What is the incumbency effect?

Incumbency refers to the state of being the current office holder, as opposed to a political challenger. For example, in the U.S. Presidential race of 2012, Barack Obama was the "incumbent"--the person holding the office of the President--and was running against Mitt Romney, the Republican challenger.

However, incumbency can also refer to an obligation or duty. An example of this usage would be a statement such as, "it is incumbent upon you to pay the taxes you owe" or "the need to pay taxes has become an incumbency".