How Much Social Pressure Shapes Altruism?

In an experiment, List sent volunteers into various chicago neighborhoods to solicit money for a children's hospital. Some households just got a cold call, a knock on the door. Others were alerted ahead of time that someone was going to be knocking on their door and asking for money. A third group was told that the knock was coming and given the choice to opt out. Liszt says this shows how much social pressure shapes generosity.