Garcia effect

In a series of studies he (John Garcia at the University of California-Berkeley) gave rats some food, and then several hours later he gave them a dose of radiation that made them sick. Although the nausea occurred several hours after they ate, the rats generally learned in a single trial never to eat that type of feed - seemingly responsible for their illness - again.

When Garcia paired the nausea with buzzers or light flashes, however, he could not train the rats to avoid them. In other words, rats seem to come into the world “preprogrammed” to learn some things easily, such as to avoid foods linked with nausea, but find it extraordinarily difficult to learn other things. —Evolutionary psychology