The fast mind and the slow mind: why we are wrong with complete confidence

Try this small question: a bat and a ball cost 1.10 euros together. The bat costs one euro more than the ball. How much does the ball cost?

Most people immediately hear in their head: "10 cents". The answer is fast, confident and wrong. If the ball cost 10 cents, the bat would cost 1.10 and together they would make 1.20. The correct answer is 5 cents.

Two ways of thinking

The psychologist Daniel Kahneman, winner of the Nobel Prize in economics in 2002, summed up decades of research in a simple distinction. He speaks of two "systems" of the mind, as figures of speech to explain, not as two real parts of the brain:

• System 1 is fast, automatic and effortless. It recognises a face, reads a word, senses danger, gives the answer "10 cents".
• System 2 is slow, deliberate and tiring. It multiplies large numbers, checks an argument, stops and asks: "Is that really so?"

System 1 is not our enemy. Without it we could not walk, speak or drive. The problem is that it works with shortcuts, and shortcuts sometimes take us far off course.

Confirmation bias

One of the most studied shortcuts is the tendency to look for, notice and remember more of the information that confirms what we already believe. The psychologist Raymond Nickerson, in a broad review in 1998, called it "a ubiquitous phenomenon in many guises".

A classic experiment shows it well. In 1960 Peter Wason gave participants three numbers, 2-4-6, and told them they followed a rule. They had to discover it by proposing other sequences and receiving the answer "yes" or "no". Most proposed sequences such as 8-10-12 or 20-22-24, which confirmed their hypothesis ("even numbers rising by two"), got a "yes" and felt sure. The real rule was simpler: any sequence of rising numbers. Few people tried sequences that could have disproved their idea, such as 1-2-3 or 5-10-50.

The question "is there evidence for this?" makes us feel sure. The question "what would disprove this?" makes us wise.

How to wake the slow mind

1. Pause on decisions that matter. To choose a coffee, System 1 is enough. For a new job, an investment or an accusation against someone, give yourself some time.
2. Look for the opposite evidence. Ask: what would make me change my mind? Then go and look for it.
3. Write your answer down before discussing it. In meetings, people are swayed by the first opinion they hear. When each person writes their own view first, the decision becomes more independent.
4. Be careful when you are tired or rushed. That is when the mind relies most on shortcuts.

We cannot switch System 1 off, and we should not. But we can learn to recognise the situations where speed costs us, and there let the slow mind have its say.