Table 1. Ten strategies tested in the model.

Strategy	Description
Always-cooperate	An always-cooperate individual always chooses to cooperate in social interaction, regardless of his opponent’s choice.
Always-defect	An always-defect individual always chooses to defect in social interaction, regardless of his opponent’s choice.
Always-trembling	An always-trembling individual randomly switches between cooperation and defection. His probability of cooperation or defection is 0.5 in each social interaction.
Tit-for-tat	A tit-for-tat individual always cooperates in the first round of social interaction with a new opponent and remembers his opponent’s choice. If he meets the opponent again, he will repeat his opponent’s choice in the previous round.
Generous tit-for-tat	A generous tit-for-tat individual basically uses the tit-for-tat strategy, but won’t retaliate on every defection. He has a certain probability ((b−c)/c) of cooperation when his opponent defects.
TFT-with-trembling-hand	A TFT-with-trembling-hand individual is basically a tit-for-tat individual except that he has a certain probability of random error (random defection) and doesn’t recall his error.
Shame-driven-hiding (self-conscious)	A shame-driven- hiding individual is basically a TFT-with-trembling-hand individual except that he remembers his error and tries to hide from it (avoid the interaction with the individual whom he defected on before).
Shame-driven-denying (self-conscious)	A shame-driven-denying individual is basically a TFT-with-trembling-hand individual except that he remembers his error and tries to deny it (deliberately defect on the individual whom he defected on before).
Guilt-driven-amending (self-conscious)	A guilt-driven-amending individual is basically a TFT-with-trembling-hand individual except that he remembers his error and tries to amend it (voluntarily cooperate with the individual whom he defected on before).
Pavlov*	A Pavlov individual uses a win-stay, lose-switch strategy. He only remembers his own choice. If he got b−c or b in social interaction, he would continue his choice. If he got–c or 0, he would switch his choice.

* The individuals who adopt Pavlov strategy will also make random errors. They have a probability of randomly switching choice.