Ivan Pavlov

In this photograph taken in 1925, Dr. Ivan Pavlov and his assistants demonstrate how conditioned reflexes work in dogs.

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Ivan Pavlov may be best known by memorable sound bites, such as "Pavlov's dogs" or the "Pavlovian response." But his sprawling impact on science can't be reduced to such concise phrases.

Pavlov won the 1904 Nobel in physiology. He's best known for his research on conditioned reflexes. In his most famous experiments, he would ring a bell every time he gave food to dogs. After repeating this process over and over again, the dogs would eventually begin salivating simply at the sound of the bell. It wasn't long before people realized that humans weren't all that different from dogs. We're all conditioned to respond certain ways -- both good and bad -- to various stimuli.

Pavlov's insights opened new doors in psychology and behaviorism, and they altered the way people perceive their own behaviors. He was so well-regarded in the Soviet Union and around the world that the Soviet government couldn't muzzle his outspoken condemnation of Communism. By the time he won the Nobel, he was already one of the most renowned scientists in the world, and his discoveries still reverberate today.