Figure 2. Stabilization of cooperation.

(a) Cooperation by round averaged over the course of a 20-game session, grouped by day. Early days (coloured red through green) show the sharpening of end-game effect (that is, initial cooperation increases but drops off further and more suddenly as the end-game approaches), after which the pattern stabilizes (green through purple). (b) Average restart effect between games (that is, difference in cooperation rate between round 10 of game j and round 1 of game j+1). Consistent with (a), the restart effect increases for several days then stabilizes. (c) The stabilization of cooperation is partly accounted for by the cross-session restart effect (orange): the jump in cooperation rate between the last game of day d and the first game of day d+1 for rounds 9 (left) and 10 (right). For comparison, the corresponding within-session effect (that is, difference in round 9/10 cooperation rate between successive games within a session) is also shown (teal).