Of course, there can be more explanations 1/ Assuming the contributors can punish anyone, participants who contributed higher in 2-3 rounds would start punishi

Of course, there can be more explanations 1/ Assuming the contributors can punish anyone, participants who contributed higher in 2-3 rounds would start punishing those who contribute less, given at some level, they are also free-riding though not to the extent of free riders. 2/ Free riders punishing the contributors hoping next time they do not get punished (cos punishment is a choice rather than an obligation, you may choose to enforce it.) 3/ A cynical way of playing this game is that when you know contributor is punishing someone, he loses a bit more, and free rider's profits are reduced(when overall pool < 20). Athens have a reverse effect when the rules are changed. A deduction is that I can risk to lose a bit of my gains, and can bully the contributors into not punishing me again and again.