Ivan Pavlov is best known for letter D. He is the one who came up with classical conditioning. If you are not familiar with this, this is the process wherein people who grow up in similar environments may be conditioned to have some similarities. Usually, there is a stimulus that will produce a reaction that will be similar among those who have experienced the same things.
He did experimentation with dogs wherein a bell would be associated with food. This will cause dogs to salivate. This can also be applied to how people may react to certain situations. For example, there are people who may start reacting in fear when they hear a loud sound because they have associated the sound with something bad.