6 Monkeys were trained to pull on a chain to get food. For a few weeks, they happily pulled on the chain whenever they felt like it.
But then, in a nearby cage, when one monkey pulled on the chain, suddenly a new monkey they didn’t know, was shocked with a painful bolt of electricity. They saw it happen. They heard the awful shriek. They watched the monkey grimace and cower in fear.
The monkeys all stopped pulling on the chain for days.
They starved themselves and settled for less food, as long as the other monkey wasn’t hurt.