January 29, 2003: Features

New brain research helps explain how we do – and don’t – reason

By Billy Goodman ’80


Photo: Jonathan Cohen, professor of psychology, and Joshua Greene *02, a former graduate student in philosophy, used a pair of ethical dilemmas — the “trolley dilemma” and the “footbridge dilemma” — to examine how people make moral judgements. (digitally enhanced photograph by Ricardo Barros)

Two months after psychology professor Jonathan Cohen came to Princeton, he received a visit from Joshua Greene, then a graduate student in the Department of Philosophy. Greene had an idea for some experiments to better understand what was happening in the brain during moral reasoning. Cohen, director of the university’s Center for the Study of Brain, Mind, and Behavior, was working to bring a powerful brain-imaging tool – rare outside medical centers – to Princeton. He was somewhat taken aback that a student in philosophy would be proposing an imaging experiment.

“It sounded crazy that a philosophy student would have anything concrete to do,” says Cohen, “but, as it turned out, it was an incredibly exciting and important project.”

The research, ultimately, was published about a year ago in Science, the most important broad-based science journal in the country, if not the world. The work addressed the role played by emotions in the moral judgments that people make. For a long time the dominant view among moral psychologists and philosophers – influenced largely by Immanuel Kant – has been that moral judgment is primarily a matter of reasoning. Recently, a different perspective has gained prominence, led by researchers such as University of Virginia psychologist Jonathan Haidt. Haidt argues that moral thinking is highly intuitive and emotional, and only appears to be a product of careful reasoning because of people’s after-the-fact rationalizations of their thinking. Greene’s experiments support the view that at least some kinds of moral judgments strongly engage the emotions.

The work was inspired by a family of ethical dilemmas that have tantalized philosophers. Here is a pair:

The trolley dilemma features a runaway trolley that will kill five people on the track ahead if it continues on its course. The only way to save the five people is for you to hit a switch that will turn the trolley onto a side track where it will kill one person. Do you hit the switch, thereby saving five people but leading to one death?

The footbridge dilemma is similar, with a trolley threatening to kill five people. You are standing on a footbridge over the tracks, next to a large stranger. If you push the stranger onto the tracks, killing him, his body will prevent the train from reaching the others, saving them. Do you push?

Most people answer yes to the first question, no to the second. Philosophers have puzzled over why people believe it is morally acceptable to sacrifice one life for five in one case, but unacceptable in the other. Greene, now a postdoctoral researcher in Cohen’s lab, notes that solving this puzzle has proven to be a formidable philosophical challenge. And this, he explains, gives rise to a distinct psychological puzzle: “The fact that it’s tough to justify in rational terms raises the question: How do people arrive at these conclusions?”

In search of an answer, Greene, working with Cohen and others in the Center and the psychology department, used the new magnetic resonance imaging scanner, which Cohen succeeded in bringing to Princeton, to measure the neural activity of people as they pondered these dilemmas. (The scanner is related to diagnostic MRI machines, with the added ability to measure brain activity as volunteers perform mental tasks; hence it is known as functional magnetic resonance imaging or fMRI.)

Research projects in psychology and neuroscience these days frequently include fMRI scanning because it provides a chance to test hypotheses about the involvement of brain regions in various cognitive activities. For example, a group of structures in the center of the brain make up the limbic system, one seat of the emotions. The outer folds of the brain are the cerebral cortex, which houses regions devoted to vision, speech, and memory. The frontal lobes of the cerebral cortex are where the most complicated forms of reasoning and problem-solving take place. All complex tasks, such as thinking or forming and responding to emotions, require the involvement of many brain regions. But finding that a region of the brain is, or is not, involved in a cognitive task opens up possibilities for understanding brain function and dysfunction.

The Princeton researchers found that problems of the footbridge type – which they called “personal” moral dilemmas because they require active, personal involvement – activated areas of the brain associated with emotion, much more consistently than problems like the “impersonal” trolley dilemma, which activated brain areas associated with memory. Moreover, says Greene, the few people who said it’s OK to push a person off a footbridge to save five others tended to take more than twice as long to make the decision.

Greene says the study is another piece of evidence that intuitive emotional reactions affect moral judgments. “In the footbridge case, most people say ‘no,’ but people who say ‘yes’ take a long time,” he says. “According to our model, you’ve got an emotional response saying, ‘no, no, no,’ so anyone who’s going to say ‘yes’ will have to fight that response. Whatever is going on in the brain that is different in these two cases is really affecting people’s judgments, and you can see it in how it slows people down when they go against emotion.”

The “Linda problem”

Photo: Professor Daniel Kahneman, winner of the Nobel Prize in economics. His research found heuristics frequently fail people, leading to errors in reasoning. (Frank Wojciechowski)

This notion that reasoning, judgment, and decision-making often can be less than rational has a long history in psychology, and it is permeating other fields. Greene’s work in philosophy is one example. Another is the work of psychology professor Eldar Shafir, whose research in behavioral economics looks at how people make choices and decisions under conditions of uncertainty or conflicting information.

Shafir’s colleague, Daniel Kahneman, the Eugene Higgins Professor of Psychology and Public Affairs, won the 2002 Nobel Prize in Economics for his trail-blazing work in behavioral economics. Kahneman and his longtime Stanford University colleague Amos Tversky, who died in 1996, wrote papers over 25 years examining reasoning and decision-making under conditions of uncertainty. They found that most people use heuristics – quick rules of thumb – when thinking about many sorts of problems. And while these heuristics often are good enough for the decisions people make every day, they also frequently lead to repeatable errors.

Kahneman and Tversky have had great influence in economics, where standard models of economic behavior assume that people will act rationally in a specific way, that is, to maximize “expected utility,” or satisfaction. Instead, the pair showed that when forced to reason about problems where relevant information is uncertain or perplexing, people fall back on heuristics that frequently fail them.

A common example that trips up the statistically naïve, as well as professionals who should know better, is the gambler’s fallacy. Let’s say a fair coin turns up heads five times in a row. Many a gambler would bet on the next toss to be a tail, despite the fact that each toss is independent and has an equal chance of being heads or tails. Kahneman and Tversky showed that scientists also fall into this sort of faulty reasoning when they have undue confidence in the results of a small experiment – such as a clinical trial with few patients – that may be due to chance alone. Such reasoning is like flipping five heads in a row and concluding that the coin was biased.

Although humans invented the concept of probability, it seems many people do not apply it properly, at least when thinking intuitively and making snap judgments. A famous demonstration of this is what Kahneman and Tversky called “the Linda problem”:

Linda is 31 years old, single, outspoken, and very bright. She majored in philosophy. As a student, she was deeply concerned with issues of discrimination and social justice and also participated in antinuclear demonstrations.

Kahneman and Tversky wrote this description and listed eight possible occupations or activities for this fictional Linda, including three of real interest to the researchers – that she is active in the feminist movement, that she is a bank teller, or that she is a bank teller active in the feminist movement – and five distracting fillers. Test subjects were asked to rank the eight possibilities from most likely to least likely. Subjects consistently rank the possibility that Linda is a bank teller active in the feminist movement as more likely than that Linda is a bank teller.

That, of course, is impossible, since the category “bank teller” includes the category “bank teller active in the feminist movement.” Kahneman discussed the problem recently in his Wallace Hall office, while leaning back in a leather easy chair, the only luxury in the spartan space. He noted that the problem had been widely debated in the 20 years since he and Tversky published it, and that much nonsense had been written about it. He and Tversky were perplexed that the overwhelming number of people failed to reason proficiently on this problem, including people who should have known better. “We never predicted that people would be blind,” he says.

As a result of the Linda problem and many others designed to elicit certain heuristic or intuitive judgments, Kahneman believes that “people will think intuitively unless they have very strong cues pointing them to apply logic.”

What’s for dinner?

Say you entered a restaurant, took your seat, and as you perused the menu the waiter sidled up and said, “You can have the fish, or else not both the soup and the salad.” You exchange befuddled looks with your companions and wonder whether waiters these days are out-of-work logicians rather than unemployed actors.

But, it turns out, the waiter is Stuart Professor of Psychology Philip Johnson-Laird, and he simply wants to know what you want to eat. What is possible? Because of the force of “or else,” it is not obvious that one possibility, according to the waiter’s instructions, is that you can have all three dishes at once. (See answers, page 31).

Johnson-Laird uses these sorts of intricate logical puzzles to help provide clues to the ways people think. A longstanding view in philosophy and psychology is that humans are intrinsically rational and follow logical laws “tacitly represented in the brain,” he says. In that view, reasoning is akin to applying formal rules of inference to propositions. (One famous rule, for example, is called “modus ponens’’: Given two propositions “if a, then b” and “a,” one can conclude, “then b.”) Johnson-Laird himself once held that view – that people followed such rules of thinking – and it was taken for granted 30 years ago.

No longer. “People discovered that the content of what you’re reasoning about can affect the conclusions that you draw,” he says. In other words, people apply general and background knowledge to reasoning tasks. This finding was, he says, “mildly embarrassing” to those who said we reason using formal rules of inference, because such rules should be blind to content.

An alternative view, promoted by Johnson-Laird and colleagues, comes close to common sense, he admits. When we reason, he says, “we think about possibilities that are compatible with the premises that we’re reasoning from.” In practice, that means that people focus on what is true or possible from a given set of premises; they often fail to consider what they believe must be false. For example, ponder this problem:

In a hand of cards, only one of the following three assertions is true: There is a king or an ace or both in the hand. There is a queen or an ace or both in the hand. There is a jack or a ten or both in the hand. Is it possible that the hand contains an ace?

Johnson-Laird and his colleagues have given this problem to many smart people and most answer, “yes.” They are wrong: If the first assertion is true, the second and third must be false – so there is no ace. The same reasoning applies if the second assertion is true.

Johnson-Laird calls this theory of reasoning, in which people focus on what’s possible, a “mental models” approach. It lends itself to various predictions, such as, the more possibilities we have to think about in solving a problem, the harder the task should be. “We can’t hold in mind many possibilities,” he says, which gets in the way of promptly understanding assertions of the sort the waiter made about the fish, the soup, and the salad.

He recently teamed up with two Princeton colleagues on an experiment using fMRI to look for regions of the brain that perform logical reasoning and to look for evidence that might favor one of the competing theories — rules of inference or mental models — for how logical reasoning is carried out in the brain.

If logical reasoning primarily involves applying rules of inference to sentences, then language areas of the brain in the left hemisphere ought to be active. If instead people reason mainly by imagining possibilities in mental models, then regions of the right hemisphere thought to be involved in spatial representation should be more active.

To study the issue, Johnson-Laird, postdoctoral researcher James Kroger (now at the University of Michigan), and Center director Cohen devised problems for volunteers to solve as their brains were scanned. The problems included both easy and hard logic problems. The researchers surmised that language areas of the brain initially would be active for both kinds of logic problems, as solvers tried to understand them. But, says Johnson-Laird, the hard problems prompted volunteers to search for counterexamples to solve them; would that be evident in a brain scan?

Here are the premises for one series of problems they used:

There are five students in a room. Three or more of these students are joggers. Three or more of these students are writers. Three or more of these students are dancers.

An easy logic problem asks: Does it follow that at least one of the writers in the room is a student? (See answers, #2.) A hard logic problem asks: Does it follow that at least one of the students is all three? (#3)

The easy problem can be answered just by understanding the premises. The hard problem prompts a search for a counter-example – any single arrangement of the five students and their activities in which none is simultaneously a jogger, writer, and dancer. When people looked for counterexamples, says Kroger, they were making mental models and checking them until a counterexample was found. By noting regions active only during counterexample problems, “we were able to see where in the brain mental models were made.”

As expected, they confirmed the finding, from physicians who treat patients with frontal lobe damage, that the frontal lobes are strongly involved in logical and mathematical thinking. More noteworthy, there were some areas in the right frontal lobe active only during logical reasoning, and areas of the left frontal lobe active only during mathematical calculation.

And, it turns out, when subjects were thinking of counter-examples while solving the hard logic problems, they appeared to use an area roughly the size of a quarter, about an inch above and behind the right eye. This region, called the frontal pole, was active only during the counterexample problems.

For Kroger, the results are exciting because they suggest the most forward parts of the brain are crucial to complex mental processes, a subject he continues to work on. And the results lend support to Johnson-Laird’s mental models theory as psychologists continue to debate how people think.

Quick thinking

Professor Susan Fiske displays a model of the brain. Fiske studies prejudice and social stereotyping. (Ricardo Barros)

Try this problem: A bat and a ball together cost $1.10. The bat costs $1 more than the ball. How much does the ball cost? (#4)

If you answer 10 cents, as many Princeton undergraduates do, you’ve given an answer that seems reasonable but that you will discover, upon checking, is wrong.

“We do a lot of thinking without checking,” says Kahneman. “My guess is, you’ll find brain differences between when people check and when they don’t. Localizing those differences will be interesting.”

Following increasingly common usage, Kahneman called quick, intuitive thinking system 1 and more controlled, reflective thinking system 2, a distinction that some trace to William James in the 19th century. Moreover, he expects to see evidence of these systems in the brain. The system-1 / system-2 distinction, he says, “is going to be tied to the brain, or it will vanish.” In other words, if scientists don’t find evidence of these two distinct processes in the brain, they will have to abandon or modify the notion of two systems of thinking.

That distinction – between rapid, intuitive thought and deliberate reasoning with feedback – crops up widely in psychology. For instance, social psychologist and Princeton professor Susan Fiske, who studies prejudice and stereotyping, says that intergroup biases are “surprisingly automatic.” People, she notes, categorize other people based on age, race, and gender in “milliseconds” and then have visceral responses to those categories in milliseconds, too.

According to Fiske, researchers have found that when people are shown faces of other racial groups, even when presented subliminally, too quickly to be consciously perceived, they have a reaction that includes a response in a part of the brain called the amygdala, centrally located in the brain’s limbic system, where emotions are generated. This stereotyping does not mean that everyone is a racist, Fiske says, but is the result of learned, socially constructed categories and living in a culture where racial stereotyping is common.

Fiske and Elizabeth Wheeler *02, then a graduate student, found that the amygdala response could be eliminated by changing the context or inviting a more careful, system 2-like deliberation. They interfered with the automatic response by flashing a vegetable name immediately before showing a face and asking subjects to say whether the person pictured would like the vegetable or not.

Fiske also has shown that automatic stereotyping reactions can be reduced or eliminated if people are sufficiently motivated. For example, a member of an athletic team that includes someone from a different racial or social group is much less likely to stereotype that person. And a person motivated by values of fairness or harmony may employ a more “individuating kind of process’’ to override the automatic response, says Fiske.

Her work tests her theory that two distinct processes exist for making sense of people – one relatively automatic, quick, and categorical, the other slower, more controlled, and susceptible to motivation and context. Before the fMRI scanner came to Princeton, Fiske’s research relied on questionnaire responses, measures of attention, and nonverbal cues to discomfort. Increasingly, she uses fMRI to look for evidence of those two systems in the brain, extending her own and others’ earlier work that found an amygdala response to unfamiliar faces of another race.

Imaging, Fiske says, “is one more tool in our toolbox” for understanding how the brain constructs, and is influenced by, social relationships. Indeed, scientists at the Center, who come from many Princeton departments as well as other universities and even pharmaceutical companies, are using a variety of research techniques to better understand the relationship of brain and mind, and how both lead to behavior.


Billy Goodman ’80 is a writer who lives in Montclair, New Jersey.

For more logic puzzles click here.


When subjects were asked to categorize pictures of faces by age (over or under 21), they generally had increased amygdala activation — a typical alarm response — when those pictures were of unfamiliar people of a difference race (left). But, if the subjects first had to concentrate on a task that distinguished one person from another, for example, predicting whether the person pictured would like a particular vegetable, the cross-race amygdala response disappears (right). (Elizabeth Wheeler and Susan T. Fiske)


Having my head examined

In the quest for knowledge, a PAW writer has his brain scanned

Photo: Do not be alarmed by the lack of activity in writer Billy Goodman’s brain. He probably was just dozing. (Princeton Psychology Department)

Phil got $10 and offered to split it with me. The “split,’’ however, was $9 for him and $1 for me. In my head I said a phrase that translates roughly as “over my dead body,” and I pressed a button with my left middle finger – which meant that I was rejecting his offer. Which meant that neither of us got a dime.

I was participating, off the books, in a study run by two postdoctoral researchers in the psychology department, Jim Rilling and Alan Sanfey. They are investigating the neural correlates of the perception of fairness, or lack of it, in social interactions. In other words, what’s going on in your brain when someone stiffs you, as Phil did me?

To visualize what is happening in the brain, Sanfey and Rilling place volunteers inside a brain scanner in the basement of Green Hall. That is how I came to be flat on my back, my head inside a big donut of a superconducting magnet, as I contemplated the insulting proposition from Phil, a Princeton undergraduate who made his offer not in person, but via a computer screen. The magnet is part of a machine that does functional magnetic resonance imaging (fMRI), which is like a diagnostic MRI scanner with the addition of being able to measure brain activity.

Lying as still as possible in the narrow confines of the magnet, I was able to view, with a mirror mounted over my head, a computer screen where I could read each offer as it was made and see a picture of who made it. I accepted or rejected offers by pressing the proper buttons on a special glove I wore. The offers are part of the well-known – to psychologists and economists – Ultimatum game. One person receives money and proposes to split it any way he wants with a responder. The catch: If the responder rejects the split, neither person gets any money.

Rationally, economists know that the responder should take any sum; it’s free money. Psychologists know, however, that grossly unfair splits are often rejected. Sanfey and Rilling want to know what’s going on in people’s heads when they receive an unfair offer. Are they indignant, and does that show up in brain activity?

The two-year-old fMRI scanner – one of the first of its kind outside a medical center – is able to measure brain activity as subjects perform tasks, such as accepting or rejecting monetary offers. Specifically, as a part of the brain becomes more active, there is an increase in blood flow to that area. The machine, which was loudly scanning my brain every three seconds, detects that blood-flow response, and computer software later turns it into a color-coded map of the brain showing regions with more activity during a particular task.

Rilling and Sanfey are finding that people who reject grossly unfair offers seem to have more activity in a part of the brain called the anterior insula than people who do not reject such offers. The anterior insula is implicated in various negative emotional states and may be activated when we feel indignant. Because the undergraduate volunteers who play the game keep any money they earn, they must be truly annoyed to reject unfair offers and forgo real money.

By Billy Goodman ’80



1. (page 29) The assertion, “You can have the fish, or else not both the soup and the salad,” is difficult to understand because “or else,” “not,” and “both” create multiple possibilities that are difficult to keep in mind at once. To understand what is possible, substitute for some of the terms — a technique that is widely used in problem-solving. If you were told you could have “the fish or else the meat,” you would know you could have one or the other, not both. If you could have “the fish or else not the meat,” you could still have one or the other, in this case “fish” or “not the meat.” You could, however, have “fish” and “meat” together. If you substitute “both the soup and the salad” for “meat,” you’ll see that you can have fish, soup, and salad together.

2. (page 30) Of course. We told you it was an easy problem.

3. (page 30) No. The easiest way to see this is to represent the five students by the numbers 1 through 5, and list their activities beneath them. Make three joggers, three writers, and three dancers, without any one being all three. That’s a counterexample.

4. (page 31) Five cents. Even if you got the right answer, you must admit that your brain immediately screamed “10 cents.” Remember what your elementary school math teachers told you: Check your work!

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