" In a set of classic experiments, the psychologist Z. Y. Kuo6 showed that almost all kittens who witness their mother killing and eating a rodent eventually do so themselves. However, when kittens are raised in the same cage with a rat, never seeing any other rat, and never seeing a cat kill a rat, then they almost never kill rats themselves. When kittens have a rat for a littermate and also witness their mothers killing rats outside the cage, about half of them learn to kill—but they tend to kill only the kinds of rat they had seen their mothers kill, and not the kinds that they grew up with. Finally, when kittens are given an electric shock each time they see a rat, they soon learn not to kill rats—indeed, to run in terror from them. "

― Carl Sagan , Shadows of Forgotten Ancestors