Hey its Kate and I currently read an article on the "groundbreaker" Leon Festinger. He lived in the 1900's and was an American social psychologist. Festinger was best known for cognitive dissonance and social comparison theory. Social comparison theory and cognitive dissonance were described by other psychologist as " the two most fruitful theories in social psychology.
I believe he is one of the most important because he created the theory that the idea starts that we seek consistency in our lives and attitudes in any situation where two cognitions are inconsistent. He was diagnosed with cancer before he was able to publish some new material, and decided not to get treatment. The time period has huge influence on medicane with how prevention of some cancers could of lead to more knowledge.
To read the article for yourself- en.wikipedia.org/wiki./Leon_Festinger