, then on what basis are we to suppose that the weaker of the two signals becomes disentangled, and comprehensible? The tapes also have a dubious conceptual rationale in their assumed therapeutic impact. Even if the message could achieve semantic representation, how or why should it affect motivation? Answering the question “How?” is important, because it provides the theoretical justification for the practice.

There are subliminally embedded messages at work. You won’t be able to hear them consciously. But your subconscious will. And it will obey [Zygon].

To gain control, it is necessary speak to the subconscious mind in a language that it comprehends-we have to speak to it subliminally. [Mind Communications Inc.]

Is there a pipeline to the id? Can we sneak directives into the unconscious through the back door? There may be a fundamental misconception at work here, consisting of equating unconscious perceptual processes with the psychodynamic unconscious (Eagle 1987; Marcel 1988). Cognitive psychologists use the term unconscious to refer to perceptual processes and effects of which we have no phenomenal awareness. Induced movement is an example of an unconscious perceptual process. Tacit knowledge of and conformity to grammatical rules is another example of unconscious processing. No one would want to argue, however, that either of these domains of activity have anything to do with the psychodynamic unconscious. Psychodynamic theorists use the term unconscious as a noun with a capital U, to refer to, for lack of a better term, the id-"a cauldron full of seething excitations,” as Freud expressed it. Because semantic activation without conscious awareness can be demonstrated, some observers have jumped to the conclusion that subliminal stimulation provides relatively direct access to the id. This assumption has neither theoretical nor empirical support.

While tape distributors often claim that their products have been scientifically validated, there is no evidence of therapeutic effectiveness (e.g., Auday et al. 1991; Greenwald et al. 1991; Merikle and Skanes, in press; Russell et al. 1991). In addition, both Merikle (1988) and Moore (1991a) have conducted studies that showed that many tapes do not appear to contain the sort of signal that could, in principle, allow subliminal perception to occur.

Quite apart from the lack of empirical support, there is little or no theoretical motivation for expecting therapeutic effects from such stimuli. The “explanation” consists of attributing to the systemic unconscious whatever mechanisms or processes would be logically necessary in order for the effects to occur. Because there is no independent evidence for such “unconscious” perceptual processes, it is not surprising that there is no evidence for the imputed effects (see Eich and Hyman 1991; Moore 199lb). Furthermore, Greenwald (1992) has recently queried the conventional psychoanalytic conception of a sophisticated unconscious processor, arguing that it is neither theoretically necessary nor empirically substantiated.

The burden of proof of the viability of these materials is on those who are promoting their use. There is no such proof, and therefore the possibility of health fraud could be raised. These tapes sometimes sell for as much as $400 a set. Of even greater concern is the fact that legitimate forms of therapy may go untried in the quest for a fast, cheap “cure.”

According to William Jarvis, president of the National Coalition Against Health Fraud, a quack is “anyone who promotes, for financial gain, a remedy known to be false, unsafe, or unproven” (Jarvis 1989: 4). Fraud, on the other hand, implies intentional deception. Consequently, not all quackery is fraud, nor is fraud synonymous with quackery. As Jarvis has pointed out, in some ways quacks may be worse than frauds. “The most dangerous quacks are the zealots who will take the poison themselves in their enthusiasm for their nostrums. Sincerity may make quacks more socially tolerable, but it goes far in enhancing their danger to the public” (Jarvis 1989: 4).

Scientists, the Media, and the Popularization of Science

The popularity and interest in the topic of subliminal influences-both inside and outside academic circles-can be attributed, in part, to media coverage (c.f., Pratkanis 1992). Conspiracy theories make good copy, and in subliminal advertising we have a large-scale technological conspiracy to control people’s minds with invisible stimuli. With subliminal tapes you can allegedly change your behavior and your personality in profound and important ways-effortlessly and painlessly. The quick fix of psychotherapy is an intriguing notion. It is therefore small wonder that it continues to be a popular topic for writers. Carl Sagan (1987) has suggested that pseudoscience flourishes because the scientific community does a poor job of communicating its findings. To propose that we can be influenced in dramatic ways by undetectable stimuli is a remarkable claim with little scientific support, but blaming journalists for promulgating the claim absolves the scientific community from any responsibility in the educational process. Relations between scientists and the press could be improved if scientists communicated more clearly. Researchers take such great pains to avoid making absolute pronouncements that they often err in the opposite direction. We sometimes speak with a tentativeness that belies the facts, understating our confidence that some propositions are true and that others are false (Rothman 1989). When we talk to the press, we need to speak plainly. For example, Phil Merikle recently observed that “there’s unanimous opinion that subliminal tapes are a complete sham and a fraud” (Rae 1991). Merikle is correct, but such candor is relatively rare. Who will distinguish science from pseudoscience if not the scientists?

Paradoxically, while negative scientific evidence continues to accumulate, the subliminal-tape industry-fueled by aggressive advertising campaigns-thrives. As Burnham (1987) has noted, advertising’s authority often derives from the use of scientific regalia. Advertising’s purpose is, however, antithetical to that of science: “Advertisers [are] engaged in remystifying the world, not demystifying it” (Burnham 1987: 247). Extraordinary claims, if they are repeated often enough, can perpetuate extraordinary beliefs. When nonsense masquerades as science and magic is disguised as therapy, the result is not always laughable. Consider the self-help tape for survivors of sexual abuse; the user is informed that lasting relief from the trauma of abuse is contingent upon the victim’s acknowledgment of their own role in causing the abuse in the first place (Moore l99lb).


Subliminal advertising and psychotherapeutic effects from subliminal tapes are ideas whose scientific status appears to be on a par with wearing copper bracelets to cure arthritis. Not even the most liberal speculations regarding the use of subliminal techniques for “practical” purposes impute any potential utility to these practices (Bornstein 1989). The interesting question to ask is not “Do subliminal advertising techniques or subliminal auditory tapes work?” but, rather, “How did these implausible ideas ever acquire such an undeserved mantle of scientific respectability?” The answer involves a complex interplay of public attitudes toward science, how social science is popularized in the mass media, and how the scientific community communicates to those outside the scientific community. Carl Sagan may be right-pseudoscience will flourish if scientists don’t take more responsibility for the accurate dissemination of scientific information.

According to Burnham (1987), superstition has triumphed over rationalism and skepticism partly because scientists no longer engage in the popularization of science-summarizing, simplifying, and translating scientific findings for lay audiences. The function of popularizing science and health is now carried out by journalists and educators. Consequently, many topics, including this one, receive coverage that is, at best, deficient in background information and meaningful context, and at worst, fragmented and misleading. Further confusion is caused by the tendency among journalists to manufacture controversies where none exists by juxtaposing the pronouncements of “authorities” who contradict one another. If all authorities (including those with financial stakes in their positions) are equally admissible, controversies abound.


I am grateful to Phil Merikle and Anthony Pratkanis for comments on an earlier draft of this paper, portions of which were presented to the American Speech and Hearing Association’s annual convention in St. Louis, November 18, 1989, to the American Psychological Association’s annual convention in Boston, August 12,1990, and to the annual convention of the Committee for the Scientific Investigation of Claims of the Paranormal, Berkeley, Calif., May 2, 1991. Inquiries should be directed to Timothy E. Moore, Department of Psychology, Glendon College, York University, 2275 Bayview Ave., Toronto, Ont. M4N 3M6. E-mail: GL2500020@YUVENUS.


Recent research in subliminal perception has provided very riffle evidence that stimuli below observers’ subjective thresholds influence motives, attitudes, beliefs or choices.

“Subliminal advertising and psychotherapeutic effects from subliminal saves are ideas whose scientific status appear to be on a par with wearing copper bracelets to cure arthritis.”

“Quite apart from the lack of empirical substantiation, there is little or no theoretical motivation for expecting therapeutic effects from such stimuli.”

Timothy E. Moore

Timothy E. Moore is in the Psychology Department, Glendon College, York University, 2275 Bayview Ave., Toronto, Ont. M4N 3M6.