Fehr and Gachter (2000), Cooperation and punishment in public goods

Fehr, E., & Gachter, S. (2000). Cooperation and punishment in public goods experiments. American Economic Review, 90, 980-994.

They did a number of public goods experiments with 24 subjects. In non-punishment conditions, it was a regular public goods game in which they could invest some amount of their initial endowment. In the punishment condition, each round had a punishment stage, in which they could contribute to punish individuals (presumably those with low contributions).
In stranger conditions, at each round, a random group of 4 players was made, so there was no correlation between rounds. In partner conditions, the same group played together round after round.

Results

In the stranger condition, no punishment eventually everyone goes to the free-riding situation. However, with punishment, average contributions are stable.

In the partner condition, no punishment eventually leads to free riding again. However, with punishment, the situation eventually goes to full cooperation.