Trolley Variants: A Taxonomy and Annotated Exemplars
Classification prompt — pick an axis
Decide which conceptual axis you want to examine. Read the short definitions and follow the section anchors below for the canonical variants that isolate that distinction.
| Canonical variant | Axes emphasized | Two-line summary |
|---|---|---|
| Classic Switch | Act/Omission · Means/Side-effect | A lever diverts a runaway trolley from five people to one on a side track; highlights causal distance and whether switching counts as acting. |
| Footbridge (Push) | Personal force · Intention | A bystander must push a large person off a bridge to stop a trolley, thereby killing them to save five; isolates physical contact and intended means. |
| Loop / Sacrifice | Collective trade-offs · Identity framing | A diversion route requires sacrificing a passenger connected to the trolley to save multiple outsiders; probes agent-relative permissions and passenger identity. |
| Bystander/Omission | Omission · Role framing | A bystander can refrain from flipping a switch, letting five die; explores whether omissions are morally distinct from acts. |
| Probabilistic Switch | Risk · Expected outcomes | Choosing a diversion with probabilistic outcomes (e.g., chance to save or kill) tests how uncertainty alters permissibility judgments. |
When to consult — Use loop/sacrifice or push variants to sharpen whether killing someone as the means to stop a trolley differs from letting them die as a side-effect.
Avoid conflating foreknowledge with instrumental use: a close causal chain that uses the person's body as a stopping-mechanism is prototypical for testing means.
When to consult — The classic switch and bystander omission variants contrast an active diversion with standing by; frame the same outcomes both ways to test the symmetry.
Be explicit about role and obligations: labeling someone as a 'driver', 'passenger', or 'bystander' changes the moral salience of omission.
When to consult — The footbridge/push example is canonical: it isolates emotional salience and whether 'pushing' differs from flipping a remote lever.
Distinguish physical contact from emotional proximity: both can increase perceived wrongness but for different explanatory reasons.
When to consult — Use probabilistic-switch variants to contrast deterministic outcomes with risky choices; useful for arguments about expected outcomes vs. rights-based constraints.
Keep the background epistemic state explicit: what the agent knows about probabilities matters for the conceptual question being isolated.
Compact comparison matrix
Reader’s guide: choosing and adapting variants
Start by naming precisely which conceptual contrast you intend to isolate (for example, 'intention vs foreseen side-effect'). Then pick the canonical variant that most directly foregrounds that contrast. If you combine features, state which axis is primary to avoid category-mixing.
Framing note: Describing agent roles (passenger, driver, bystander) changes the argument frame; prefer neutral labels when the role should not be the focus.
Limitations & ethics
Thought experiments isolate conceptual distinctions; they do not establish empirical claims about how people decide. Avoid overgeneralizing from a single variant: different variants test different principles and cognitive contrasts. Be explicit about which distinction you treat as primary.
Further reading: seek canonical discussions under the labels used in this page (classic trolley, footbridge/push, loop/sacrifice, probabilistic-switch) and primary sources that introduce the doctrine of double effect and act/omission debates.