The behavior resulting from conditional cooperation
is consistent with at least three theoretical
approaches: first, people may want to
behave in an appropriate way and to conform to
a social norm (David M. Messick, 1999); second,
people have some level of fairness preferences,
such as reciprocity (Matthew Rabin,
1993); or third, contributions by others may
serve as a signal of the quality of the public
good, or of the organization that provides the
good (e.g., a charity) (Lise Vesterlund, 2003).