Dunning-Kruger And The Calamitous Lemon Juice Bank Robbery That Launched A New Cognitive Bias

For Clifton Johnson and McArthur Wheeler, the masterminds behind a 1995 hit on several Pittsburgh banks, lemons were essential. The key ingredient that would shield their identity as they boldly went about committing crime in broad daylight.

You see, they thought lemon juice would make their faces invisible to security cameras. A genius strategy. Masterful, even, except for one, niggling detail: lemons don’t work like that.

Writing in invisible ink

So confident was Wheeler in the trick that he’s reported to have even exclaimed, “But I wore the juice!” when arrested. His confused logic stemmed from the true fact that lemon juice can be used as a kind of invisible ink.

You can try it for yourself by following these steps:

• Take a paintbrush, dip it in lemon juice, write your secret message.
• Let the lemon juice message dry until the paper appears clear.
• Heat the paper with a hairdryer or iron and – hey presto! You secret is revealed.

The trick works on paper because lemon (and other citrus fruit juice) contains carbon compounds. When heated, these compounds break down and the released carbon reacts with oxygen, becoming darker.

It’s a fun activity to try out with kids, but it’s also had some impressive real-world applications. After they were imprisoned in the Tower of London, Guy Fawkes’s associates – perpetrators of the failed Gunpowder Plot – sent secret letters to each other written in orange juice.

From the Houses Of Parliament to a Pittsburgh bank

If it’s good enough for blowing up the Houses of Parliament then it’s surely enough to take on a few banks in Pittsburgh, right? But Wheeler is no fool so, naturally, he does a test run.

Wielding a Polaroid camera, he slapped some lemon juice on his face and snapped a picture. To his amazement and delight there was no discernible face in the subsequent picture. It was time to rob some banks.

You can imagine their surprise, then, when the police presented them with the security camera evidence of their crime spree. They had been betrayed by science, but little did they know they were about to contribute to a whole new scientific concept.

When the news of Johnson and Wheeler’s botched bank job hit the news, it caught the attention of Cornell psychology professor David Dunning. 

It’s certainly regrettable to have your worst mistake catch the attention of a psychologist. Worse still when it inspires a paper titled, “Unskilled and unaware of it: how difficulties in recognizing one's own incompetence lead to inflated self-assessments”.

The dawn of Dunning-Kruger

Authored by Dunning and graduate student Justin Kruger, it explored the idea that people may overestimate their capabilities because they don’t know enough to realize how little they know. 

This, the authors write, creates a dual burden: “Not only do these people reach erroneous conclusions and make unfortunate choices, but their incompetence robs them of the metacognitive ability to realize it.”

They ran four studies investigating how participants scored in tests relating to humor, grammar, and logic, and then looked at how those participants rated their own performance. What they saw was that those scoring the worst had a tendency to review their own performance as being among the best.

This miscalibration was linked to deficits in metacognitive skill – that is, the ability to think about your own thinking. Without metacognition, and the necessary knowledge on a subject to realize how little you know about it, these people couldn’t realistically assess their own performance.

What was interesting was that when these same participants were given a little training in the given task, their capacity to review their performance accurately improved. And so, the cognitive bias known as the “Dunning-Kruger Effect” was born.

What is the Dunning-Kruger effect, really?

Ironically, the Dunning-Kruger effect often gets erroneously thrown around as way of saying, “stupid people think they’re geniuses”. The actual takeaway, as they so often are, is a little more nuanced than that.

The Dunning-Kruger effect isn’t about how intelligent you are, it’s about how well- or misinformed you are and how this influences your confidence. Situations you only know a little on you are more likely to think you know a lot about. 

As such, it’s a reminder to be humble when speaking up on things we aren’t an expert in.

“The effect is about us, not them,” said Dunning in a conversation with Jonathan Jarry published by the McGill University Office for Science and Society. “The lesson of the effect was always about how we should be humble and cautious about ourselves.”

So, laugh at the lemon juice robbers if you wish. It is, undeniably, a very funny story (made even better by the fact the pair said the lemon juice in their eyes actually made it difficult to see during the bank robberies).

Just don't get too cocky. After all, we're all vulnerable to not realizing how little we know until there is already egg (read: lemon) on our faces.