 Field studies have been used to examine the relationship between an individual’s contributions to a public good and others’ contributions. Depending on what one thinks individuals’ primary motivations are, one would expect an individual’s and others’ contributions to be substitutes or complements. If an individual is primarily motivated by altruism, he or she will care about the consumption of others and therefore contribute less when others or the government already contribute enough. In this case we would expect the two kinds of contributions to be substitutes. If, by contrast, an individual is primarily motivated my social norms such as fairness and reciprocity, he or she will contribute when others do their bit. In that case, we would expect the two kinds of contributions to be complements. Whether they are on average substitutes or complements is an issue that is difficult to determine in laboratory experiments due to the relatively small sample sizes and because the mix of motivations may differ between experiment and field.