Ask HN: Do social sites with karma need a decay function?

If karma in a social site is a reward for good behavior, it seems like it always becomes harder and harder for new users to "get ahead" compared to those who have been around forever. Perhaps this lessens the incentive to good behavior over time?

Do such sites need to implement some kind of "karma decay" system, to prevent the leader-board from becoming more static over time and encourage new users to behave themselves?