The trolley problem is a thought experiment in moral philosophy and moral psychology with many variations, involving hypothetical ethical dilemmas about whether to sacrifice one person to save a larger number of people. The series usually begins with a scenario in which a runaway trolley (tram) or train is on course to collide with and kill a number of people (traditionally five) tied to the tracks, but a driver or bystander can intervene and divert the vehicle to kill just one person on a different track. Then other variations of the runaway vehicle, and analogous life-and-death dilemmas (medical, judicial, etc.) are posed, each containing the option either to do nothing—in which case several people will be killed—or to intervene and sacrifice one initially "safe" person to save the others. Opinions on the ethics of each scenario turn out to be sensitive to details of the story that may seem immaterial to the abstract dilemma. The question of formulating a general principle that can account for the differing judgments arising in different variants of the story was raised in 1967 as part of an analysis of debates on abortion and the doctrine of double effect by the English philosopher Philippa Foot. Later dubbed "the trolley problem" by Judith Jarvis Thomson in a 1976 article that catalyzed a large literature, the subject refers to the meta-problem of why different judgements are arrived at in particular instances. Thomson and the philosophers Frances Kamm and Peter Unger have analyzed the trolley problem extensively. Thomson's 1976 article initiated the literature on the trolley problem as a subject in its own right. Characteristic of this literature are colourful and increasingly absurd alternative scenarios in which the sacrificed person is instead pushed onto the tracks as a way to stop the trolley, has his organs harvested to save transplant patients, or is killed in more indirect ways that complicate the chain of causation and responsibility. Earlier forms of individual trolley scenarios antedated Foot's publication. Frank Chapman Sharp included a version in a moral questionnaire given to undergraduates at the University of Wisconsin in 1905. In this variation, the railway's switchman controlled the switch, and the lone individual to be sacrificed (or not) was the switchman's child. The German philosopher of law Karl Engisch discussed a similar dilemma in his habilitation thesis in 1930, as did the German legal scholar Hans Welzel in a work from 1951. In his commentary on the Talmud, published in 1953, Avrohom Yeshaya Karelitz considered the question of whether it is ethical to deflect a projectile from a larger crowd toward a smaller one. Similarly, in The Strike, a television play broadcast in the United States on 7 June 1954, a commander in the Korean War must choose between ordering an air strike on an encroaching enemy force, at the cost of his own 20-man patrol unit; and calling off the strike, risking the lives of the main army of 500 men. Beginning in 2001, the trolley problem and its variants have been used in empirical research on moral psychology. It has been a topic of popular books. Trolley-style scenarios also arise in discussing the ethics of autonomous vehicle design, which may require programming to choose whom or what to strike when a collision appears to be unavoidable. More recently, the trolley problem has also become an Internet meme.
Original dilemma Philippa Foot's version of the thought experiment, now known as the "trolley driver", 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 classical utilitarianism, such a decision would be not only permissible, but, morally speaking, the better option (the other option being no action at all). This fact makes diverting the trolley obligatory. An alternative 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 doing nothing would be considered an immoral act.
Empirical research
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