 A large number of factors appears to affect the size of the contributions. Apart from repetition (one-shot vs repeated games and number of rounds), experience and learning, and communication, the marginal payoff of contributions, the size of the group, provision points (in some experiments the public good is provided only if contributions reach a certain threshold), the heterogeneity of payoffs and endowments, and moral suasion (i.e., the priming of experimental subjects by experimenters) are among the factors that make a difference (Ledyard 1995: 36). Some results are quite surprising. For example, contributions increase with increasing thresholds at which the public good is provided while the probability that the threshold will be reached goes down (Isaac, Walker, &amp; Thomas 1984). However, with heterogeneous endowments, there are no significant differences of contributions at different levels (Rapoport &amp; Suleiman 1993). Similarly, contrary to economists’ expectations, group size can have a positive effect on contributions while it dilutes the effect of marginal returns (Isaac, Walker, &amp; Williams 1994). That causal factors affect experimental results in unsystematic and quite unexpected ways appears to be a fairly general feature of experimental economics (Reiss 2008: Ch. 5).