Would you pull a lever to kill one person to save five? The trolley problem is philosophy's most famous thought experiment, revealing deep tensions between utilitarian and deontological ethics.
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Imagine you're standing beside a railway track when you notice a runaway trolley hurtling toward five people tied to the tracks. You're standing next to a lever that can divert the trolley onto a side track—but there's one person tied to that alternative track. Do you pull the lever, killing one person to save five? Or do you do nothing, allowing five people to die?
This is the trolley problem, one of philosophy's most famous thought experiments. Since philosopher Philippa Foot introduced it in 1967, the trolley problem has become central to discussions of ethics, moral psychology, and even the programming of autonomous vehicles. It's simple yet profound, revealing deep tensions in how we think about right and wrong.
Related: Learn more about Utilitarianism: The Greatest Good
Related: Learn more about Kant's Categorical Imperative: A Guide to Moral Philosophy
Related: Learn more about Ethics 101: How Do We Know What's Right and Wrong?
British philosopher Philippa Foot originally presented the trolley problem in her paper "The Problem of Abortion and the Doctrine of Double Effect." She was investigating the moral principle known as the doctrine of double effect—the idea that it's sometimes permissible to cause harm as a side effect of pursuing a good outcome, even when causing the same harm as a means to that outcome would be wrong.
Foot's original scenario involved a trolley driver who could either continue on the main track, killing five workers, or divert to a side track, killing one. Her purpose was to explore why this diversion seemed morally permissible while other scenarios involving killing one to save many seemed wrong.
In 1985, philosopher Judith Jarvis Thomson expanded the trolley problem with new variations, most famously the "fat man" variant: You're on a footbridge over the trolley tracks. A runaway trolley threatens five people below. Next to you stands a large man whose weight, if pushed onto the tracks, would stop the trolley, saving the five but killing him. Should you push him?
This variation produces a striking inconsistency in most people's moral intuitions. The majority say they would pull the lever in the original scenario but wouldn't push the fat man, even though both actions involve killing one person to save five. This inconsistency has fascinated moral philosophers and psychologists for decades.
The trolley problem brilliantly illuminates the conflict between two major approaches to ethics: consequentialism (particularly utilitarianism) and deontology.
Utilitarian ethics, associated with philosophers like Jeremy Bentham and John Stuart Mill, judges actions by their consequences. The morally right action is the one that produces the greatest overall happiness or well-being for the greatest number of people.
From a utilitarian perspective, the trolley problem seems straightforward: five deaths create more suffering than one death, so you should pull the lever (or even push the fat man) to minimize total harm. The math is simple—five lives saved minus one life lost equals a net positive of four lives.
Utilitarian reasoning:
However, pure utilitarian calculations can lead to unsettling conclusions. If killing one innocent person to save five is justified, what about harvesting organs from a healthy person to save five dying patients? Most people recoil from this conclusion, suggesting our moral intuitions involve more than simple consequentialist math.
Deontological ethics, championed by Immanuel Kant and others, emphasizes duties, rules, and rights rather than consequences. From this perspective, some actions are inherently wrong regardless of their outcomes.
Deontologists often distinguish between killing and letting die, or between intended and foreseen consequences. In the trolley scenario, diverting the trolley may be seen as causing a foreseen but unintended death (the one person wasn't your target; saving the five was your goal). Pushing the fat man, however, involves using him as a means to an end—treating him as a tool rather than as a person with inherent dignity and rights.
Deontological considerations:
The doctrine of double effect, which Foot originally investigated, captures deontological thinking: It may be permissible to cause harm as a side effect of pursuing good, but not to cause harm as a means to achieving good.
One of the most fascinating aspects of the trolley problem is the inconsistency in people's moral judgments across different variants. Research reveals several factors that influence our ethical intuitions.
People are more reluctant to cause harm when it involves direct physical contact. Pushing someone off a bridge feels more personal and morally worse than pulling a distant lever, even when the outcomes are identical. This may reflect evolved psychological mechanisms—direct violence requires overcoming stronger inhibitions than indirect harm.
Psychologists have identified "personal force" as a key variable. Actions involving personal force (pushing, strangling, stabbing) trigger different moral intuitions than impersonal actions (lever-pulling, bomb-dropping). This distinction may have evolutionary roots: throughout human history, up-close violence required different moral restraints than distant harms.
We distinguish between using someone's death as a means (pushing the fat man to stop the trolley) and causing death as a side effect (diverting the trolley, where the one person's death isn't the mechanism that saves the five). This tracks the deontological intuition that people have inherent dignity and shouldn't be used merely as tools.
People generally judge harmful actions more severely than harmful omissions. Actively switching the trolley feels different from simply allowing it to continue on its current path, even when doing nothing is itself a choice with foreseeable consequences.
Modern neuroscience has examined the brain processes underlying trolley problem judgments, revealing that moral reasoning involves both emotional and cognitive systems.
Research by Joshua Greene and others suggests that moral judgments emerge from the interaction of two systems:
Emotional/intuitive processes: Quick, automatic emotional responses associated with brain regions like the amygdala and ventromedial prefrontal cortex. These generate deontological intuitions against directly harming others.
Cognitive/reasoning processes: Slower, deliberative reasoning associated with the dorsolateral prefrontal cortex. These support utilitarian calculations of costs and benefits.
In the footbridge variant, the personal nature of pushing activates strong emotional responses that oppose the action. In the lever variant, emotional responses are weaker, allowing utilitarian reasoning greater influence.
Brain imaging studies show that people who make utilitarian judgments (willing to push the fat man) often show greater activity in cognitive control regions, suggesting they're overriding emotional aversions. Meanwhile, those making deontological judgments show stronger responses in emotion-related areas.
Not everyone responds to trolley problems the same way. Research has found variations based on:
While the trolley problem might seem like a pure thought experiment, it has practical implications for several contemporary issues.
Self-driving cars must be programmed to make split-second decisions in potential accident scenarios. Should a car sacrifice its passenger to save multiple pedestrians? Should it swerve to avoid a group of people if doing so would kill a single bystander?
These aren't hypothetical questions—engineers and policymakers must decide how to program moral decision-making into AI systems. The trolley problem provides a framework for thinking through these choices, though there's no consensus on the right answers.
Healthcare often involves trolley-problem-like dilemmas:
The COVID-19 pandemic made these dilemmas urgently real, as hospitals developed protocols for rationing ventilators and ICU beds.
Warfare constantly presents trolley-type scenarios:
The doctrine of double effect, which the trolley problem helps illuminate, plays a significant role in just war theory and international humanitarian law.
Despite its influence, the trolley problem faces several criticisms.
Critics argue that abstract thought experiments strip away crucial context that matters morally. Real ethical decisions involve:
Bernard Williams argued that focusing on abstract dilemmas misses how real moral agents are constituted by their commitments, relationships, and integrity.
Some philosophers question whether our intuitions about bizarre thought experiments should guide moral theory. Peter Unger showed that people's moral judgments are highly sensitive to seemingly irrelevant details, suggesting our intuitions may be unreliable guides.
The trolley problem emerges from Western philosophical traditions emphasizing individual rights and consequences. Other cultural frameworks might approach moral dilemmas differently, focusing on relationships, social harmony, virtue, or role-based duties.
The trolley problem reveals tensions between utilitarian and deontological thinking, but other ethical frameworks offer alternative perspectives.
Rather than asking "What should I do?" virtue ethics asks "What kind of person should I be?" From this perspective, the trolley problem's focus on isolated decisions misses how character develops through habits and practices over time. A virtuous person cultivates qualities like courage, compassion, and practical wisdom that guide action.
Feminist care ethics emphasizes relationships, empathy, and context rather than abstract rules or calculations. From this view, the trolley problem's reduction of people to numbers neglects the relational dimensions of moral life.
African philosophy's concept of ubuntu—"I am because we are"—emphasizes interconnectedness and community. This framework might approach the trolley problem by considering how decisions affect social bonds and collective flourishing, not just individual welfare.
Ultimately, the trolley problem's value lies not in providing answers but in revealing the complexity of moral reasoning. It shows that:
The trolley problem remains relevant because it concisely captures fundamental tensions in moral philosophy. It forces us to confront difficult questions: Are consequences all that matter? Do some acts remain wrong regardless of outcomes? How should we balance competing moral considerations?
While philosophers continue debating these questions, the trolley problem serves a crucial function: it makes abstract ethical theories concrete and tests them against our moral intuitions. Whether those intuitions are reliable guides remains controversial, but examining them helps us understand the psychological and philosophical foundations of morality.
In an increasingly complex world facing challenges from AI ethics to global resource allocation, the kind of moral thinking the trolley problem provokes becomes ever more essential. We may never resolve the trolley problem definitively, but wrestling with it makes us more thoughtful moral agents—and that might be its greatest value.
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