The trolley problem is a thought experiment in ethics. The general form of the problem is this: There is a runaway trolley barreling down the railway tracks. Ahead, on the tracks, there are five people tied up and unable to move. The trolley is headed straight for them. You are standing some distance off in the train yard, next to a lever. If you pull this lever, the trolley will switch to a different set of tracks. However, you notice that there is one person on the side track. You have two options: (1) Do nothing, and the trolley kills the five people on the main track. (2) Pull the lever, diverting the trolley onto the side track where it will kill one person. Which is the most ethical choice?
The problem was first introduced by Philippa Foot in 1967, but also extensively analysed by Judith Thomson, Peter Unger, and Frances Kamm as recently as 1996, and has also been revisited in 2015 (Larman and Oates). The German Hans Welzel discussed a similar problem as early as 1951. Outside of the domain of traditional philosophical discussion, the trolley problem has been a significant feature in the fields of cognitive science and, more recently, of neuroethics. It has also been a topic in popular books dealing with human psychology. The problem is also discussed with regards to the ethics of the design of autonomous vehicles.
Foot's original structure of the problem ran as follows:
Suppose that a judge or magistrate is faced with rioters demanding that a culprit be found for a certain crime and threatening otherwise to take their own bloody revenge on a particular section of the community. The real culprit being unknown, the judge sees himself as able to prevent the bloodshed only by framing some innocent person and having him executed. Beside this example is placed another in which a pilot whose airplane is about to crash is deciding whether to steer from a more to a less inhabited area. To make the parallel as close as possible it may rather be supposed that he is the driver of a runaway tram which he can only steer from one narrow track on to another; five men are working on one track and one man on the other; anyone on the track he enters is bound to be killed. In the case of the riots the mob have five hostages, so that in both examples the exchange is supposed to be one man's life for the lives of five.
A utilitarian view asserts that it is obligatory to steer to the track with one man on it. According to simple utilitarianism, such a decision 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, moving to another track constitutes a participation in the moral wrong, making one partially responsible for the death when otherwise no one would be responsible. An opponent of action may also point to the incommensurability of human lives. Under some interpretations of moral obligation, simply being present in this situation and being able to influence its outcome constitutes an obligation to participate. If this is the case, then deciding to do nothing would be considered an immoral act if one values five lives more than one.
The initial trolley problem also supports comparison to other, related, dilemmas:
The fat man
One such is that offered by Judith Jarvis Thomson:
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 putting something very heavy 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; when asked, a majority of people will approve of pulling the switch to save a net of four lives, but will disapprove of pushing the fat man to save a net of four lives. 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 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 considers, and ultimately rejects, in The Limits of Morality.
A claim can be made 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.
Act utilitarians deny this. Peter Unger (a non-utilitarian) 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.[clarification needed]
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 only morally justifiable but perhaps even imperative. This is essentially related to another thought experiment, known as ticking time bomb scenario, which forces one to choose between two morally questionable acts.
The loop variant
The claim that it is wrong to use the death of one to save five runs into a problem with 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. However, this diversion loops back around to rejoin the main track, so diverting the trolley still leaves it on a path to run over the five people. But, on this track is a single fat person who, when he is killed by the trolley, will stop it from continuing on to the five people. 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, if we originally decided that it is permissible or necessary to flip the switch, intuition may suggest that the answer should not have changed. However, in this case, the death of the one actually is part of the plan to save the five.
The rejoining variant may not be fatal to the "using a person as a means" argument. This has been suggested by Michael J. 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 had been absent – or even with no track changes, if a change is made such that the one person is high on the gradient while the five are low, so that the trolley cannot reach the one. The question has therefore not been answered. It is also possible to change the points to "1A, 2B", causing the trolley to continue straight ahead towards the 5, avoiding the 1, but then be derailed on purpose where the loop rejoins the main track, before reaching the 5, killing no-one.
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. Do you support the morality of the doctor to kill that tourist and provide his healthy organs to those five dying persons and save their lives?
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.
In cognitive science
The trolley problem was first imported into cognitive science from philosophy in a systematic way by Hauser, Mikhail, et al. They 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 have been claimed by Noam Chomsky et al. to support ordinary language use. The data in the 2007 paper by Hauser, Mikhail et al. only contains 33 individuals brought up in a non-English-speaking educational system. The main author, Marc Hauser, was subsequently sanctioned by his then employer, Harvard University, in eight unrelated cases of gross research malpractice and data falsification. Subsequent cross-cultural research has found many apparent counterexamples to this idea of 'Universal Moral Grammar'. Further evidence against the idea of a universal moral grammar was presented in a meta-analysis with 6,100 participants showing that women have stronger deontological inclinations than men, while men exhibit only slightly stronger utilitarian inclinations than women.
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 and a reasoned cognitive response, and that these two responses 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.
The trolley problem has been the subject of many surveys in which approximately 90% of respondents have chosen to kill the one and save the five.  If the situation is modified where the one sacrificed for the five was a relative or romantic partner, respondents are much less likely to be willing to sacrifice their life.
In 2012, participants made their choices while wearing a head mounted display device that displayed virtual avatars of the trolley victims, and gave a real time simulation of the approaching vehicle. As the vehicle approached, the virtual avatars in the path would begin to scream until impact. Subjects who were more emotionally aroused during the test were less likely to kill the one.
A 2009 survey published in a 2013 paper by David Bourget and David Chalmers shows that 68% of professional philosophers would switch (sacrifice the one individual to save five lives) in the case of the trolley problem, 8% would not switch, and the remaining 24% had another view or could not answer.
As urban legend
In an urban legend that has existed 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. There is a 2003 Czech short film titled Most or The Bridge (USA) which deals with a similar plot. This version is often given as an illustration of the Christian belief that God sacrificed his son, Jesus Christ.
Implications for autonomous vehicles
Problems analogous to the trolley problem arise in the design of autonomous cars, in situations where the car's software is forced during an accident to choose between multiple courses of action (sometimes including options which include the death of the car's occupants), all of which may cause harm. A platform called Moral Machine was created by MIT Media Lab to allow the public to express their opinions on what decisions autonomous vehicles should make in scenarios that use the trolley problem paradigm.
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