Reciprocation
People generally feel obliged to return favours offered to them. This trait is embodied in all human cultures and is one of the human characteristics that allow us to live as a society.
Compliance professionals often play on this trait by offering a small gift to potential customers. Studies have shown that even if the gift is unwanted, it will influence the recipient to reciprocate.
A variation on this theme is to ask for a particularly big favour. When this is turned down, a smaller favour is asked for. This is likely to be successful because a concession on one side (the downscaling of the favour) will be reciprocated by a concession by the other party (agreement to the smaller favour).
Reciprocation is an application of Reciprocity (social psychology).
Read more about this topic: Influence: Science And Practice