What Does Psychology Tell Us About Using CAPTCHA On Websites?

In his book Thinking, fast and slow, Daniel Kahneman outlines how the human brain uses two different mental systems for making decisions:

  • System 1 – The fast, automatic, little or no effort, intuitive, but largely unconscious mind.
  • System 2 – The slow, disciplined, effort hungry, largely conscious mind that monitors system 1 and allocates attention to  more complex mental problems that require it. However, this system is lazy and will rely on system 1 whenever it thinks it is adequately handling decision making.

As an example of how these systems work here are some simple puzzles. The answers are at the bottom of this page.  Do not try to calculate the answers, but listen to your intuition:

1. A bat and ball cost $1.10 (one dollar and ten cents).

The bat costs $1 more than the ball.

How much does the ball cost?

2. If it takes 5 machines 5 minutes to make 5 widgets , how long

would it take 100 machines to make 100 widgets?

  • 100 minutes OR 5 minutes

3. In a lake, there is a patch of lily pads. Every day, the patch doubles in size.

If it takes 48 days for the patch to cover the entire lake, how long would it take for the patch to cover half of the lake?

  • 24 days OR 47 days

These 3 questions made up the Cognitive Reflection Test that experimenters recruited students from Princeton to take. The questions were selected because they all suggest an immediate intuitive answer that is incorrect. For the bat and ball puzzle the number 10 (10 cents) tends to come to one’s mind. This is the intuitive answer but is wrong. If the ball did cost 10 cents and the bat is $1 more than 10c ($1.10)  that would make the total cost of the bat and the ball $1.20, not $1.10 as stated in the puzzle.

When the test was administered, half the students were given a test paper with a small font and washed out print that was legible, but difficult to read. The other half were given a test paper in normal print. Interestingly, 90% of the students who received the test paper in normal font got at least one question wrong. However, only around a third of those given the difficult to read paper got any questions wrong. This is because the difficult to read paper caused cognitive strain (i.e. shit we have a problem!) which automatically activates system 2. This mobilizes our full attention and allocates resource that is more likely to reject answers suggested by our intuition (system 1).

What this demonstrates is how easily we are happy to rely on our intuition (system 1) when things appear to be going well (i.e. no complex problems to solve). We rely on system 1 for making most our decisions, but this can sometimes cause us to jump to conclusions that are incorrect.


Implications for Conversion Optimisation:

  • If you want to ring alarm bells and activate your customer’s system 2 then make the font small and difficult to read. This may occur during the registration process for websites that use the Captcha security test (Completely Automated Public Turing test to tell Computers and Humans Apart). No wonder this may seriously damage your conversion rate! If you have been using persuasive copy to encourage sign-up this security mechanism could undo all your hard work as it causes cognitive strain and activates a person’s system 2. This system is more likely to reject the reason for an impulsive decision and abandon a transaction.
  • This model of decision making also suggests sites should avoid asking visitors to remember instructions or promotional codes etc (e.g. displaying codes as images that cannot be copied and pasted). The more information that a website expects visitors to remember for future use the more likely it will lead to cognitive strain which will activate system 2. If mental effort is needed for storing information there will be less available for other activities and people become prone to missing messages or instructions during a website journey.
  • The use of two different mental systems also challenges the way organisations use traditional research and usability testing for assisting website design. In a previous post, Why should you stop using focus groups?, I outlined why focus groups can be a misleading research tool.
  • However, standard usability testing, particularly in labs, are prone to some of the same kinds of bias. What Kahneman’s work suggests is that direct questions often engage the wrong system (system 2) and that observation of behaviour is more likely to provide true insights. It also supports the benefits of ethnographic research where people are observed undertaking a behaviour in their natural environment (e.g. in their home) rather than in a user lab. Ultimately though the most reliable way of understanding what affects visitor behaviour will be an online experiment.


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  • About the author:  Neal provides digital optimisation consultancy services and has worked for  brands such as Deezer.comFoxybingo.com, Very.co.uk and partypoker.com.  He identifies areas for improvement using a combination of approaches including web analytics, heuristic analysis, customer journey mapping, usability testing, and Voice of Customer feedback.  By  aligning each stage of the customer journey  with the organisation’s business goals this helps to improve conversion rates and revenues significantly as almost all websites benefit from a review of customer touch points and user journeys.
  • Neal has had articles published on website optimisation on Usabilla.com  and as an ex-research and insight manager on the GreenBook Blog research website.  If you wish to contact Neal please send an email to neal.cole@outlook.com. You can follow Neal on Twitter @northresearch and view his LinkedIn profile.

Answers: 5 cents, 5 minutes, 47 days

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