Philosophers, like Plato, have already asked this old question. Thomas Hobbes and Adam Smith, also. The question is: what makes a country less corrupt while others extremely corrupt? Plato answered this by the simple proposition: If power handed to Philosopher-King (maybe like himself), a country would be better, in many ways. Including less corrupt Because this king knows how to govern well. He has the wisdom to do so. And he is also clean.
But, democracy rise because Plato was fatally wrong. There is no Philosopher-King, at least, in reality. An individual can corrupt and also incompetent while in power.
We see nowadays, less corrupt countries exist. Comparing countries all over the world, political scientists search for the explanation. What makes a country less corrupt? Naci Mocan, a professor from Lousiana State University, has a simple answer: uninterrupted democracy, i.e., freedom to punish leaders who corrupt by not reelect him or her, and the rule of law created by common people, i.e., Common Law based on British Legal System. He found this argument on robust data: interviews of 54 thousand people from 29 countries in five continents. His answer seems very hard to refute.
What makes these research great? Because it uses data from more than 50,000 respondents from 29 countries across continents, something that Plato would envy as scholars.