In the 1950s and 60s, John Garcia discovered a form of classical conditioning that violated then-accepted principles of learning — and initially couldn't get published because it challenged established theory so directly.

The Finding

Rats that consumed a novel-flavored drink and then experienced nausea (via radiation or lithium chloride injection) — even hours later — developed a powerful, lasting aversion to that flavor after a single pairing.

Why This Was Revolutionary

Classical conditioning was thought to require:

• Temporal contiguity — the conditioned stimulus and unconditioned stimulus must occur close together in time
• Many pairings to establish the association

Garcia's rats developed aversions from a single trial, with a gap of hours between the taste and the nausea. This violated both rules.

Biological Preparedness

The explanation: evolution has built in specific learning pathways for biologically relevant associations. Nausea and taste are a highly relevant combination for survival — so the brain learns this connection rapidly and indelibly, outside the standard rules.

Applied Uses

Taste aversion conditioning is now used to protect livestock from predators and reduce coyote attacks — by lacing bait with nausea-inducing agents.

Reference:

Wikipedia: Taste Aversion

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