stage-game equilibrium, Selfish players have a lower expected utility compared with

Other-Regarding players, since the latter produce lower efforts, which increases the

overall expected payoffs. Hence, Selfish players have more to gain by coordinating on

the Pareto-dominant equilibrium. This yields our next hypothesis:

Hypothesis 2. Selfish subjects are more likely to emerge as leaders

Since a leader is most likely needed for achieving the Pareto-dominant outcome

(e.g., see Kreps 1986, and Hermalin 2012), and we expect Selfish people are more likely

to become a leader (Hypothesis 2), a group with no Selfish subjects is less likely to

collude (i.e., coordinate on minimal effort) than a group with a Selfish player:

Hypothesis 3. With communication, collusion is more likely for a group with a

Selfish player than one with no Selfish players.

Ideally, we would like to “turn off” and “turn on” social preferences to identify

their effects on efforts. We can possibly achieve this by pairing individuals with

subjects that behave like human subjects but do not incur payoffs. Specifically,

other-regarding concerns should not play a role when partners are machines. Thus, if

we pair subjects knowingly with computer-simulated subjects, we expect Selfish and

Other-Regarding subjects to behave similarly.

Hypothesis 4. Selfish and Other-Regarding subjects behave similarly when paired

with computer simulated subject.

5 Experimental Results

We begin by classifying subjects in terms of social preferences derived from their

giving behavior. We then use these results to study the relationship of individual and

the group composition of social preferences and effort, the emergence of leaders, and

collusive outcomes.

5.1 Categorizing Social Preference Types from Giving Menus

Table 2 summarizes the mean choices of our subjects under all 9 price vectors in

treatments: 1) Chat and 2) No Chat. 10 We analyze the Robot treatment in section


We see that regardless of the price of giving, subjects keep on average just above

70% of their endowment. Using these choices, we sort our subjects into Selfish and

Other-Regarding. A subject is categorized as Selfish if he or she does not allocate

any tokens to the other group members in any of the nine periods. All subjects who

work under the simplifying assumption that Other-regarding players’utility is a weighted sum of

individual payoffs and weights add up to one.

10 These vectors (a, b, c) represent the price a of giving to one’s self, the price b of giving to player

1, and the price c of giving to player 2.


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