The two lines

About this video

Look at two lines, one above the other. The top one ends in fins that open outwards, like the feathers at the tail of an arrow; the bottom one ends in arrowheads. Almost everybody says the top line is longer. The picture was first drawn by the German scientist Franz Müller-Lyer in 1889, and it still fools nearly everyone. Measure the two lines and they are exactly the same length. Then look again: for almost everyone, the top one still looks longer. Knowing the answer doesn't change what you see.

Scientists still debate which shortcut the eyes are using here. What matters is the kind of mistake it is. It isn't random: people don't guess the top line longer one day and shorter the next. Nearly everyone gets it wrong, in the same direction. That is what psychologists call a bias. A slip is like a dart that misses in any direction; a bias is a dart that misses the same way every time, because the throw itself is pulled.

A cognitive bias is a mistake in thinking, not only in seeing, that most people make again and again in a predictable direction. It is not stupidity: clever, careful people have biases too, because they have the same mental shortcuts. Confirmation bias and anchoring are two well-known ones.

Like the lines, a bias doesn't switch off when you learn its name. What you can do is learn where it lives, and then use a ruler: something outside your head that doesn't share your shortcut. Write a decision down, count instead of guessing, or ask someone else. This video explains cognitive bias on Knaph.

Transcript

1. 0:00Two lines. Which one is longer, the top or the bottom?
2. 0:06Measure them. They're exactly the same length.
3. 0:12And the top one still looks longer. Knowing doesn't fix it.
4. 0:18A slip misses any way. A bias misses the same way, every time.
5. 0:24A cognitive bias is a mistake in thinking that leans one way, in nearly everyone.
6. 0:31So use a ruler. Something outside your head.
7. 0:37Knowing its name won't switch it off. Use a ruler.