Public Goods with punishment
Instructions
You belong to a randomly
formed group of five players. You are given 20 points each and must decide
simultaneously on how much to invest to the common pool. The total contribution
is multiplied by ½ and added to EACH players payoff  independently
of whether this player contributed or not. After this one round game, you
will be able to punish your partners by reducing their score by a certain
number of points at the cost of half the points.
Check:
Question: Suppose
that players 1, 2 and 3 give 10 points each and players 4 and 5 contribute
nothing. What is the common pool? How many points will each player have?
Answer: 1, 2, and
3 end up with 25 points, the others with 35 points.
Question: What if
only player 1 gives 10 points and the others nothing?
Answer: Player 1 ends
up with 15 points, the others with 25.
Evaluations
Number of players: 25 (5 games)
Mean investment: 9.6 ±4.6
Mean payoffs: 34.4 ± 5.0 before punishment
18.6 ± 15.9 final
Notes

Punishment increases investment from an average of 4.0 to 9.6.

Final payoffs, however, are lower! (18.6 instead of 26.1)

The minimal investment was 2.

The biggest differences between intermediate and final payoffs occur for
very high and very low investors.
Written by Christoph Hauert