We may all realize that there are donation boxes at the counters of McDonalds, Starbucks, and many other cafes. Most donation boxes that I have seen so far are transparent and are at least half full. Many years ago, I passed by McDonalds when the staff was trying to clear the coins out of a full charity box. Surprisingly, he only cleared half of the coins in the box and put the box back in place. This idea is brilliant as it shows the targets that many other people are donating their coins. They are trying to leverage the power of social proof and social influence so as to convince people to donate.
Alpizar, Carlsson, & Stenman (2008) conducted a natural field experiment on voluntary contributions to a national park. International tourists were asked if they would like to donate to the park. The tourists were either given information about other people’s contributions ($2, $5, or $10), or were asked to donate without any information. The results of their study showed that the likelihood of contributing anything at all is significantly higher in the $2 reference contribution.
This suggests that people will be more willing to donate when they see (1) many other people are donating (keeping the box at least half full), and (2) the amount that people usually contribute (using a transparent box).
F. Alpizar, F. Carlsson, & O.J. Stenman (2008). Anonymity, reciprocity, and conformity: Evidence from voluntary contributions to a national park in Costa Rica. Journal of Public Economics, 92, 1047-1060.