Is corruptness an inate quality of a politician or do they just join the dark side after being elected?

When someone graduates high-school and must pick a major in College: what are the characteristics of those who enroll in political classes? Are they corrupt to begin with and just looking for a safe haven? Are they honest until they graduate and get a job in politics and then go with the flow and become dishonest, self-absorbed liars? After being elected to a position and receive their first paycheck: does that flip a switch in their heads and turn them against the people who voted them in?