Trolley problem
The trolley problem is a thought experiment
Thought experiment
A thought experiment or Gedankenexperiment considers some hypothesis, theory, or principle for the purpose of thinking through its consequences...

 in ethics
Ethics, also known as moral philosophy, is a branch of philosophy that addresses questions about morality—that is, concepts such as good and evil, right and wrong, virtue and vice, justice and crime, etc.Major branches of ethics include:...

, first introduced by Philippa Foot
Philippa Foot
Philippa Ruth Foot was a British philosopher, most notable for her works in ethics. She was one of the founders of contemporary virtue ethics...

, but also extensively analysed by Judith Jarvis Thomson
Judith Jarvis Thomson
Judith Jarvis Thomson is an American moral philosopher and metaphysician, best known for her use of thought experiments to make philosophical points.- Career :...

, Peter Unger
Peter Unger
Peter K. Unger is a contemporary American philosopher and professor at New York University. His main interests lie in the fields of metaphysics, epistemology, ethics, and the philosophy of mind. He attended Swarthmore College at the same time as David Lewis, earning a B.A. in philosophy in 1962,...

, and Frances Kamm
Frances Kamm
Frances M. Kamm is a philosopher specialising in normative and applied ethics. Kamm is currently the Littauer Professor of Philosophy and Public Policy at the John F. Kennedy School of Government at Harvard University and Professor of Philosophy in the Faculty of Arts & Sciences at Harvard...

. Outside of the domain of traditional philosophical discussion, the trolley problem has been a significant feature in the fields of cognitive science
Cognitive science
Cognitive science is the interdisciplinary scientific study of mind and its processes. It examines what cognition is, what it does and how it works. It includes research on how information is processed , represented, and transformed in behaviour, nervous system or machine...

 and, more recently, of neuroethics
Neuroethics is the ethics of neuroscience, and the neuroscience of ethics.The ethics of neuroscience deals with matters as a subclass of bioethics...



Foot's original formulation of the problem ran as follows:
A trolley
A tram is a passenger rail vehicle which runs on tracks along public urban streets and also sometimes on separate rights of way. It may also run between cities and/or towns , and/or partially grade separated even in the cities...

 (i.e. in British English a tram) is running out of control down a track. In its path are five people who have been tied to the track by a mad philosopher. Fortunately, you could flip a switch, which will lead the trolley down a different track to safety. Unfortunately, there is a single person tied to that track. Should you flip the switch or do nothing?

A utilitarian
Utilitarianism is an ethical theory holding that the proper course of action is the one that maximizes the overall "happiness", by whatever means necessary. It is thus a form of consequentialism, meaning that the moral worth of an action is determined only by its resulting outcome, and that one can...

 view asserts that it is obligatory to flip the switch. According to simple utilitarianism, flipping the switch would be not only permissible, but, morally speaking, the better option (the other option being no action at all). An alternate viewpoint is that since moral wrongs are already in place in the situation, flipping the switch constitutes a participation in the moral wrong, making one partially responsible for the death when otherwise the mad philosopher would be the sole culprit. An opponent of action may also point to the incommensurability
Commensurability (ethics)
In ethics, two values are incommensurable when they do not share a common standard of measurement.Philosophers argue over the precise nature of value incommensurability, and discussions do not always exhibit a consistent terminology...

 of human lives. Under some interpretations of moral obligation
Duty is a term that conveys a sense of moral commitment to someone or something. The moral commitment is the sort that results in action and it is not a matter of passive feeling or mere recognition...

, simply being present in this situation and being able to influence its outcome constitutes an obligation to participate. If this were the case, then deciding to do nothing would be considered an immoral act if one values five lives more than one.

Related problems

The initial trolley problem becomes more interesting when it is compared to other moral dilemmas.

The fat man

One such is that offered by Judith Jarvis Thomson
Judith Jarvis Thomson
Judith Jarvis Thomson is an American moral philosopher and metaphysician, best known for her use of thought experiments to make philosophical points.- Career :...

As before, a trolley is hurtling down a track towards five people. You are on a bridge under which it will pass, and you can stop it by dropping a heavy weight in front of it. As it happens, there is a very fat man next to you - your only way to stop the trolley is to push him over the bridge and onto the track, killing him to save five. Should you proceed?

Resistance to this course of action seems strong; most people who approved of sacrificing one to save five in the first case do not approve in the second sort of case. This has led to attempts to find a relevant moral distinction between the two cases.

One clear distinction is that in the first case, one does not intend harm towards anyone - harming the one is just a side effect
Unintended consequence
In the social sciences, unintended consequences are outcomes that are not the outcomes intended by a purposeful action. The concept has long existed but was named and popularised in the 20th century by American sociologist Robert K. Merton...

 of switching the trolley away from the five. However, in the second case, harming the one is an integral part of the plan to save the five. This is an argument Shelly Kagan
Shelly Kagan
Shelly Kagan is the Clark Professor of Philosophy at Yale University and the former Henry R. Luce Professor of Social Thought and Ethics. Originally a native of Skokie, Illinois, he received his B.A. from Wesleyan University and his Ph.D. from Princeton University under the supervision of Thomas...

 considers, and ultimately rejects, in The Limits of Morality.

So, some claim that the difference between the two cases is that in the second, you intend someone's death to save the five, and this is wrong, whereas in the first, you have no such intention. This solution is essentially an application of the doctrine of double effect, which says that you may take action which has bad side effects, but deliberately intending harm (even for good causes) is wrong.

On the other hand, Thomson argues that an essential difference between the original trolley problem and this version with the fat man, is that in the first case, you merely deflect the harm, whereas in the second case, you have to do something to the fat man to save the five. Thomson says that in the first case, nobody has any more right than anyone else not to be run over, but in the second case, the fat man has a right not to be pushed in front of the trolley.

Act utilitarians deny this. So do some non-utilitarians such as Peter Unger, who rejects that it can make a substantive moral difference whether you bring the harm to the one or whether you move the one into the path of the harm. Note, however, that rule utilitarians do not have to accept this, and can say that pushing the fat man over the bridge violates a rule to which adherence is necessary for bringing about the greatest happiness for the greatest number.

Another distinction is that the first case is similar to a pilot in an airplane that has lost power and is about to crash and currently heading towards a heavily populated area. Even if he knows for sure that innocent people will die if he redirects the plane to a less populated area - people who are "uninvolved" - he will actively turn the plane without hesitation. It may well be considered noble to sacrifice your own life to protect others, but morally or legally allowing murder of an innocent person in order to save five people may be insufficient justification.

The fat villain

The further development of this example involves the case, where the fat man is, in fact, the villain who put these five people in peril. In this instance, pushing the villain to his death, especially to save five innocent people, seems not just moral, but, to some, also just and even an imperative. This is essentially related to another famous thought experiment, known as ticking time bomb scenario
Ticking time bomb scenario
The ticking time bomb scenario is a thought experiment that has been used in the ethics debate over whether torture can ever be justified.Simply stated, the consequentialist argument is that nations, even those such as the United States that legally disallow torture, can justify its use if they...

, which forces one to choose between two morally questionable acts. Several papers argue that ticking time bomb scenario is a mere variation trolley problem.

The track that loops back

The claim that it is wrong to use the death of one to save five runs into a problem with "loop" variants like this:
As before, a trolley is hurtling down a track towards five people. As in the first case, you can divert it onto a separate track. On this track is a single (fat) person. However, beyond that person, this track loops back onto the main line towards the five, and if it weren't for the presence of that (fat) person, who will stop the trolley, flipping the switch would not save the five. Should you flip the switch?

The only difference between this case and the original trolley problem is that an extra piece of track has been added, which seems a trivial difference (especially since the trolley won't travel down it anyway). So intuition may suggest that the answer should be the same as the original trolley problem – one may flip the switch. However, in this case, the death of the one actually is part of the plan to save the five.

The loop variant may not be fatal to the "using a person as a means" argument. This has been suggested by M. Costa in his 1987 article "Another Trip on the Trolley", where he points out that if we fail to act in this scenario we will effectively be allowing the five to become a means to save the one. If we do nothing, then the impact of the trolley into the five will slow it down and prevent it from circling around and killing the one. As in either case, some will become a means to saving others, then we are permitted to count the numbers. This approach requires that we downplay the moral difference between doing and allowing.

However, this line of reasoning is no longer applicable if a slight change is made to the track arrangements such that the one person was never in danger to begin with, even if the 5 people were absent. Or even with no track changes, if the one person is high on the gradient while the five are low, such that the trolley cannot reach the one. So the question has not been answered.

Even in the situation where the people aren't tied down due to a criminal act, but simply happen to be there without the ability to warn them, the out-of-control trolley is similar to the out-of-control airplane. Either 5/500 or 1/100 people are going to die as a result of the accident already in progress, and it is important to minimize the loss of life, despite the fact that the 1/100 are effectively being "used" to spare the life of the 5/500. The 100 people (and their property) in the less-densely-populated area do in fact stop the plane too. Responsibility for this goes back to any criminal negligence that caused the accident to occur in the first place.


Here is an alternative case, due to Judith Jarvis Thomson, containing similar numbers and results, but without a trolley:
A brilliant transplant surgeon has five patients, each in need of a different organ, each of whom will die without that organ. Unfortunately, there are no organs available to perform any of these five transplant operations. A healthy young traveler, just passing through the city the doctor works in, comes in for a routine checkup. In the course of doing the checkup, the doctor discovers that his organs are compatible with all five of his dying patients. Suppose further that if the young man were to disappear, no one would suspect the doctor.

The man in the yard

Unger argues extensively against traditional non-utilitarian responses to trolley problems. This is one of his examples:
As before, a trolley is hurtling down a track towards five people. You can divert its path by colliding another trolley into it, but if you do, both will be derailed and go down a hill, and into a yard where a man is sleeping in a hammock. He would be killed. Should you proceed?

Responses to this are partly dependent on whether the reader has already encountered the standard trolley problem (since there is a desire to keep one's responses consistent), but Unger notes that people who have not encountered such problems before are quite likely to say that, in this case, the proposed action would be wrong.

Unger therefore argues that different responses to these sorts of problems are based more on psychology than ethics – in this new case, he says, the only important difference is that the man in the yard does not seem particularly "involved". Unger claims that people therefore believe the man is not "fair game", but says that this lack of involvement in the scenario cannot make a moral difference.

Unger also considers cases which are more complex than the original trolley problem, involving more than just two results. In one such case, it is possible to do something which will (a) save the five and kill four (passengers of one or more trolleys and/or the hammock-sleeper), (b) save the five and kill three, (c) save the five and kill two, (d) save the five and kill one, or (e) do nothing and let five die. Most naïve subjects presented with this sort of case, claims Unger, will choose (d), to save the five by killing one, even if this course of action involves doing something very similar to killing the fat man, as in Thomson's case above.

This scenario is similar to the fact that whenever a crime is in progress and someone calls the police, even though it is known well in advance that calls to police each year end up creating pedestrian and motorist deaths due to accidents, very few people would consider disbanding the police to ensure that no innocents should die en-route to a crime scene. (reasonable assumption however citation needed) In the case where the five aren't tied down due to a criminal act, it still falls into the category of diverting a crashing plane into a less-densely-populated area.

The mother

In another variant, the irrationality of human ethics is explored when the cost has a personal consequence.
As before, a trolley is hurtling down a track towards five people. You can flip a switch and divert the train to run one person over instead of five, but that person is your mother. Would you flip the switch?

Most people agree that they would not sacrifice their own mother to save five strangers.

In cognitive science

The trolley problem was first imported into cognitive science from philosophy in a systematic way by John Mikhail, who began testing trolley problems on different groups of people, including children and people from non-Western cultures, when he was a visiting graduate student in MIT’s Department of Brain and Cognitive Sciences. Mikhail hypothesized that factors such as gender, age, education level, and cultural background would have little influence on the judgments people make, in part because those judgments are generated by an unconscious “moral grammar” that is analogous in some respects to the unconscious linguistic grammars that support ordinary language use. Preliminary results pointed in that direction, and Mikhail’s initial findings have been confirmed and expanded to more than 200,000 individuals from over 100 countries.

In neuroethics

In taking a neuroscientific approach to the trolley problem, Joshua Greene under Jonathan Cohen decided to examine the nature of brain response to moral and ethical conundra through the use of fMRI. In their more well-known experiments, Greene and Cohen analyzed subjects' responses to the morality of responses in both the trolley problem involving a switch, and a footbridge scenario analogous to the fat man variation of the trolley problem. Their hypothesis suggested that encountering such conflicts evokes both a strong emotional response as well as a reasoned cognitive response that tend to oppose one another. From the fMRI results, they have found that situations highly evoking a more prominent emotional response such as the fat man variant would result in significantly higher brain activity in brain regions associated with response conflict. Meanwhile, more conflict-neutral scenarios, such as the relatively disaffected switch variant, would produce more activity in brain regions associated with higher cognitive functions. The potential ethical ideas being broached, then, revolve around the human capacity for rational justification of moral decision making.


Daniel Bartels of Columbia University found that individual reactions to trolley problems is context sensitive and that around 90% would refuse the act of deliberately killing one individual to save five lives. Further study by Daniel Bartels and David Pizarro focused on those 10% who made utilitarian choice. The study asked participants to series of value statement. The experiment found that those who had stronger utilitarian leaning had stronger tendency to psychopathy
Psychopathy is a mental disorder characterized primarily by a lack of empathy and remorse, shallow emotions, egocentricity, and deceptiveness. Psychopaths are highly prone to antisocial behavior and abusive treatment of others, and are very disproportionately responsible for violent crime...

, Machiavellianism
Machiavellianism is, according to the Oxford English Dictionary, "the employment of cunning and duplicity in statecraft or in general conduct", deriving from the Italian Renaissance diplomat and writer Niccolò Machiavelli, who wrote Il Principe and other works...

 or tended to view life as meaningless. The economist magazine who reported this finding stated that "utilitarians, ... may add to the sum of human happiness, but they are not very happy people themselves."

As urban legend

In an urban legend
Urban legend
An urban legend, urban myth, urban tale, or contemporary legend, is a form of modern folklore consisting of stories that may or may not have been believed by their tellers to be true...

 that has been making the rounds since at least the mid-1960s, the decision must be made by a drawbridge keeper who must choose between sacrificing a passenger train or his own four-year-old son. This version is often drawn as a deliberate allegory to the Christian belief that God sacrificed his son, Jesus of Nazareth.

See also

  • Consequentialism
    Consequentialism is the class of normative ethical theories holding that the consequences of one's conduct are the ultimate basis for any judgment about the rightness of that conduct...

  • Deontology
  • Principle of double effect
    Principle of double effect
    The principle of double effect; also known as the rule of double effect; the doctrine of double effect, often abbreviated as DDE or PDE; double-effect reasoning; or simply double effect, is a set of ethical criteria for evaluating the permissibility of acting when one's otherwise legitimate act...

  • The Case of the Speluncean Explorers
    The Case of the Speluncean Explorers
    The Case of the Speluncean Explorers is a famous hypothetical legal case used in the study of law, which was written by Lon Fuller in 1949 for the Harvard Law Review....

  • Violinist (thought experiment)
    Violinist (Thought Experiment)
    The Violinist is a famous thought experiment first posed by Judith Jarvis Thomson in 1971.-The "famous violinist" thought experiment:The Stanford Encyclopedia of Philosophy describes her thought experiment as follows:...

  • Virtue ethics
    Virtue ethics
    Virtue ethics describes the character of a moral agent as a driving force for ethical behavior, rather than rules , consequentialism , or social context .The difference between these four approaches to morality tends to lie more in the way moral dilemmas are...

  • Experiments in Ethics
    Experiments in Ethics
    Experiments in Ethics is a 2008 book by the Princeton philosopher Kwame Anthony Appiah.Many philosophers have been sceptical about the relevance of empirical moral psychology to ethics. But Appiah points out that philosophy has almost always had an experimental side...

External links

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