woman in black long sleeve shirt using macbook




Try for 30 seconds to multiply these two numbers in your head, not using pen and paper or a calculator:

17 x 24

It’s difficult to do in your head—most people I ask this of give up after a few seconds.

Here’s another task. Look at the photo in Figure 10.1 and decide what it’s a photo of.

Portrait of a cute boy feeling sad with Soft focus. This image is originally a Black & White negative. Film grain no added. Scanned film with grain. Some scratches and grain due to the age of the photo. Photo taken in 1980.

FIGURE 10.1 What is this a photo of?

Most people say it’s a picture of a little boy who is sad. Why did I ask you these questions?

System 1 And System 2 Thinking

Those two experiences—doing a multiplication problem in your head, and identifying the photo as a picture of a sad boy, feel very different. According to Daniel Kahneman, author of Thinking, Fast and Slow (2013), these are examples of the difference between System 1 and System 2 thinking.

When you looked at the photo of the little boy, and decided what it was, that was a quick and easy task. You didn’t have to think hard about it. That’s an example of System 1 thinking. System 1 thinking, Kahneman says, is quick, intuitive, easy, and effortless.

System 2 thinking is different. When you had to multiply the two numbers in your head, that was an example of System 2 thinking. System 2 thinking is difficult. It takes a lot of effort.

You can tell when people are doing System 2 (hard, effortful) thinking because their pupils dilate while they’re doing it.

System 1 Is The Normal Mode

Kahneman asserts in his book that System 1 thinking (quick, intuitive, easy thinking) is people’s natural or normal mode. Most of the time, most people are in System 1 mode.

System 2 thinking is activated only when people encounter something difficult to do (like multiplication in their heads). When that happens, System 1 gives up very quickly and turns to System 2 to take over.

If you are designing a product that doesn’t require people to think much, (browsing through their social media feed, looking at pictures of friends) you have a better chance at designing a product that is easy to use since most people are in System 1 mode most of the time. But if your product requires people to do System 2 thinking (filing their tax return, applying for car insurance, and so on) watch out. You will likely be designing for System 2 mode and it’s easy to forget that most people most of the time aren’t thinking that hard.

Triggering System 2 Thinking

So what do you do if you want people to think carefully about something before they take action on it? How can you get them into System 2 mode?

Most designers usually don’t make things difficult for the people using their product, For example, if someone is landing a plane, doing surgery, or operating a nuclear power plant, you may want her to be in System 2 mode for at least part of the task.

If it’s important that people carefully think something through, then you have to get them to switch into System 2 mode. You can kick people out of System 1 mode and into System 2 mode fairly easily.

Here are some examples of ways to do that:

  • Show them text in a font that is moderately hard to read, for example:
    Because this text looks like script and because there isn’t much contrast between the text and the background, it is harder to read. (This might not work well in the browser).
  • Give them a moderately hard math problem to do without any external aids, like the multiplication at the beginning of this chapter.
    Ask them to solve a word problem, such as:

    A bat and a ball cost $1.10 in total. The bat costs $1 more than the ball. How much does the ball cost?

The System 1 answer is that the ball costs $0.10 ($1.00 for the bat and another $0.10 for the ball). But if the bat costs $1.00 more than the ball, and if the ball is $0.10, then the bat would be $1.10 and the total would be $1.20. So the correct answer is that the ball costs $0.05 and the bat is $1.05, for a total of $1.10.

  • Here’s another one:
    • If it takes 5 machines 5 minutes to make 5 widgets, how long would it take 100 machines to make 100 widgets?

The System 1 answer is that it would take 100 minutes. But if you analyze the problem from a System 2 mode, if 5 machines make 5 widgets in 5 minutes, then it’s taking just 5 minutes to make the widgets. So if you had 100 machines making 100 widgets, it would still just take 5 minutes.

System 1 mode makes a lot of errors like this. If you want people to think carefully about something and not make these errors, you have to get them to switch to System 2 mode. You do that by making them think hard.

Design For Errors

When you’re working on a product, you’re usually immersed in the design. This means that you’re not looking at the product from the point of view of someone who is seeing or using it for the first time, or someone who hasn’t seen it or used it for three months. You know how everything works. The user doesn’t.

It’s easy in the design phase to underestimate how many errors people will make. They won’t find the button. They won’t remember where a link is located. They won’t remember what a label means.

This is exacerbated by System 1 thinking. People are just not thinking that hard. Assume they aren’t, and assume they’ll be making lots of mistakes. Your job then, as a designer, is to make sure that it’s easy to recover from mistakes.


  • Since most people use System 1 thinking most of the time, assume that they’ll make errors. Give them good feedback when they make mistakes and let them undo or easily change what they’ve done.
  • When you want people to think about something carefully, give them something difficult to do first, so they switch to System 2 thinking. Ask a difficult question or use a harder to read font so that System 2 will kick in.


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