A few years later, Judith Thomson, a philosopher at the Massachusetts Institute of Technology, coined the term “trolley problem” and created what would become its two most famous variants, the “footbridge” and the “switch.” In the “footbridge” scenario (also known as “fat man”), the streetcar is heading towards five workers, but this time you’re on a footbridge over the track. Standing precariously close to the edge of the bridge next to you is a very large man, who, if he happened to topple onto the track below, could stop the trolley before it reaches the five. Do you push him? Thomson’s writing sparked so much interest in the philosophical community that a sub-discipline of “trolleyology” emerged during the ‘70s and ‘80s.

The trolley dilemmas vividly distilled the distinction between two different concepts of morality: that we should choose the action with the best overall consequences (in philosophy-speak, utilitarianism is the most well-known example of this), like only one person dying instead of five, and the idea that we should always adhere to strict duties, like “never kill a human being.” The subtle differences between the scenarios provided helped to articulate influential concepts, like the distinction between actively killing someone versus passively letting them die, that continue to inform contemporary debates in law and public policy. The trolley problem has also been, and continues to be, a compelling teaching tool within philosophy.

By the late ‘90s, trolley problems had fallen out of fashion. Many philosophers questioned the value of the conclusions reached by analyzing a situation so bizarre and specific.

It wasn’t clear trolleys could ever find a life out of the pages of academic journals until one philosophy graduate student, Joshua Greene, revived them with the modern techniques of neuroscience.

Greene decided to slide people into an fMRI machine to glimpse what happened in their brains when faced with different trolley-problem scenarios. He ultimately found that the answers people gave correlated with how emotionally engaged they felt with the dilemma. The decision to pull the switch was related to activity in the prefrontal cortex (associated with cool, conscious deliberation), while the decision not to push the fat man involved areas like the amygdala, associated with strong emotional reactivity.

The paper that reported these results in 2001 inspired hundreds more studies using the trolley problem to study moral decision-making—and also piqued interest from people in other fields, like sociologists, economists, and anthropologists. Greene and others have used trolleys to conclude that strong mental imagery and visceral emotions make us more likely to make an intuitive decision (“never kill a human being”), as opposed to a more mathematical calculated decision (five lives versus one). Greene’s research also suggests that the reason people are less likely to push the fat man than to flip a switch is because we all possess a biologically pre-programmed emotional aversion to delivering harm personally (touching a man as you shove him off a bridge) as opposed to impersonally (yanking a lever).