What is cause and effect chain?

Example: In September 1854, physician John Snow traced a cholera outbreak in London's Soho district by mapping deaths and walking the causal chain backward. He started with an observation: deaths clustered around the Broad Street water pump. Working with Reverend Henry Whitehead, Snow traced the chain link by link — from cholera deaths, to contaminated water consumption, to the Broad Street pump, to a leaking cesspool beneath a nearby house, to the soiled diapers of an infant with cholera who lived at 40 Broad Street. Each link was a relationship: the cesspool contaminated the well, the well fed the pump, the pump served the neighborhood, the neighborhood drank the water, the water carried the pathogen. Remove any single link and the chain breaks. Snow didn't just identify the cause — he traced the entire mechanism. That is what a causal chain does: it converts a mysterious outcome into a sequence of individually verifiable relationships.

This concept is part of Relationship Mapping in the How to Think curriculum.