The game is a classic prisoner's dilemma, but the framing—introduced by Andreoni and Varian (1999)1—may obscure this fact. Each student is matched with another over a number of rounds. In each round, each chooses between Pull (akin to defect) or Push (akin to cooperate). Actions and earnings are revealed after each round.
In a one-shot prisoner's dilemma, defect is the dominant strategy. The Nash equilibrium is therefore for each player to defect (choose Pull) despite the fact that cooperation (both choosing Push) maximizes overall earnings.
Communication and indefinite repetition may facilitate cooperative outcomes (choosing Push).
With the default parameters, Pull increases own earnings by 300 while Push instead increases the match's earnings by 400. Each pair plays 5 rounds, which should lead to Nash-equilibrium play for most pairs in the final round, particularly if the number of rounds is announced in advance.
Allowing Chat will generally lead to more cooperative behavior. Not revealing the number of rounds will also lead to an increase in students choosing Push, the cooperative action.
Another way to increase cooperative behavior is to increase the Push amount (400 by default) relative to the Pull amount (300 by default).
The results screen presents a table and a chart, each focusing on the likelihood of each action.
The table (Figure 1) indicates the percentage of students choosing each action in each round.
The chart (Figure 2) presents this information in a slightly different way. It shows for each round the incidence of the three outcomes: both played Pull; both played Push; and one played Push while the other played Pull. It is likely that groups playing Push/Pull in early rounds will convert to Pull/Pull in later rounds.
Note: The chart has dynamic elements. Click the checkboxes in the legend to hide a particular category. Also, mousing over a label in the legend highlights the corresponding bars in the chart.
By default, robots (i.e. automated players) choose randomly between Push and Pull (i.e. Always Random). With the Robot Strategies drop-down menu, you can choose alternative strategies, including:
By selecting All Play Only Robots in a series of indefinitely repeated games with Tit-for-Tat robots, students can experience the organic emergence of cooperative outcomes.