Sex Differences in In-group Bias using a PD Game with Minimal Groups

Nobuhiro Mifune

Toshio Yamagishi

(Hokkaido University)

The 13th International Conference on Social DilemmasKyoto, 20-24th August 2009

Ingroup bias

Ingroup bias: Cooperation or altruism toward ingroup members and/or aggression or discrimination against outgroup members

Ingroup: the group which one belongs toOutgroup: the group which one does not belong to

Ingroup bias

Ingroup bias is ubiquitous in social life (e.g., wars and conflict between nations, religions, or races; corporations; schools, etc.)

Social psychologists have conducted many studies, and have shown that ingroup bias occurs even in Minimal Groups (Tajfel et al., 1971).

Kandinsky groupKlee group

Minimal groups

A B

A

B

A

B AA

A

B

BB

Groups formed on a trivial criterion, No past interaction, no interdependence between participants

Does ingroup bias occur when participants play one-shot PD games with members from the same minimal group (ingroup) or members from the other minimal group (outgroup)?

0

5

10

15

20

25

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35

Coo

pera

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leve

ls %

Ingp Outgp

Did ingroup bias occur?Cooperation levels in one-shot PD game (Jin & Yamagishi, 1997)

Ps cooperated more with ingroup members than outgroup members: ingroup bias occurred even in minimal groups.

Why does ingroup bias occur?

Social Identity Theory:

When people are categorized into a group, they like members of their own group, and cooperate with ingroup members accordingly.

Simply having a partner from the same group is sufficient for ingroup bias to occur.

Is this “true”?

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Coo

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Ingp Outgp

Did ingroup bias occur?Cooperation levels of one-shot PD game (Jin & Yamagishi, 1997)

just having a partner from the same group is NOT sufficient for ingroup bias to occur

Common Knowledge

Private Knowledge

Common Knowledge? Private Knowledge?

Common Knowledge

Ps

My partner is from the same (or

different) group.

My partner is from the same (or

different) group.

Participants could expect cooperation from their ingroup partner. → ingroup bias occurred

Common Knowledge? Private Knowledge?

Private Knowledge

Ps

The partner is from the same (or different)

group, but…

Which group the partner is

from?

Ps

Participants could not expect cooperation from their ingroup partner. So, ingroup bias did not occur

Did Ingroup bias disappear in the private knowledge condition?

It has been repeatedly shown that ingroup bias completely disappeared in the private knowledge condition using several different kinds of games.

PD game (Jin & Yamagishi, 1997; Kiyonari, 2002; Yamagishi, Mifune, Liu & Pauling, 2008)

Allocator choice game (Suzuki, Konno & Yamagishi, 2007)

Dictator game (Hashimoto, Mifune & Yamagishi, the 2nd day presentation; Yamagishi & Mifune, 2008)

Did ingroup bias disappear in the private knowledge condition? (cont’d)

Some evolutionary psychologists have predicted that there should be sex differences in ingroup bias.

Males’ adaptive tasks in evolutionary history included dealing with intergroup conflict. So, males should have the tendency to cooperate with ingroup member and attack or compete with outgroup member.

Male Warrior Hypothesis (van Vugt, de Cremer & Janssen, 2007)

Primal Warrior Hypothesis (Yuki & Yokota, 2008)

Did Ingroup bias disappear in the private knowledge condition? (cont’d)

However, no sex difference in ingroup bias was found in the past experiments (e.g., Jin & Yamagishi, 1997; Yamagishi, Mifune, Liu & Pauling, 2008).

Is this because of the mixed-sex situation? (i.e., in the Jin & Yamagishi (1997) or other experiments, both males and females participated at the same time)

How about in groups of only male or female Ps?

Experiment: Replication of Jin & YamagishiKlee

Klee

Klee

Klee

Kandinsky

Kandinsky

Kandinsky

Kandinsky

Only males

Experiment: Replication of Jin & Yamagishi

Klee

Klee

Klee

Klee

Kandinsky

Kandinsky

Kandinsky

Kandinsky

Only femalesWhen we replicate Jin & Yamagishi (1997) using same-sex groups, can we find any sex difference in ingroup bias?

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Common Private Common Private

Female Male

% Ingp Outgp

Cooperation levels

< .01Result of Jin & Yamagishi (1997)

Common Private

Cooperation levels relative to the control condition

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30

40

Control Common Private Control Common Private

Female Male

Coo

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tion

leve

ls %

Ingp Outgp

ns. ns.

ns. ns. ns.

< .01 < .01< .01

Aggression or discrimination against outgroup members did not occur

Control Common Private

Result of Jin & Yamagishi (1997)

Control: Ps did not know their partner’s group membership

Ingroup bias in minimal groups is based on ingroup cooperation, not aggression toward outgroup members (e.g., Mummendey & Otten)

Correlation of Cooperation between Ingroup and Outgroup

Partial correlation coefficients between cooperation with ingroup members and outgroup members in the common knowledge condition and private knowledge condition, controlling for the default cooperative tendency (i. e., cooperation level in the control condition)

Female

Private

　In-group

Out-group

0.38

p = 0.002

Common

　In-group

Out-group

0.34

p = 0.007

Male

Private

　In-group

Out-group

0.21

p = 0.078

Common

　In-group

Out-group

0.10

p = 0.430

Correlation of Cooperation between Ingroup and Outgroup

Partial correlation coefficients between cooperation with ingroup member and that of outgroup member in the common knowledge condition and that in the private knowledge condition with controlling the default cooperative tendency (i. e., cooperation level of the control condition)

Female

Private

　In-group

Out-group

0.38

p = 0.002

Common

　In-group

Out-group

0.34

p = 0.007

Male

Private

　In-group

Out-group

0.21

p = 0.078

Common

　In-group

Out-group

0.10

p = 0.430

Outgroup cooperation was not negatively correlated with ingroup cooperation.

Ingroup cooperation does not entail aggression toward outgroup members (cf. Brewer, 1999)

Summary

People cooperated with ingroup members even in the minimal groups.

Ingroup cooperation occurred only in the common knowledge condition, but not in the private knowledge condition.

In a same-sex situation (groups of all males or all females), males showed ingroup cooperation regardless of the knowledge condition.

The results for all-female groups was completely same as in the past experiment (Jin & Yamagishi, 1997).

How can we explain these results?

Previous studies (e.g., Jin & Yamagishi, 1997)

Current study

Discussion

Why did participants show ingroup cooperation only in the common knowledge condition in the past studies and among females in this study?

Group heuristic: People have a “default decision rule” to cooperate with ingroup members.

The “Group” is a container of generalized exchange (Yamagishi & Kiyonari, 2000). People assume “If I cooperate with an ingroup member, the other ingroup member may also cooperate with me.” In other words, when they can expect the cooperation from ingroup members, they will cooperate with ingroup members.

Discussion

Why did males show ingroup cooperation even in the private knowledge condition (i.e., when they could not expect cooperation from their ingroup partner), and yet, they did not show any aggression toward the outgroup?

=> Male-specific intergroup aggression (Male Warrior Hypothesis or Primal Warrior Hypothesis) cannot explain the two findings simultaneously.

Discussion

Why did they cooperate with ingroup members unconditionally?

Display of Solidarity Hypothesis (Gould, 1999, 2000)

One reason to form groups is to prevent aggression from others. Individuals who successfully show solidarity toward a group can avert aggression. On the other hand, individuals who are in less cohesive groups can become targets of aggression.

The unconditional nature of cooperation makes the ingroup solidarity credible.

Considering the costs of being attacked (e.g., death), the incentive for free-riding may not be so high.

Discussion

Why are displays of solidarity specific to men?

Men are the primary targets of inter-group aggression (Sidanius &

Veniegas, 2000). Because of this, it is adaptive for men to be sensitive to detecting intergroup conflict situation in order to avert aggression.

If the benefits of displaying solidarity (i.e., reducing costs of conflict/warfare) are greater among men than among women, such benefits should exceed the costs of displaying group solidarity (unconditional cooperation to the ingroup, etc.).

Thank you

Yamagishi, T., & Mifune, N. (2009). Social exchange and solidarity: in-group love or out-group hate? Evolution and Human Behavior, 30, 229-237.