DW: Well, all of these terms are a little bit blurry. I use it to mean two related but different things. First, there's common sense knowledge, by which I mean the collection of facts and rules and pieces of received wisdom that we collect in the course of everyday life, such as knowing that when you walk into the elevator you don't face the rest of the people, or you don't go to work naked.

But the other kind of common sense I refer to is what you might call common sense reasoning, which is how we reason about cause and effect and how we explain the things we observe.

The two kinds of common sense are related because a lot of things that sound like statements of fact that turn out to contain hidden reasoning processes about cause and effect. For example, it sounds like common sense fact to say that the police are more likely to investigate incidents of serious crime than non-serious crime. But it turns out that there's a huge correlation between the crimes that police investigate and the socioeconomic status of where the neighborhood where the crime takes place. It's not surprising when you think about it: wealthy people in wealthy suburbs are more likely to report crimes that happen, police are more likely to respond to those crimes because they come from people who are wealthy and who are likely to vote, and who have resources at their disposal if they're not happy.

As a result, the crimes that are reported by those people are more likely to get classified as serious, because the police investigated them. Thus the common sense statement that people are more likely to investigate serious crimes is true, but the causal arrow is the opposite of what commonsense reasoning would suggest (i.e. that police investigate crimes because they are more serious).

This is one of the problems of common sense. It blurs the distinction between statements about how things are, and statements about how things come to be.

CF: So everything is obvious if we agree with it?

DW: Almost. We can agree that something is common sense as long as we share the same set of assumptions. I think people really don't understand how much of a problem this is, because they assume that what they think is common sense is right.

Someone was just telling me the other day that she was mystified at how we couldn't just use common sense to make policy. It seemed completely ridiculous to her--why isn't there someone in politics who could say that It's common sense that everyone should have equal access to healthcare? But that's a belief that is deeply disputed! And people who disagree with her also feel that they are using common sense.

So common sense is actually very good for resolving everyday situations, where everyone shares the same set of assumptions. The problem is that it feels so effective to us in these circumstances that we're tempted to use it to make decisions and plans and predictions about situations that are not everyday situations. We want to use it to make decisions that are about people who are very different from us, who are interacting with each other in complex ways over extended intervals of time.

This sounds abstract, but it's precisely what you encounter when you make policy. For example: How can we change the incentives that doctors face when they deal with patients to improve the efficiency and effectiveness of health care?

To many politicians, this all sounds like an exercise in common sense. As Bill Frist, former United States senator from Tennessee, put it in an op-ed piece that I cite, "This is not rocket science." But that's really, really misleading. Because you're not talking about a problem that's like you disciplining your child, or you trying to figure out how to get a person whom you know very well to do something that you want him to do. This is about you making a set of rules that are going to apply to hundreds of thousands of individuals who are all very different, and who have different types of circumstances and live in different parts of the country. As soon as you try to apply your common sense to solve this problem, you're going to fail, and you're going to generate all kinds of unanticipated consequences.

CF: So common sense is very adaptive for certain situations but maladaptive for others, and we don't understand the distinction?

DW: Right. The problem is that when thinking about a very large-scale, abstract problem, what we tend to do is reduce it down to a single scenario, like, Oh, I go to the doctor. And how would my doctor respond differently if he were being paid this way or that way? But very often there is some other factor that we didn't put into our simulation that turns out to be important, and that means our prediction will be wrong.

CF: How else can common sense go wrong?

DW: If our model of individual behavior is flawed in this way, our model of collective behavior is even worse. When marketers think about their demographic, they actually construct stories about individual people. For example, when they're trying to come up with ideas for how to sell a particular product, they might say something like "Emily is a 27-year-old woman who lives in Chicago and she has a four-year college education, and she's just gotten engaged to Doug." Then they ask, "How are we going to sell this product to Emily?"

But of course Emily doesn't exist. In reality you have this very diverse population of people who have all kinds of needs and incentives and who are also interacting with each other in ways that are hard to anticipate. Stories like the one about Emily paper over all of that complexity, effectively replacing the whole system with a single individual, a "representative individual" and then try to reason about their behavior as if it were an actual person. It's a big error.

CF: Why do advertisers and policy makers keep making this error, then?

DW: It's a good question: If we're making such obvious mistakes with common sense, why are the mistakes not obvious? When we study physics, we have all of this common sense intuition that we bring to physics class. But very quickly, you realize that your intuition sucks. You can't guess the answers. You have to work through the math.

But the social world is different. To begin with, we have a lot more intuition, so we're much better at coming up with explanations than we are in physics. Even then, we still get things wrong for the reasons I've been describing, but this is where the third problem of common sense reasoning comes into play: there's something about how we learn from history that prevents us from realizing what it is that we're doing wrong.

Take what's happening now in the Middle East. Once something has come to an end, we'll construct a story about what happened. It's probably going to involve Facebook, and it's probably going to involve the Google exec Wael Ghonim and his cohort of tech-savvy young leaders who got together in Egypt, and some rebel leaders we haven't yet named, and very likely some critical battle or protest--and it going to involve these things not because they actually explain anything but because that's how we tell stories. Stories have critical moments where things change, they have critical characters around whom the action revolves, and they have a beginning, middle, and end.