From Barking Up The Wrong Tree:

What determines reciprocity in employment relations? We conducted a controlled field experiment and tested the extent to which cash and non-monetary gifts affect workers’ productivity. Our main finding is that the nature of the gift, not its monetary value, determines the prevalence of reciprocal reactions. A gift in-kind results in a signicant and substantial increase in workers’ productivity. An equivalent cash gift, on the other hand, is largely ineffective or even though an additional experiment showed that workers would strongly favor the gift’s cash equivalent.

It probably has nothing to do with reciprocity.  If I pay you money you have to share it with your family and then buy a car out of your share.  If I give you a car it is all yours.

This logic also often provides a psychology-free explanation of the endowment effect.  You are willing to pay at most $10,000 for a car.  But if I give you that car for free and offer to buy it back from you, you require $20,000, because you will get to keep only half of that money.

(inspired by discussions with my Behavioral Economics class.)

Update: See Ben’s comment below for another variation on the theme which also came up in class.  If you have present-biased preferences you have an endowment effect because cash will be shared with future selves, whereas instantaneous consumption is all for your present self.