Friday, March 1, 2013

Skepticism about subliminal priming

A piece written by Wolfgang Stroebe and Miles Hewstone in defense of social priming research appeared yesterday as the cover story of THE (Times Higher Education). The piece highlights how the fallout from the Stapel fraud case in social priming has tainted the reputations of others who are not suspect. That undoubtedly is true—fraud cases have fallout well beyond the fraudster, and that fallout can be unfair and unjustified. Unfortunately, in making its case, the piece unfairly impugns the motives and agenda of at least some critics of those findings. Yes, some are motivated by the belief that the effects themselves are false positives, but many have a more theoretical and historical perspective that leads to skepticism about the interpretation of these findings rather than of the findings themselves. 

The commentary describes what the authors feel is a concerted attack against social psychology by a small cadre of clueless cognitive psychologists. 
Having used priming exclusively to test hypotheses about associative memory, cognitive psychologists could not believe either that priming could have such a pervasive influence on behaviour or that people were not aware of this influence
Next, the authors attribute the motivation for skepticism to resentment that cognitive psychology effects garner less media coverage
Whereas the press often reported the findings of cognitive social psychologists, reporters were less interested in the work of their non-social colleagues.
I find both the characterization of cognitive psychologist and the attribution of motives both misleading and unhelpful to the discussion. Yes, many of those who are skeptical of social priming claims are cognitive psychologists. But, the reasons for their skepticism have nothing to do with jealousy or any interdisciplinary vendetta. Rather, at least for many of the criticsthey are based on a long history of battles over subliminal perception and the effects of non-conscious processing.  These battles predate the present social priming kerfuffle are just the latest in over a century of disputed claims about subliminal influences.

What I find both disturbing and remarkable about the THE piece is that its authors appear unaware of how this history motivates the skepticism about more recent claims. A critical quotes from the THE piece make clear my concern:
But things changed dramatically towards the end of the 20th century with the rediscovery (the notion had already struck Freud and the behaviourists) that people can be influenced by stimuli in their environment without being aware of it.
When, exactly, was this "knowledge" lost and in need of rediscovery? The debate over the power of subliminal perception has been enduring, started before Freud's influence (with experimental claims dating to the mid-late 1800s), and continued unabated throughout the 20th century. The reason for skepticism comes from that history. If there is one consistent, recurring pattern in that literature, it is that new claims of powerful subliminal or implicit influences on behavior are later shown to have occurred with awareness, to have been subject to uncontrolled demand characteristics, or to be unreplicable.
  • Have you heard that subliminally flashing "eat popcorn" or "drink coke" leads you to buy more concessions at the movies? That one was an admitted hoax perpetrated by advertising executive James Vicary executive in the 1950s.  Even after it was debunked over the decade that followed, it was featured prominently in Wilson Bryan Key's 1973 bestseller, Subliminal Seduction, and is still a common myth. (see Snopes coverage. Chris Chabris and I detail this case in our book, The Invisible Gorilla as well.)
  • Remember the claims that subliminal self-help tapes could help you weight or stop smoking? Those were repeatedly debunked throughout the 1980s. (e.g, this paper by Merikle. This paper suggests that many such tapes didn't even have messages embedded.)
  • How about subliminal messages planted in rock music albums leading people to violence or suicide? Not so much, although claims of subliminal persuasion led to a lot of lawsuits against musicians (wikipedia covers that one).

Those are public examples of questionable claims about subliminal influences, but what about more rigorous methods used to study implicit perception in the lab? When you look at the history of implicit perception effects in domains ranging from dichotic listening (1950s - 1960s) to masked priming (1970s - present), there is one consistent pattern: The more rigorous the measurement of awareness, the smaller the effects. In perhaps the most influential review paper on this topic, Daniel Holender's Behavioral and Brain Sciences article in 1996 analyzed 30+ years of the history of semantic priming without awareness and found that almost no studies adequately excluded awareness. He identified a set of criteria necessary to objectively rule out awareness in order to document a truly implicit effect. Not everyone agrees with his criteria, but in the perception world, even the hard-core proponents of semantic priming without awareness acknowledge that the effects typically are small and short lived. They often try to find qualitative differences between explicit and implicit priming in order to further rule out awareness (e.g., see work by Merikle, Reingold, Snodgrass, and others).

When people from the implicit perception world see claims of huge effects of subliminal primes on behavior, they are skeptical because of this history. When the measures used to rule out awareness consist of post-experiment questioning (an inadequate approach because it does not rule out awareness at the time of presentation), they are skeptical that the effect is truly implicit. When the studies make no attempt to compare the size of an implicit effect to the same effect with varying degrees of awareness, they are skeptical that it is truly implicit. When studies do not use signal detection methods to test whether people were sensitive to the presence of a prime, they are skeptical that it is truly implicit.

The irony is that the history of social psychology is filled with robust effects of unrecognized influences on decisions and behavior, cognitive dissonance chief among them. Social psychology has documented the many ways in which we are unaware of the reasons for our beliefs, attitudes, and actions. Nisbett and Wilson's classic critique of think-aloud protocols highlights that dissociation between the factors that influence us and our awareness that those factors are influencing us. But, claims of implicit influences on behavior are different than claims that we can't intuit the mechanisms of mind.

What makes many of the social priming claims interesting, and the reason they garner attention, is that they purportedly occur entirely without awareness. If I told subjects to envision themselves as elderly and then they walked more slowly, that would be interesting, but it wouldn't imply a powerful unconscious influence—it would just be a cool example of induced method acting. Most cognitive psychologists I know reacted to that social priming result by assuming the effects occurred with awareness, making the findings interesting but not as provocative or ground breaking. They were more traditional social effects in which people were unaware of how their experiences influenced their behavior even if they were aware of the existence of those influences. For some social priming effects, subjects are admittedly aware of the prime but not of its influence. From the perspective of implicit perception, those aren't as controversial.

When the THE authors claim that these influences are entirely outside of awareness, that triggers the same skepticism that arose in light of earlier subliminal perception and persuasion debates. To my knowledge, no study in the social priming literature has applied the criteria identified by Holender to demonstrate that the primes truly were outside of awareness. I haven't seen any papers in that literature discuss the issue of what counts as implicit in light of these debates in the implicit perception world. For example, a target article by Huang and Bargh in Behavioral and Brain Sciences that is now open for peer commentary doesn't cite Holender's paper or any of the other critiques of implicit perception claims.

The latest trend, both in that paper (in a footnote) and in a new paper by Hassin in the current issue of Perspectives on Psychological Science, seems to be to claim that the priming effects can be implicit without being implicit. In effect, Hassin is claiming that you can claim unconscious influences without actually showing that the stimuli were processed outside of awareness. In effect, these papers are suggesting that the lack of an ability to attribute behaviors to the prime is all you need in order to claim that the prime subliminally influenced behavior. It's a clever reframing, but that isn't what many of these papers have claimed.

There is a crucial difference between stating that people misattribute the reasons for their behaviors and claiming that the actual reasons are themselves implicit. The first point is uncontroversial. The second is unsupported. To my knowledge, no study in the social priming literature, however you want to define social priming, has met the criteria Holender and others have set out for documenting subliminal persuasion. When the claim is that stimuli that fall outside of awareness influence behavior, the claim is one of subliminal persuasion. And, such claims depend on demonstrating that the primes themselves fall entirely outside of awareness.

My point is that, at least for most of the people in cognitive psychology with interests in implicit perception, the source of skepticism has nothing to do with a vendetta against social psychologists or jealousy about media coverage. Rather, these new findings intruded on a long-standing debate within the literature on implicit perception, one that has seen repeated claims of strong implicit effects on behavior, only to to have those claims shot down. The past 20 years have seen steady improvements in the methods used to test for implicit perception. Strong claims of the powerful influences of implicit processes are nothing new in cognitive (or social) psychology.  And, the past two decades have seen significant refinements in how we must measure awareness in order to make claims about implicit perception. Social priming research either must adopt those more rigorous controls for awareness or it must avoid claims that these influences are truly implicit. The findings might still be interesting even if the stimuli are not implicit. But, demonstrating that an influence is truly implicit has proven much more challenging than you might think.

An aside: It is less clear what to make of one-off failures to replicate some of these effects (as opposed to tests of whether the effects occur outside of awareness). Within cognitive psychology, we have seen examples of failures to replicate claims of implicit perception as well (e.g., Marcel's priming work from the 1980s). Perhaps the replication studies were conducted poorly or lacked adequate manipulation checks. Perhaps they are false negatives or lacked power to find the documented effect. Perhaps the original was a false positive. Without repeated replication using a common, accepted protocol, it's hard to determine what the true size of these effects might be. Individual failures to replicate can tar a finding and researcher unfairly. That is one reason I have been pushing for multiple independent replications of important findings, all conducted using a shared and vetted protocol. That will provide a better measure of the actual size of an effect in reality.

*update* In case you're interested in reading more about the conditions required to claim that a stimulus was processed implicitly, I have co-authored a couple of review pieces. You can get them here and here.

*update 2* - I have written a follow-up post (Link) in which I explore the nature of goal priming: Wh/at, exactly, must be implicit in these claims, and are there other theoretical reasons for skepticism


  1. Very interesting and even-handed explanation of this debate.

    On the replication point, you may be interested in Uri Simonsohn's new work on the statistical criteria that should be applied to replications (it doesn't work to simply apply the same p<0.05 standard as we apply to new results, for various reasons). He presented this at SPSP in January but I don't know of a version online yet.

  2. Thanks Leigh. Yes, I've corresponded with Uri about his arguments, and I largely agree that if the goal of a replication is to use .05 as a decision criterion, it's essential to have much greater power than the original study (that's the crux of what his analysis shows). I will be posting more on replications, hypothesis testing, and the use of NHST in replications soon (in short, I think using p-values to determine replication "success" and "failure" isn't the best way to go in general. More broadly, I'm not convinced it does much good to talk about replication successes and failures in general. I'd rather focus on estimating effect size and, when necessary, pointing out when a replication study shows a substantially different effect size. THe goal should be to understand the true effect in the world rather than to reject null hypotheses. Stay tuned.

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  4. I've followed some of the related lines of research for a long time and I remember it being a difficult but successful road to demonstrate marginal perception originally, the very idea that we detect meaningful patterns that we are not even remotely aware of detecting. Having also established in various ways that perception is not just direct apprehension of the world but involves an elaborate chain of processing of some sort, subject to various influences, the idea of subliminal priming seems to have naturally become more and more taken for granted. I have a lot of respect for John Bargh's work though I think the framing of some of the questions can indeed be done differently. I also think it is very helpful here that you point out that there is an important distinction to be made between our demonstrated inability to explain our actions accurately and attributing them to specific implicit reasons.

    I think you make it more clear that we end up taking a lot for granted in our tacit acceptance of the specific effects of implicit stimuli. Certainly it makes sense to continue to dig more into the actual process of perception and the staging of action involved and I think setting a higher bar on what constitutes truly implicit reasons for behavior makes good sense as well. Thanks for clarifying this issue in a useful way. Thanks for this very helpful post.