Fat Man Variant

Overview and Origin

The Fat Man Variant (often called the Footbridge Case) is a prominent form of the trolley problem, a set of ethical thought experiments used to probe moral intuitions about killing, letting die, and the permissibility of sacrificing one person to save many. While the original trolley problem is commonly attributed to Philippa Foot, the Fat Man Variant was developed and made famous by Judith Jarvis Thomson in the 1970s and 1980s as part of her critical engagement with utilitarianism and with Foot’s initial formulation.

In the standard trolley case, an agent can flip a switch, diverting a runaway trolley from a track where it will kill five onto a side track where it will kill one. Many respondents regard flipping the switch as morally permissible. The Fat Man Variant modifies the situation in a way that preserves the numerical trade-off—one death versus five—but alters the means by which the harm is inflicted. Thomson’s variant has become a canonical example in both philosophical ethics and empirical moral psychology.

Structure of the Thought Experiment

In Thomson’s Fat Man case, the scenario usually runs as follows:

You are standing on a footbridge over a trolley track. A runaway trolley is racing down the track toward five people who will be killed if the trolley is not stopped. Next to you stands a very large man. You know that if you push this man onto the track, his body will stop the trolley. He will be killed, but the five people further down the track will be saved. You cannot stop the trolley by jumping yourself; only the large man’s mass is sufficient. You have only two options: (1) do nothing and allow the trolley to kill the five, or (2) push the man, killing him but saving the five.

Formally, the case preserves the following elements:

• Constant outcome numbers: inaction leads to five deaths; action leads to one death.
• Agent control: the agent can intervene in only one specific way—pushing the man.
• Causal role of the victim: the large man’s body is used as the instrumental means to stop the trolley.
• No consent or prior fault: the man is typically described as innocent and unconsenting, with no special moral status different from the five.

The key comparison is with the switch case, where the agent merely redirects the trolley from one track to another. In that scenario, many judge it permissible to turn the switch, even though this foreseeably causes the death of the one. In the Fat Man Variant, by contrast, many judge that pushing the man is wrong. This divergence in intuitions, despite identical numbers of deaths and lives saved, is the central puzzle.

Philosophical Significance and Debates

The Fat Man Variant is used to test and refine several major ethical concepts and theories.

1. Utilitarian and consequentialist perspectives

From a classical utilitarian standpoint, what matters morally is the overall balance of happiness or the minimization of suffering. Since pushing the man saves five at the cost of one, a straightforward utilitarian calculus strongly favors pushing.

Proponents of consequentialism often use the Fat Man Variant to illustrate the counterintuitive implications of their view and then try to defend or revise the theory:

• Some argue that our reluctance to push is driven by emotional biases, such as aversion to personal violence, rather than by sound moral reasoning.
• Others adopt more sophisticated forms of consequentialism that incorporate rules, indirect effects, or trust and social stability, which might count against acts like pushing even if, in isolation, they appear utility-maximizing.

2. Deontological constraints and the Doctrine of Double Effect

By contrast, deontological theories, especially influenced by Kant, emphasize constraints on action: there are some things we must not do to others, even for good outcomes. The Fat Man Variant is often cited as supporting such constraints.

A central tool here is the Doctrine of Double Effect (DDE), which distinguishes between:

• Intended harm as a means to a good end, and
• Foreseen harm as a side effect of pursuing a good end.

In the switch case, the death of the one on the side track is commonly characterized as a foreseen side effect of redirecting the trolley, not as something used as a means. In the Fat Man case, the man’s death appears to be an intended means: his body is deliberately used to stop the trolley. Deontological theorists and defenders of DDE claim this grounds a moral difference: it is impermissible to use a person as a mere means, even to save more lives, whereas it may be permissible to allow a foreseen but unintended harm.

Critics respond that:

• The intended/foreseen distinction is unclear or unstable; intentions can be redescribed in ways that blur the line.
• From the victim’s perspective, whether their harm was intended or foreseen seems morally irrelevant; what matters is that they were killed for the sake of others.

3. The ‘personal force’ distinction and moral psychology

Empirical studies in moral psychology have used the Fat Man Variant to investigate how people actually make moral judgments. Researchers such as Joshua Greene and colleagues report that many people:

• Are willing to flip the switch in the original trolley case, but
• Are unwilling to push the man in the Fat Man case.

One influential interpretation is the personal vs. impersonal distinction:

• The Fat Man Variant involves personal force—direct, physical contact causing harm.
• The switch case is impersonal, involving distant, mechanical intervention.

Some cognitive scientists argue that emotionally charged, personal harms trigger strong aversive reactions that shape our intuitions, while more abstract, impersonal harms allow for more “utilitarian” reasoning. On this view, the divergence of intuitions across variants may reflect psychological mechanisms rather than deep moral truths.

4. Rights, agency, and using persons as means

The Fat Man Variant also features in debates about rights and human dignity. Critics of pushing often claim that:

• The large man has a right not to be killed or used against his will as a tool for the benefit of others.
• Respect for persons includes a prohibition on treating individuals merely as resources for social utility.

Some theorists draw on Kantian ideas: pushing the man is said to violate the imperative to treat persons as ends in themselves, not merely as means. Others seek to derive the prohibition from rights-based or contractualist frameworks, arguing that no rational person could reasonably accept a principle allowing themselves to be sacrificed in such a way.

Defenders of more outcome-focused ethics challenge these claims, questioning whether such absolute prohibitions can be sustained in light of extreme cases where enormous numbers of lives might be saved by sacrificing one.

5. Variants and refinements

A range of modifications to the Fat Man Variant test the robustness of people’s intuitions and the theories used to explain them. For example:

• Cases where the large man volunteers to jump, exploring the role of consent.
• Situations where the large man is somehow responsible for the threat, testing whether culpability changes moral judgments.
• “Trapdoor” versions where pulling a lever drops the man rather than physically pushing him, examining whether physical contact or psychological distance drives our responses.

These variants reveal that intuitions are sensitive to framing, which complicates the use of the Fat Man case as a straightforward argument for or against any single moral theory.

Overall, the Fat Man Variant functions less as a settled argument than as a testing ground for ethical principles. It presses the question of whether we may ever deliberately use one person as a sacrificial means to save others, and it remains a central case in contemporary moral philosophy and moral psychology.