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Indelible initial impressions: the anchoring effect

July 25, 2012
In science, when you’re on a good thing, should you stick to it – or is that a red flag?

How to recognise this tactic

Anchoring is the human tendency to rely almost entirely on one piece of evidence or study, usually one that we encountered early, when making a decision.

Single experiments practically never settle an important scientific debate – it is the preponderance of evidence among researchers in the field (who must be able to replicate each others’ findings) that determines the currently accepted explanation for any given phenomenon.

Barry L. Beyerstein, Canadian psychologist and skeptic, 1995

Why do people use this tactic?

Anchoring is a cognitive bias – it’s something that we do to ourselves. It shows up in all sorts of places but, in science-related situations, it’s our tendency to rely on and hang on to our first source of exposure to a topic. We then try to fit all new information into the model we have developed around that source, rejecting anything that doesn’t fit. We don’t do this consciously, it’s just a normal human bias.

What’s wrong with this tactic

Anchoring prevents us from considering alternative models. It makes us less likely to abandon models that have been falsified or modify ideas that don’t fit the real-world evidence. So we end up having faulty explanations for all sorts of phenomena. The trouble is that some people don’t realise they have this bias. They continue to harangue us with arguments derived from outdated ideas (that is, they ply us with PRATT), and often succeed in persuading some that they are right.

What to do when confronted by this tactic

Learning to guard against anchoring and other biases is an important part of a scientific education. You need to consciously remain open to new evidence and, if it’s supported by the science, be prepared to revise your thinking. When someone is trying to persuade you, look out for the absence of recent evidence and a reliance on very dated studies. Of course, many scientific explanations refer to long-standing principles such as the law of conservation of energy or the particle model of matter, but these have been repeatedly verified over a long time and this is well-known. If someone continually refers to a relatively obscure idea or study, you should suspect that anchoring is at work.

Variations and related tactics

Anchoring bias is also known as the focusing effect.


  • Anti-vaccine campaigners continue to anchor on a 1998 paper by Dr. Andrew Wakefield and others. The paper reported on intestinal disorders found in autistic children and strongly suggested that these disorders had resulted from the measles-mumps-rubella (MMR) vaccine. This was not supported by other studies. After an investigation by journalist Brian Deerpublished by the British Medical Journal, and an inquiry conducted by the UK General Medical Council,  The Lancet  retracted the paper in 2010.  In the same year, Wakefield was struck off the Medical Register for serious professional misconduct.
  • Anchoring is very common among deniers of anthropogenic global warming. One classic anchor focuses on the Vostok ice cores. They show that in the past, rises in atmospheric carbon dioxide concentrations have lagged behind temperature increases. The deniers use this to resist all evidence showing that increasing carbon dioxide from burning fossil fuels is at present the principal driver of global warming. The carbon dioxide – warming connection is, in fact, a self-reinforcing system, so an increase in either CO2 or temperature causes an increase in the other.
Add to the list of examples by leaving a comment.

Barry L. Beyerstein’s quote from Distinguishing Science from Pseudoscience.

This is one of ScienceOrNot’s Science red flags. See them all here.
  1. Toxoplasma gondii is important in sheep industries worldwide, killing lambs. A large body of serological and bioassay evidence led to the consensus view that parasite transmission is mainly through cat faeces. A series of studies challenged that, presenting PCR evidence of T gondii in lambs’ umbilical cords. That led to a new hypothesis stating that T. gondii in sheep is mainly transmitted from ewe to lamb.
    I worked on this topic myself and in my opinion, the consensus view is robust. The new hypothesis did need checking out, though. If it had been supported by accumulating evidence, advice to farmers about disease prevention would have needed to change.

    • I’m not sure what you’re getting at here, argylesock. Are you referring to the fact that Age of Autism is currently trying to use a Toxoplasma gondii study to support Andrew Wakefield’s original paper? I notice that on the same page, they try to use the arsenic-based lifeforms study to support their ideas about heavy metals and autism, despite the fact that replication has failed (here and here)

      • In fact no, I wasn’t commenting on the autism story. You seemed to have asked for examples of hypotheses changing in response to new evidence. The story about ovine Toxo includes a claim that the hypothesis needed to change, a claim which (in my opinion) doesn’t hold up.
        The story about autism and vaccines is compelling, I find, but not in a good way. As a scientist I’m firmly pro-vaccination.

Trackbacks & Pingbacks

  1. The anti-GMO bad science checklist
  2. Science Red Flags – autisticagainstantivaxxers

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