Trolley problem

The trolley problem is a series of thought experiments in ethics, psychology and artificial intelligence involving stylized ethical dilemmas in a scenario of whether to sacrifice one person to save a larger number. The series usually begins with a scenario in which a runaway trolley or train is on course to collide with and kill a number of people down the railway track, 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 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 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

's version of the thought experiment, now known as the "trolley driver", ran as follows:
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. 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

In 2001 the psychologist Joshua Greene and colleagues published the results of the first significant empirical investigation of people's responses to trolley problems. Using functional magnetic resonance imaging, they demonstrated that "personal" dilemmas preferentially engage brain regions associated with emotion, whereas "impersonal" dilemmas preferentially engaged regions associated with controlled reasoning. On these grounds, they advocate for the dual-process account of moral decision-making. Since then, numerous other studies have employed trolley problems to study moral judgement, investigating topics like the role and influence of stress, emotional state, impression management, levels of anonymity, different types of brain damage, physiological arousal, different neurotransmitters, and genetic factors on responses to trolley dilemmas.
Trolley problems have been used as a measure of utilitarianism, but their usefulness for such purposes has been criticized.
In 2017 a group led by Michael Stevens, a YouTuber also known as Vsauce, performed the first realistic trolley-problem experiment. Subjects were placed alone in what they thought was a train-switching station, and shown footage they believed to be real of a train going down a track, with five workers on the main track, and one on the secondary track; the participants had the option to pull the lever to divert the train toward the secondary track. Five of the seven participants did not pull the lever.

Survey data

The trolley problem has been the subject of many surveys in which about 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 the one life.
A 2009 survey by David Bourget and David Chalmers shows that 68% of professional philosophers would switch in the case of the trolley problem, 8% would not switch, and the remaining 24% had another view or could not answer.

Criticism

In a 2014 paper published in the Social and Personality Psychology Compass, researchers criticized the use of the trolley problem, arguing, among other things, that the scenario it presents is too extreme and unconnected to real-life moral situations to be useful or educational.
In her 2017 paper, Nassim Jafari Naimi lays out the reductive nature of the trolley problem in framing ethical problems that serves to uphold an impoverished version of utilitarianism. She argues that the popular argument that the trolley problem can serve as a template for algorithmic morality is based on fundamentally flawed premises that serve the most powerful with potentially dire consequences on the future of cities.
In his book On Human Nature rarity, 2) inevitability, 3) safety zone, 4) possibility of becoming a victim, and 5) the lack of perspective of the dead victims who were deprived of freedom of choice.
In a 2018 article published in Psychological Review, researchers pointed out that, as measures of utilitarian decisions, sacrificial dilemmas such as the trolley problem measure only one facet of proto-utilitarian tendencies, namely permissive attitudes toward instrumental harm, while ignoring impartial concern for the greater good. As such, the authors argued that the trolley problem provides only a partial measure of utilitarianism.

Applicability in legal reasoning

The trolley problem has been widely discussed in legal theory as a means of exploring how law distinguishes between intentional harm, omission, and necessity. In criminal law, the scenario illustrates the tension between utilitarian and deontological approaches to culpability: while a utilitarian might justify diverting the trolley to minimise deaths, legal systems often treat deliberate killing as impermissible regardless of the net outcome. This distinction reflects the act–omission doctrine, under which a failure to prevent harm is generally less blameworthy than actively causing it.
In English common law, courts have traditionally rejected the defence of necessity in cases involving the deliberate taking of life, even where doing so would save others. The leading precedent is R v Dudley and Stephens, in which shipwrecked sailors were convicted of murder after killing a cabin boy to survive. The decision is often cited as a legal analogue to the trolley problem, emphasising that moral calculations of greater good do not justify intentional killing under the law.
In tort law and regulatory policy, however, trolley-like reasoning appears in cost–benefit analyses where harm is balanced against risk reduction—such as in product design, environmental regulation, or accident prevention. The U.S. decision in United States v. Carroll Towing Co. articulated a formula weighing the burden of precautions against the probability and gravity of potential harm, effectively institutionalizing a utilitarian calculus within negligence law.
More recent legal discussions have introduced the Trolley Problem into the legal regulatory framework for autonomous vehicles. As autonomous driving technology advances, legal scholars have begun to focus on how vehicle decisions in unavoidable collision scenarios are coded, and the potential legal liabilities that such coding may trigger.