The trolley problem is a philosophical thought experiment — not a laboratory study — but it has generated more empirical psychological research than almost any other single scenario. Proposed by philosophers Philippa Foot and Judith Jarvis Thomson, it has been tested on hundreds of thousands of people worldwide.

The Classic Version

A runaway trolley is headed toward five people tied to the tracks. You can pull a lever to divert it to a side track where one person is tied. Do you pull the lever?

Most people say yes — sacrifice one to save five.

The Footbridge Variant

Same trolley, same five people. But now you're on a bridge above the tracks with a large man beside you. If you push him off the bridge, his body will stop the trolley, saving five but killing him. Do you push?

Most people say no — even though the math is identical.

What This Reveals

• Moral intuitions are not consistently utilitarian — personal action feels different from impersonal action
• The emotional system (pushing someone) and rational system (save five) conflict
• Joshua Greene's dual-process theory of morality: gut reactions vs. deliberate reasoning produce systematically different moral judgments

Reference:

Wikipedia: Trolley Problem

https://en.wikipedia.org/wiki/Trolley_problem