 A field experiment involving an on-air fundraising campaign for a public radio station found support for the complementarity hypothesis (Shang &amp; Croson 2009). Specifically, if social information is provided about a high contribution of another donor, pledges increase and the likelihood that a donor contributes again next year goes up. The same has been found in a study of voluntary contributions to an information good (a newsletter) on the Internet (Borck et al. 2006). There have also been field studies of alternative provision mechanisms, comparing a voluntary contribution mechanism for a pure public environmental good to a green tariff mechanism, which can be interpreted as an impure public good (Kotchen &amp; Moore 2007).