system one and two. Can you give us an example or tell us a bit about the characters in the book?

Well, the characters are indeed system one and system two. System one corresponds to

a distinction that everybody recognizes in their own thinking, that there are some thought

that just happen to you and there are some thought that you must generate. There is a

lot of mental life that is completely effortless, and then there is some of mental life that

feels like work. That distinction is obvious, and people recognize it.

When I say the word "mother," you have images probably of your mother and you certainly

have an emotional reaction, and that's something that happens to you. When I say, "Two plus

two," a number comes to your mind. You didn't bring it there. It just came. It happened

to you. There are many, you know—in fact, most of mental life is like that. The words

that I utter when I say this sentence, they just come to me. Sometimes I will stop and

choose which word—that's system two—but most of the time, when I speak the words just

come, so that's system one.

System two is—well, there are really two types of operations that system two performs,

and one is complex computations. That is where the pupil dilates and this is mental work.

Mental work is involved in short-term memory tasks. If I ask you, "What was your previous

telephone number," you'll work, and your pupil will expand by about 30 or 40 percent of its

area as you retrieve those. Then there is self-control, the inhibition of impulses.

When you are indeed choosing your words carefully because you don't want to offend, those are

situations in which system two is hard at work, and you feel it, so it corresponds...

System one and System two really correspond to experiences that are readily available

and that everybody recognizes. That distinction between something happening and something

that you do is, I think, pretty compelling to most people.

The dichotomy that you've drawn between system one and two, how does that relate to the previous

work you've done on heuristics and biases?

Well, it turns out we had—Amos Tversky and I, when we started our work, we had something

in mind that was fairly similar to that. We were interested in intuitive statistics, so

in estimates that come to people's mind about probabilities and so on. Now in many of these

cases—we were both teachers of statistics, so we were testing our own intuitions, but

we knew that we could compute. So in our very first paper, we distinguished intuition from

computation, and our point was that intuition is in some cases surprisingly error-prone

and that people should rely on computation.

That was the beginning, but we never studied what I now call system two. Then our work

became controversial, and people attacked it and criticized it. There was something

that essentially all the criticisms and all the experimental criticisms of our work had

in common, in that they created a situation in which people could figure out the answer

by working on it. That was really the background.

Amos Tversky and I in the very last paper that we wrote together, we answered one of

our very persistent and well-known critic, Gerd Gigerenzer. We pointed out that in his

experiments, typically people would see—well, how would I describe it? One of our best-known

examples in heuristics, and it's one of the best examples in the heuristics literature,

is the Linda example. Linda is that not-so-young woman. She's about 30 years old now, but I'm

telling you that when she was a student she was an activist, the feminist, marched in

all the marches. I didn't say feminist, actually. Then we asked people how likely it is that

Linda now is a bank teller, or how likely it is that she is now a bank teller and is

active in the feminist movement. Now there's no question that when you ask different people

those two questions, they will invariably say that it's more likely that she's a feminist

bank teller rather than a bank teller.

When you ask them the two questions to compare the two options, you're allowing system two

to check logic. By priming logical reasoning and by creating some—you can sensitize people

so that they will detect that obviously she is more likely to be a bank teller than a

feminist bank teller, but that seems to be a different process. When people see only

one example, they evaluate the fit of that example. When you show them two things together,

they can also compare, them and you provide another cue.

That was really the background to the distinction between the two systems with the controversy

around our work. It was an attempt to resolve that controversy by pointing out if you do

it between subjects and if you do it the way the world is—so you make judgments intuitively

about things, that they happen—you get those effects, and you can make them disappear by

allowing logic to play.

I've worked a lot with anchoring. That's a phenomenon. So somebody puts a number in your

head, and it looks plausible after a while. I mean, in fact, this is the way our mind

works. We hear something strange, we try to make sense of it. Trying to make sense of

it makes us more prone to believe it, so anchoring is a suggestion. In fact that's very powerful.

You can recognize when you're being anchored. So if you are in a negotiation situation and

the other side has an outrageous number, you know there is—you could become anchored,

and that is worth resisting.