In an office area, there is a coffee machine and a jar in which people
are asked to put voluntary contributions for the coffee they consume.
There is a picture above the jar.
Each week the picture is changed. Some weeks it is a picture of flowers and other weeks it is a picture of eyes staring at whomever is at the coffee machine.
When it is eyes, people on average put four times as much money into the voluntary jar.
So, if you want more tips, draw eyes on your tip jar.
- Melissa Bateson, Daniel Nettle, and Gilbert Roberts, "Cues of being watched enhance cooperation in a real-world setting"