Score contribution per author:
α: calibrated so average coauthorship-adjusted count equals average raw count
We study a voluntary contributions mechanism in which punishment may be allowed, depending on subjects' voted rules. We found that out of 160 group votes, even when groups had no prior experience with unrestricted punishment, no group ever voted to allow unrestricted punishment and no group ever allowed punishment of high contributors. Over a series of votes and periods of learning we found a distinct reluctance to allow any punishment at the beginning, with a gradual but clear evolution toward allowing punishment of low contributors. And groups allowing punishment of only low contributors achieved levels of cooperation and efficiency that are among the highest in the literature on social dilemmas.