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were higher in the first 20 periods, but they showed a rapid decrease in the last
20 periods. Increasing the amount of information about other agents made this
decrease steeper, even if this pattern remained stable for different group sizes.
This is due to the fact that playing “Defect” triggered even more defections,
thus reducing the overall cooperation rate and making the decrease even steeper
when the amount of available information was higher.
defect, no gossip
defect, 1 gossip
defect, 10 gossips
0.8
0.6
0.4
groups of
5
10
25
50
0.2
refuse, no gossip
refuse, 1 gossip
refuse, 10 gossips
0.8
0.6
0.4
0.2
0
25
50
75
100 0 5 0 5 00 5 0 5 0
step
Fig. 2.
Cooperation rates over time for Defect (top) and Refuse (bottom) strategies for
Gossipers. In larger groups (25 and 50 agents) the amount of information has a negative
effect on cooperation rates when the reaction is based on defection, as compared to the
Refuse strategy. In this latter case, cooperation rates are stable and quite high.
On the contrary, when gossipers played “Refuse”, cooperation levels were quite
high for all group sizes. Small groups showed higher cooperation rates (Figure
2, bottom panel), because refusing dangerous interactions in small groups was
effective in isolating cheaters and in making cooperators interact with other
cooperators. Taking into account the first periods of the simulation experiments,
we saw that there are interesting differences in the cooperation rates, not only
between groups of different sizes, but also among situations with zero, one or
ten items of information (3).
In a scenario in which all strategies are loaded, we compared the performance
of populations in which all the strategies were present (C, FR, TFT, NP, MP,
NG and MG) for different combinations of costs (HpHc, HpLc, LcHp, LcLp), for
different group size (5 and 50). Figure 3 (4) shows that punishment and reputa-
tion can have a combined effect that boosts cooperation to 1 in large groups in
every situation but in the LpHc. The negative effect of Defect is confirmed also
when there are both Punishers and Gossipers, and it is also made more relevant
by the higher amount of information.
