Social priming: Of course it only kind of works

Social priming is the field of research about how thinking about or interacting with something (like warm coffee, or old age) can affect later, vaguely related behaviour. (Rolf Zwaan has a useful summary of the theoretical background here and here, and a recipe for how to whip one of these up for yourself here.) It has been a top target for replication efforts in psychology. Although social priming effects in general have been widely demonstrated, many specific results (e.g. priming people to think about old age makes you walk slower; Bargh, Chen & Burrows, 1996) have failed to reliably replicate (even thought the effect sizes for individual studies are often surprisingly large). Most of the attention has been on the work of John Bargh because he basically invented the field (all discussed in this profile from January 2013).  Last year Bargh exploded all over the internet with a bit of a tantrum about these failures on his Psychology Today blog (now deleted, but archived for posterity here and here and discussed in detail by Ed Yong here). This made him something of a punching bag on Twitter, etc and so people are a bit excited that another Bargh social priming result has failed to replicate (oh and hey, here's another, non-Bargh one). Cue panic, gnashing of teeth and reflexive defensive moves by social psychologists (plus coverage of the topic in the NYT).

I'm a bit bemused by it all, really. I am not at all surprised that while social priming works in general, there is wide variation in how well specific social priming tasks work out. Of course priming works - it couldn't not work. But the lack of control over the information contained in social priming experiments guarantees unreliable outcomes for specific examples. Let me see if I can explain what I mean.

Canalisation - Why Priming Basically Works
A useful concept from the developmental biology literature is canalisation. The idea is that systems that have any kind of structure (e.g. genetic or cognitive) are able to use that structure to smooth out variation in outcomes caused by variation in input. This isn't a rigid 'having this gene makes you only able to do one thing'. The idea is more dynamic - the structure acts as a guide to the preferred outcome.

The word canalisation should evoke the formation of a canal by the activity of flowing water. Over time the flow of water creates a preferred path (the canal) and any water that flows along that canal will generally follow the path laid out. Minor variations at the start of the water flow (say, in the exact location of the source of the water) lead to the water taking slightly different paths down the river, but this variation is still structured by the form of the canal so that by the end, the water comes out at the same place. This word is used to describe the kind of robustness you see in complex, nonlinear dynamical biological systems; there is both stability and variability in the overall behaviour of the system as it unfolds over time but (if functioning properly) it's all within the systems's tolerance for error and the net result is still good.

In biological systems, canalisation is like forming a habit, a disposition to act in a certain way. In psychology, most of our dispositions are acquired through experience. Priming is like taking an empty canal formed by the prior water flow of experience and running a little water down it to see what happens. Where the water goes tells you about the form the canal has from its prior history. 

In the social priming literature, therefore, where the 'water' goes should depend on the connections and associations you have in your social knowledge about the world, and it should therefore reveal those connections to the social psychologist. That's what, experimentally, priming is for.


The limits of social priming

People are complex dynamical systems with numerous dispositions and connections lying around based on prior experience. We are also sensitive to all kinds of information in all kinds of formats. Some of that information is social in nature. If you poke this kind of system with a social prime that the system can detect, then that information will indeed flow somewhere, by definition. 

But where it will flow is underconstrained. Primes are, by definition, merely nudges to a trained system which you then just let run and respond how it likes. Social primes are nudging systems with a lot of room to move; none of our social associations and connections are compulsory that way, say, gravity is. The same people will respond in different ways at different times to the same social prime 'nudge' because at any given moment people are differently 'shaped' systems. This gets worse because social priming studies are typically between-subject designs, and (shock!) different people are even more different from each other than the same people at different times! 

Then there's also the issue of whether the social primes used across replications are, in fact, the same. It is currently impossible to be sure, because there is no strong theory of what the information is for these primes. In more straight forward perceptual priming (see below) if I present the same stimulus twice I know I've presented the same stimulus twice. But the meaning of social information depends not only on what the stimulus is but also who's giving it and their relationship to the person receiving it, not to mention the state that person is in.

In social priming, therefore, replicating the design and the stimuli doesn't actually mean you've run the same study. The people are different and there's just no way to make sure they are all experiencing the same social stimulus, the same information (and remember, information is absolutely where it's at for explaining behaviour). While the system will respond, exactly how and how much will vary over time. 

So frankly I am not surprised that social priming works as a basic effect but that the results of specific attempts to prime produce highly variable results. It just couldn't be any other way, once you'd thought about it for a bit.

A contrast: motor priming
I did a post-doc at the University of Warwick with Friederike Schlaghecken & James Tresilian using masked priming to investigate motor control processes (Wilson, Tresilian & Schlaghecken, 2010, 2011). In this kind of priming, you briefly present a stimulus (say an arrow) that cues the participant about a response option (e.g. press a button with the left vs right hand). You then replace the stimulus with a randomised mask, and then present another stimulus to which the participant actually has to respond. 

Depending on the timing of all this, there are two responses to the priming. There's the positive compatibility effect (PCE) where you are faster to respond if the prime cued the same action. Different timing produces the negative compatibility effect (NCE) where you are actually slower to respond if the prime and the response match. You get a PCE when the prime can be seen or when the response is required right after the mask, and an NCE when the prime is subliminal or there is a longer delay after the mask.

The full details of the NCE are still a little up for grabs, but the basic setup is that it reflects the dynamics of motor cortex as it prepares the response. The prime makes you begin to prepare one movement; the mask interrupts that preparation, making you slam on the brakes. This inhibition then makes it harder to actually do that response if needed. This braking takes time to occur, however, so if you ask a person to respond right after the mask you get in before the brakes hit and you get a PCE. If you give the system just a little more time, however the inhibition has begun and you get the NCE.

So the basic idea is the same as social priming. You give a trained system some incomplete information and then let it just do it's thing. You then poke it at different times to see what's going on, and when you do you find an unfolding pattern of activation and inhibition which relates nicely to the flow of information you are presenting with your stimuli. 

These effects are robust and easy to generate if you do the timing correctly (if you have Matlab and can read instructions you can try it yourself using the toolbox I wrote for Friederike). Why? Because we've exercised a lot of control over the stimuli so we know what is going in, and because the system we're tapping is fairly straightforward and highly canalised so that it responds fairly consistently. This makes priming a useful technique for studying motor control, and suggests that because social priming isn't constrained in this way, priming may simply be too variable a method to find out anything useful.

Some concluding thoughts
The problem with social priming is conceptually similar to the small effects size effect problem (although note that one of the actual problems with social priming is the occasional unrealistically large effect). The problem is that there's just not enough control being exerted on what's going in, and we just don't have enough of a formal description of the system being primed. The solution is the same; more targeted poking of more carefully described systems. I describe two examples at the end of the small effects post; how I broke coordinated rhythmic movement by targeting very specific information variables (Wilson & Bingham, 2008) and how Geoff can turn visual metric shape perception on and off by controlling access to the required large scale (>45°) changes in perspective (e.g. Bingham & Lind, 2008).


Pretty much everything we talk about on the blog these days boils down to this; get as precise as possible (preferably to the point of a decent model; Golonka & Wilson, 2012) about what the hell it is you're doing, and use that to generate testable hypotheses about the kind of poking that should and shouldn't matter. Poking something usually means manipulating information, and so you'd better be prepared to get serious about that too. Until then, social priming (and psychology in general) will just continue working as a general sort of principle, but not work reliably in specific cases. We can probably do better than that.

References
Bargh J.A., Chen M. & Burrows L. (1996). Automaticity of social behavior: Direct effects of trait construct and stereotype activation on action., Journal of Personality and Social Psychology, 71 (2) 230-244. DOI: 10.1037//0022-3514.71.2.230 Download

Bingham, G.P. & Lind, M. (2008). Large continuous perspective transformations are necessary and sufficient for perception of metric shape. Perception & Psychophysics, 70(3), 524-540. Download

Golonka, S., & Wilson, A. D. (2012). Gibson’s ecological approach – a model for the benefits of a theory driven psychology. Avant, 3(2), 40-53. Download

Wilson, A. D., & Bingham, G. P. (2008). Identifying the information for the visual perception of relative phase. Perception & Psychophysics, 70 (3), 465-476. Download 

Wilson, A. D., Tresilian, J. R., & Schlaghecken, F. (2010). Continuous priming effects on discrete response choices. Brain and Cognition, 74, 152-159. Download
 
Wilson, A. D., Tresilian, J. R., & Schlaghecken, F. (2011) The masked priming toolbox: an open source Matlab toolbox for masked priming researchers. Behavior Research Methods, 43, 210-214. Download Link to the toolbox