But if you have a gut feeling about whether you should buy Google stock, that's not useful at all, because intuitions are only reliable in the areas of life where you get good feedback. And you know just from being a human being and from interacting with people your whole life what the cues are for trustworthiness. Am I sitting there with my eyes shifting all over the place? Am I drumming my foot on the floor? Do I not look at you when I talk to you? Do I immediately ask you for your credit card number? Those kinds of things just set off fire alarms in your head, as they should, but there's no way to do that in the stock market—it's just much too complicated an organism. And every time you think you've got some cue that predicts something, the problem is there are a hundred million other people combing through the same data looking for it, and they've already been there. By the time you notice it, it either isn't really there or other people have already used it. In either case, it's not useful to you, but you'll think it will be.

JoI: You make the point in the book about how making money produces a similar reaction in the brain to when an addict takes drugs or a gambler wins. Did you see any studies or experiments along these lines with fund managers or other financial professionals who are dealing with other people's money?

Zweig: Well, there's very little reason to believe that professionals and individual investors' brains are much different. There's been a lot of psychological research done on this. There isn't much in neuroeconomics yet, but based on 20 years of observing the financial markets, I certainly don't see any evidence that professionals are more rational investors than individuals. There's certainly a fair amount of anecdotal evidence that they're less rational, but they're certainly not more. And there's no real reason why you would expect them to be.

JoI: You make the point in the book about how making money produces a similar reaction in the brain to when an addict takes drugs or a gambler wins. Did you see any studies or experiments along these lines with fund managers or other financial professionals who are dealing with other people's money?

Zweig: The really surprising thing is how little we know about how we think. J.P. Morgan once said that every man has two reasons for everything he does: the reason he states and the real reason. I think he meant something a little different by it, but what a neuropsychologist or a neuroeconomist would say is that most of us don't even know why we do things, and we can often be in the grip of unconscious emotion or unconscious biases, feelings and inclinations that are in our mind but we have no awareness of. You feel it; you just can't articulate it, and you may not be aware that it's there until after it passes. This is one of the hardest ideas you can ever get someone to admit.

For example, if you're watching CNBC and the market is plunging and Jim Cramer is throwing furniture and biting the heads off live chickens, you may be sort of watching it saying, ''Oh wow, something really bad is happening; the market is crashing.'' But while you're watching it, your palms are sweating, your breath is coming fast, your pulse is racing, your muscles are tensing, your entire body is on red alert. You're intensely upset by what's happening in front of you, but the thinking part of your brain is so busy trying to make sense of it that it's not aware of what 18 July/August 2008 the emotional part of your brain is experiencing. And if in that moment you are suddenly called on to make a choice, ''Should I sell this stock or should I hold it?'' ... If you're making that choice at that moment while Jim Cramer is screaming in your face, you will not buy and it's highly unlikely that you'll hold ... because all of that screaming, all the red, all the downward-pointing lines are so upsetting that you will make a negative decision, even if you're not aware at that moment of how upset you are.

The flip side of this is unconscious bias. Just as you can have a feeling that you're not aware of having, you also can have preferences that you don't realize you have. The simplest example is what psychologists call ''implicit egotism,'' which is a really bad term for liking whatever is closest to you in some way or another. For example, people are 65 percent more likely to marry someone whose surname begins with the same initial as their own. Psychologists have looked at hundreds of thousands of data points and demonstrated very clearly that this is true, and that people named Dennis and Denise are much more likely to become dentists than you would expect by random chance. People named George are more likely to become a geoscientist then you would expect by chance alone. We all come with these strange, unconsciousness preferences. We don't think we think that way, but we do.

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