Reinforcer

[Britney]

A thing, event or action given to someone to ensure that they [behave] the way you want them to. The term may be applied to the person who does it.

Not to be confused with enforcer which is a person paid to [punish] those who fail to do something such as [abide] by a rule or agreement

Usually used to describe negative or unpleasant, when the person knows they are being controlled, but equally applies to [pleasant] or positive event, when the person usually doesnt know they are being controlled but thinks they are doing it because they want to. Positive reinforcers are more subtle and more effective.