Dec 04 2012

The Power of Confirmation Bias

It is my contention that scientific skepticism is an intellectual discipline and a cognitive skill set more than anything else. It is also a philosophy, a value system, and an approach to knowledge – but these are hollow without the knowledge and skills to apply that philosophy.

This is especially true in our complex world, with sophisticated pseudoscience alongside mature and highly technical real science, ideologies of every stripe pushing their agenda, governments with power to protect, and markets and corporations with a profit motive to deceive. The internet is also drowning us in information, much of it dodgy.

It is therefore not enough to have a generally skeptical outlook, or even to call oneself a skeptic. Skepticism is a journey of self-knowledge, exploration, and mastering the various skills that comprise so-called metacognition – the ability to think about thinking. <shameless plug> For a thorough discussion of metacogntion, you can check out my Teaching Company course: Your Deceptive Mind: A Scientific Guide to Critical Thinking Skills. I also understand it makes a wonderful gift.</shameless plug>

As an example of the need for metacognitive skills in navigating this complex world there is confirmation bias. This is definitely on my top 5 list of core skeptical concepts, and is a major contributor to faulty thinking. Confirmation bias is the tendency to perceive and accept information that seems to confirm our existing beliefs, while ignoring, forgetting, or explaining away information that contradicts our existing beliefs. It is a systematic bias that works relentlessly and often subtly to push us in the direction of a desired or preexisting conclusion or bias. Worse – it gives us a false sense of confidence in that conclusion. We think we are following the evidence, when in fact we are leading the evidence.

Part of the illusion of evidence created by confirmation bias is the fact that there is so much information out there in the world. We encounter numerous events, people, and bits of data every day. Our brains are great at sifting this data for meaningful patterns, and when we see the pattern we think, “What are the odds? That cannot be a coincidence, and so it confirms my belief.” Rather, the odds that you would have encountered something that could confirm your belief was almost certain, given the number of opportunities.

Another factor that plays into confirmation bias is using open-ended criteria, or ad-hoc or post-hoc analysis. This means that we decide after we encounter a bit of information that this information confirms our belief. We retrofit the new data into our belief as confirmation.

Confirmation bias is further supported by a network of cognitive flaws – logical fallacies, heuristics, and other cognitive biases – that conspire together to reinforce our existing beliefs. In the end you have people who, based on the same underlying reality, arrive at confidently and firmly held conclusions that are directly opposing and mutually exclusive.

I encounter examples of confirmation bias every day. (My now favorite quote about this is from Jon Ronson, who said, “After I learned about confirmation bias I started seeing it everywhere.”) Of course, at first it is easy to see confirmation bias in others, and only later do we learn to detect it in ourselves, which forever remains challenging. A recent e-mail provides an excellent example, and was the prompt for this post. The following comment is from a forum dedicated to body asymmetry disorders as an explanation for chronic conditions such as multiple sclerosis and chronic fatigue syndrome:

“I personally have serious doubts if there is anything like “MS” or MS being an “autoimmune disease”.

Literally everyone I have seen and treated with “MS” was actually a TMJ dysfunction patient. I am anxiously looking for my first “Real MS Patient”.

I am currently treating at least 20 “MS” patients. Many of them have had short lived benefits from CCSVI. They all appear to be substantially improving with TMJ correction and most symptoms previously labelled “MS related” have disappeared.

I believe that most “MS lesions” are a consequence of CSF leaking into the brain stroma – they are not lesions in the neural tissue as such otherwise I could not so rapidly correct the symptoms.

The immune system tries to get rid of this fluid which is in the wrong place but is unable to do so and the deposits calcify leading to the typical opaque lesions seen on MRI and CAT scans. At autopsy the increased immune function is wrongly diagnosed as an auto immune disorder.

Imagine taking a slice of pathological tissue around any infective lesion one is bound to see an increased immune function. To go onto interpret it as an autoimmune disease would be pure folly. The immune system needs help – not an assault on its function which the drugs do.

As per your question I have treated hundreds of patients with Myalgic Encephalomyelitis (ME), Chronic Fatigue Syndrome (CFS) and others who present just with jaw problems for very many years. I have never found a real ME or CFS patient either. The common denominator is tooth/Jaw dysfunction and some who also have their Atlas out of line.

I remain to be corrected on my hypothesis”

Sounds pretty confident. It is also all utter nonsense. The commenter, a practitioner of dental therapy, may be just promoting their practice, but let’s assume they are sincere in the beliefs they state above. For background, we have over five decades of research telling us that MS is an auto-immune disease. There are literally thousands of published studies supporting this conclusion, with multiple independent lines of evidence. The commenter, who clearly knows very little about immunology or autoimmune disease, tries to dismiss some of this evidence, such as from autopsy examination. They argue that seeing inflammation around MS lesions in the brain does not prove the lesion is autoimmune. It could just be the immune system doing its job.

Whenever you hear a claim like this about a mature science you have to ask yourself – so all the researchers dedicated to this discipline over decades missed this simple idea? If something apparently obvious occurs to you as a non-expert, it’s a good bet that it has occurred to people who have spent their careers thinking about the issue and researching it. The difference between reactive immune activity and auto-immune activity is a basic concept – it is something that is thought of every single time a pathologist looks at immune activity on a slide – is this reactive immunity, or a primary inflammatory lesion?

MS is an autoimmune disease. There is evidence of chronic central nervous system immune activity, lesions are inflammatory, and immunosuppressant treatment works.

The commenter claims they never met a real MS patient. What are their criteria? This sounds like confirmation bias through definition. They imply that if someone who is diagnosed with MS has improvement in any of their symptoms with TMJ correction, then they don’t have MS. They do not seem to be aware of the illusory power of placebo effects and confirmation bias. Any treatment, if looked at in an open-ended way, will seem to work for almost any condition. That is the power of confirmation bias. Only by carefully studying a treatment is a blinded fashion, to eliminate the effect of all biases, can we get reliable information about the real effects of the treatment.

What published research is there to support the claims of this commenter? Very little – a couple of pilot studies (meaning preliminary type studies that are not blinded, controlled, or otherwise rigorous). Preliminary studies themselves are little more than confirmation bias – they have a huge positive bias and are likely to support the researcher’s hypothesis. They are not confirmatory – meaning they really are not evidence at all, but just an exploration to guide later research.

The commenter is further impressed by the fact that they can speculate wildly about an “explanation” for apparent MS lesions. This is just another form a confirmation bias – we think that because we can think of an explanation, this in itself is a sort of confirmation (the entire field of astrology, for example, is based upon this process). We underestimate our ability to invent explanations for things post-hoc. The ability to do so says absolutely nothing about the viability of our beliefs, because we can invent post-hoc explanations for anything. The real question is – is there any objective evidence for the hypothesis. In this case, regarding CSF leakage, reactive immunity, and MS lesions, the answer is no.


One primary difference between science-based medicine and practices on the fringe, like TMJ treatment for diseases like MS, is that SBM is based as much as possible on objective rigorous evidence. It recognizes that failings of anecdotal or uncontrolled observations. Fringe practice, by contrast, is little more than a massive exercise in confirmation bias. Disconnected from the reality check of science, there is no apparent limit to the nonsense in which humans will confidently believe.

22 responses so far

22 thoughts on “The Power of Confirmation Bias”

  1. Andrew Cooper says:

    Confirmation bias was very much in evidence during the Republican Presidential campaign, during which they appeared to interpret their own polling data as confirmation that they were doing all the right things and were bound to win. They started believing their own spin. As we know, and as this piece from today’s Guardian points out the Democrats went about the job rather more scientifically.

    It is said that during their triumphal processions ancient Roman Emperors and generals had a slave whisper to them ‘remember you are only a man’ to remind them of their fallibility. I think actual and aspiring leaders need people close to them who will help them be aware of confirmation bias by telling truth to power, rather than advisers who tell them what they think they want to hear.

  2. Skeptico says:

    “After I learned about confirmation bias I started seeing it everywhere.”

    I just learned about the forer effect. It describes me perfectly.

  3. ccbowers says:

    “Confirmation bias was very much in evidence during the Republican Presidential campaign”

    In early November, Steve had a post “Moderating political Opinions” in which I brought this point up. I would not characterize this as a feature of the Republican Party as much as it is about how motivated a person is ideologically. It does appear that in this case Romney supporters were more motivated to skew their interpretation of the facts, because they were not in his favor. This bias was very obvious in real time, as I could see the most ideological prognosticators ‘doubling-down’ on their (incorrect) predictions.

    “After I learned about confirmation bias I started seeing it everywhere”

    One of my favorites as well. I do wonder where this originates – I heard a version of this quote when Jon was on ‘Point of Inquiry’ back in October, but I’m sure that this was said or written previously. I should go back and listen to the interview with him on the SGU from last year- I’m sure it was an intersting interview and I have only a vague recollection of it.

  4. fromLondon says:

    I am a PhD student working on multiple sclerosis and hence I can feel your pain about all the bogus claims and dodgy theories circulating on the internet. However, whilst I share your believe that MS is autoimmune, there are some very competent people within the field who doubt this and who (in a more sophisticated manner than your commenter) also view lymphocyte activity in the disease as merely a secondary process (see, e.g. Stys, Zamponi, van Minnen, and Geurts 2012 or Trapp and Nave 2008). I am not saying this because I want to start a discussion about MS but because for me this highlights one of the biggest problems that skeptics face: The more you truly know about an issue, the more nuanced your view of it. If someone asked the person in your post whether he or she thought MS was autoimmune, they would probably reply: “Nonsense. Here’s why it is not.” whereas I am more likely to state that I do believe so, because … but then again …

    I wonder which one comes across as the stronger statement.

  5. gr8googlymoogly says:

    I find it fortunately coincidental that the ‘practitioner of dental therapy’ sees TMJ – something that falls within his area of specialty – as the cause. It seems to me to be the old “when my only tool is a hammer, everything looks like a nail” scenario.

  6. London – I disagree with your characterization. Even the references you provide do not dispute the role of inflammation in MS – “This concept that the immune system plays a central role in the pathogenesis of MS is indisputable.”

    First, even these opinions are minority opinions. But what they are talking about is that the inflammatory mediated damage in MS is triggered by an underlying degenerative process. The inflammation, however, is still pathological.

    What the commenter is referring to is very different – that the inflammation is secondary and appropriate (not pathological, and not causative of CNS damage). That’s not a minority opinion – that’s a fringe crank opinion. Very different.

    There are always nuances, and caveats, and side points when discussing any scientific issue. It definitely requires judgement in terms of how much of that nuance to delve into in any article. I always try to make statement that, even if simplified, are correct as far as they go, and go far enough for the point being made. But you can’t explore every nuance to complete current understood detail – that would require a tome for every simple point you try to make.

  7. fromLondon says:

    Hi Steven.

    This was not meant to be a criticism nor did I try to say that the two papers support the commenter’s view. My point is simply that the frustrating thing about confirmation bias is that it leads to seemingly strong statements which can easily overshadow any scientific discussion of a topic.

  8. ccbowers says:

    “I find it fortunately coincidental that the ‘practitioner of dental therapy’ sees TMJ – something that falls within his area of specialty – as the cause. It seems to me to be the old “when my only tool is a hammer, everything looks like a nail” scenario.”

    True – that is another demonstration of how confirmation bias works. An important quality of a good clinician is being able to recognize when his/her expertise is relevant or sufficient to evaluate a given situation. This concept can be applied more broadly across any area of knowledge. This skill really requires applied skepticism, and like most skills varies from person to person. BTW – gr8googly- TMJ is an anotomical term, not a diagnosis as implied by the comment. Yes, it is pedantic to point that out (and perhaps you know that), but its a common error made.

  9. BTW – right after the first time I heard about the Baader-Meinhof phenomenon, I quickly encountered it two more times.

  10. MikeB says:

    A “Doctor” Suzanne Humphries recently appeared on a local radio station–WERU in Maine, I state it explicitly to shame them–to extol the virtues of mega-doses of Vitamin C. HUGE doses, that would make Linus Pauling blanch.

    Everything she saw was evidence of “vitamin C deficiency.” She even declared that everyone is walking around with low-grade rickets.

    What was disturbing was hearing the huge, complicated chunks of verbiage flowing out of her. As a lay person, I was bowled over: I couldn’t possibly refute anything she was saying, as I literally did not know what she was talking about.

    What convinced me that I was listening to a Quack of the Highest Order–an uberKwak–was the sheer ease with which she “diagnosed” everything as vitamin C deficiency.

    She had piles of anecdotes from her “practice”–my gosh, she has a “practice” and “patients”!–about coronary artery disease, diabetes, nose bleeds, all which she “treated” with vitamin C.

    She teaches people how to make something called “liposomal vitamin C” at home, some kind of mega-dose that “goes directly into the cells.” She tells patients to “listen to the gurgling in their stomachs” to tell them whether they’ve taken enough vitamin C. Maybe you all know what the F she’s talking about, but I didn’t, and I kept thinking: If I didn’t know better than to take this stuff with a grain of salt–a whole cellar of salt–I might fall for it because it sounds so….scientific!

    She saw vitamin C deficiency in cancer patients. She advised people to stop taking cholesterol drugs. She told diabetics to take mega-doses of vitamin C. She said that vaccines were “criminal.”

    If someone thinks the vitamin C isn’t working, then obviously they aren’t taking enough! The studies claiming to show that mega-doses of vitamin C don’t have the effects advocates claim they have are wrong because they didn’t use enough vitamin C!

    Her “confirmation bias” had run so amok that she was claiming there was a Grand Conspiracy by the FDA, the whole medical establishment, to suppress the effects of mega-doses of vitamin C, because if word got out and people learned they could make it at home and CURE EVERYTHING, then the “hospitals would empty out” and modern medicine would be out of business.

    This is an instance of confirmation bias in MEGA DOSES.

  11. Andrew Cooper says:

    @ccbowers I take your point about ideological motivation. It seems to me (in the UK) that the neocons/libertarians are far more driven by ideology and dogma than the rest of us. Their belief in free markets, the efficient markets hypothesis and the need to reign back government are based entirely on a political creed rather than on the facts. In the real worlds unrestrained competition leads to monopolies and government is a necessary feature of an advanced, civilised society.

    Apologies for going off topic but I do feel that those with dogmatic beliefs – whether they relate to alternative medicine or politics – are, by definition, more likely to be prone to confirmation bias and less likely to question their beliefs and engage in objective analysis.

    Lastly, I think that necons and libertarians commit a category error by assuming that everyone who doesn’t agree with their world view is a ‘socialist’. I’m neither a libertarian or a socialist. However, I’m happy that the right – who I certainly disagree with – engage in such fallacious thinking and I’m keen that we should encourage them to continue to do so.

  12. tmac57 says:

    MikeB-Sounds like that “Doctor” Suzanne Humphries is a one note “doctor” ,that note being a High C.

  13. ccbowers says:

    “It seems to me (in the UK) that the neocons/libertarians are far more driven by ideology and dogma than the rest of us”

    This could be because you are identifying a distinct group which is idelogically defined. What I mean is that they are ideologically motivated by definition, and “the rest of us” are more diverse in our biases since we are “the rest.” There are ideological biases on the left for sure, they just tend to be more scattered in their biases since the left is more diverse in general, at least in the US.

    There are fewer hot button issues on the left that are widely problematic due to this diversity, and ideologies tend to occur in smaller subgroups of the left. Some examples can even be found in the comments section of this blog (and this audience is likely less ideologically driven than the general population), for example take a look at his recent post on organic farming. I haven’t read that comments section in quite a while (avoiding it at this point), but I can see it is quite active still. Hydrofracking is another topic which the left has difficulty. Vaccine denial is more broad, but clearly not a rightwing or strictly libertarian problem.

    So I think that your perception that NeoCons/Libertarians are more motivated by ideology is at least partially a result of how the groups are defined and that those groups are more homogenous (therefore their interests align more easily). Perhaps you are still right even after correcting for these two issues, but I’m not sure. Any thoughts?

  14. ccbowers says:

    “Everything she saw was evidence of “vitamin C deficiency.” She even declared that everyone is walking around with low-grade rickets.”

    Rickets is primarily a Vitamin D issue, and one would have to do some serious mental gymnastics to connect that to Vitamin C in the US population. Although to correct her comment to “low grade scurvy” is even stranger.

  15. Davdoodles says:

    Falling into the trap of confirmation bias is perhaps understandable when the information, at least coincidentally, supports the conclusion.

    One of the oddest forms of confirmation bias (if indeed it qualifies as CB?) occurs in a conspiracy theorist’s brain, where any information that CONTRADICTS the conclusion, is evidence for the conclusion.

    Where “this brick does not float” is evidence that there is a widespread coverup of the existence of floating bricks…

    Brains are kooky things, at times.

  16. MikeB says:

    ccbowers, thank you for the correction. That’s what happens when a lay person works from a crappy memory from a crappier radio program.

    I’ll take your word for it that the idea of everyone walking around with “low-grade scurvy” is stranger.

  17. Therion2012 says:

    After I heard about the Kluger-Hans-effect I found out that my horse is also called Hans!
    Seriously, I tried encrypting and translating the name and then listed them up among other names and it always started stomping when the Hans-equivalent came up!
    Now I’m not really sure how it always managed to recognize it, because had it been me, I wouldn’t have recognized them that reliably. I’m a bit startled about it, but while my horse tried to explain me how it managed to do it I sadly was unable to understand it. I suspect it’s secretly using google glasses.

  18. ccbowers says:

    “I’ll take your word for it that the idea of everyone walking around with “low-grade scurvy” is stranger.”

    This idea piqued my interest enough to look into the idea briefly, since I had not heard of it being expressed this way, but it appears that this idea is connected to the megadosing of vitamin C that has waxed and waned over the years. Everything from cardiovascular disease to immune deficiencies are blamed on what is described as “chronic scurvy,” with the name Linus Pauling used as an appeal to authority.

    Typical of many alt. med pseudosciences, half-baked ideas are propped up as evidence and this is intertwined with conspiracy theories of a systematic suppression of the truth, all while ignoring how implausible these ideas are. This is really old news, but apparently someone is still out there promoting these ideas

  19. erroll says:

    It’s sad that we can’t criticize those in our own community for confirmation bias, without it turning into a complete mess. If we can’t criticize ourselves what right do we have to criticize CAM, psychics, ghost hunters, etc. Seems like a well timed article though.

  20. On the island says:

    It is also important to realize that there are hypotheses that cannot be rigourously examined in a double blind RCT of the highest standard. As a physiotherapist, I strive to practice in a manner that is evidence-based as much as possible, but with things like the placebo effect and the complex and multifactorial nature of pain, it is a difficult task. It is often impossible to get an accurately standardized and sufficiently large group of people with pain in a specific area, and it is nearly always impossible to do a double-blind treatment of any hands-on therapy. Throw in the placebo effect and other sorts of psychosocial factors and things get really complicated. RCTs certainly have a place in evidence-based medicine, however it is important to remember that valuable and valid information can be gleaned from studies even if they aren’t randomized, double-blind, controlled, level 1 evidence. Just because something can’t be objectively measured or fit into a RCT doesn’t necessarily mean it doesn’t have value. And not all studies that fit the bill convey meaningful or particularly useful information. I think the key is to be aware of bias and logical fallacy (as much as is possible) and to never stop questioning what you’re doing and why you’re doing it.

  21. bachfiend says:


    Michael Egnor has again written one of his usual threads attacking your arguments (in his usual clueless style).

    Have a look at it. It’s hilarious.

    He manages to confuse evoked potential with action potential. Process dualism (which I gather is the same as materialism) with structure dualism (which is what he accepts). And then accuses you of being a dualist too.

  22. ebohlman says:

    “After I learned about confirmation bias I started seeing it everywhere.”

    I just learned about the forer effect. It describes me perfectly.

    Unlike most people, when I’m wrong it’s never due to fundamental attribution error.

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