In Chinese culture, it is said that ‘the fish rots from the head’. Is the 5,000-year-old cultural saying wrong? If not, what is it that makes institutions become corrupt over time?

What has made civilisations themselves corrupt, leading to their extinction, such as that of Babylonia or Egypt? What has made the Christian culture and beliefs of the founding fathers of America become so corrupt that, today, secularism and liberalism drives much of the US agenda?