You don't think like you think you think

Humans are amazing creatures. We can paint murals, write concertos, and identify subtle changes in complex environments. As children, we can accurately infer someone's emotional state from facial micro-expressions, and even at just 12 months old, we can pick out faces of unfamiliar animals.

And yet we are also pretty stupid at times.

Actually, stupid is the wrong word because the mistakes of reasoning, the things that stop us thinking better, are common even amongst very smart people. Lots of education and a high IQ are no obstacle to succumbing to cognitive biases and logical fallacies!

So let's see susceptible you are to reasoning errors with a simple test - write your answers down before looking at the answers:

  1. Some months have 30 days and some have 31 days. How many months have 28 days?

  2. A bat and a ball cost $1.10 together. The bat costs $1.00 more than the ball. How much does the ball cost?

  3. If it takes 5 machines 5 minutes to make 5 widgets, how long would it take 100 machines to make 100 widgets?

  4. 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?

Check your answers at the bottom when you are done!

Did you get 4 out of 4? Unless you've come across these questions before or you have strong deductive intuitions, you probably got all three of these simple questions wrong.

And even if you did get them right, as soon as you read the question, your brain was screaming out the wrong answer before you could slow down and think deliberately.

That's because you don't think like you think you think

Most people think that they are deliberate, generally rational thinkers. For many years, that's exactly how psychologists thought we thought. But more recent work, especially that by Daniel Kahneman and Amos Tversky, shows that humans think primarily via heuristics - mental short cuts or rules of thumb.

Most of the time, we are thinking fast or what Kahneman calls System 1 - our quick and easy, always on default mode of thought.

System 1 is fantastic for things like pattern matching, recall, and association. It's these heuristics or short cuts of System 1 that allow infants to do things like inferring the emotional state of a person from their facial expression - tasks that even the best computer system or AI struggle with today.

In the case of the questions above, it is the availability heuristic that generated the immediate answer from the association of February to 28 days, ยข10 to the $1.10, and half of size to half of time. (These were the first things that jump into your mind weren't they?)

To get the right answer though, you need to think slowly.

Slow thinking, or System 2, is focused, deliberate analysis and computation. For many situations, we need to use System 2 to avoid jumping to hasty conclusions and making reasoning errors.

The problem however, is that using System 2 is hard. This type of thinking requires deliberate effort, sustained concentration, and uses significantly more energy than relying on heuristics. Contrary to popular belief, we are not multi-taskers. When we are using System 2 for one activity, we struggle to direct that deliberate focus to other simultaneous activities.

Not only that, but there is some evidence to suggest that will power and self control also rely on this focused attention. Research into ego depletion shows that whenever we are using System 2 to perform a task, our ability to regulate self control is diminished and we become more passive to choices offered to us.

Luckily there is a solution. By identify common patterns of poor reasoning and cognitive biases, we can develop new heuristics for the situations when our existing heuristics lead us astray. We can learn to think better by turning what once required focused effort into fast and easy intuition. Unfortunately, this requires deliberate practice over time. When it comes to thinking better, there are no short cuts.


  1. All of them. Every month has 28 days, some have more.

  2. $0.05. The bat costs $1.00 more than the ball. $1.05 + $0.05 = $1.10

  3. 5 minutes. 5 machines 5 minutes to make 5 widgets = 5 minutes per widget per machine.

  4. 47 days. The lilies double every day so work backwards from 48.


Baumeister, R., Bratslavsky, E., Muraven, M., and Tice, D. (1998). Ego depletion: Is the active self a limited resource? Journal of personality and social psychology, 74(5), 1252.

Wegener, D. and Petty, R. (1998). The Naive Scientist Revisited: Naive Theories and Social Judgment. Social Cognition: Vol. 16, Special Issue: Naive Theories and Social Judgment, pp. 1-7.

Frederick, S. (2005). Cognitive reflection and decision making. Journal of Economic perspectives, 19(4), 25-42.

Kahneman, D. (2011). Thinking, fast and slow. Macmillan

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Dave Kinkead

G'day, I'm Dave!

I work at the intersection of philosophy and technology. I'm a computational philosopher at the University of Queensland where I teach critical thinking, research digital pedagogies, and manage the UQ Critical Thinking Project. When I'm not thinking about thinking, I'm working on becoming a millionaire ski bum.

So far I've nailed the ski bumming bit.