1. Sex Differences in In-group Bias using a PD Game with Minimal Groups Nobuhiro Mifune Toshio Yamagishi (Hokkaido University) The 13 th International Conference on Social Dilemmas Kyoto, 20-24 th August 2009

2. Ingroup bias Ingroup bias: Cooperation or altruism toward ingroup members and/or aggression or discrimination against outgroup members Ingroup: the group which one belongs to Outgroup: the group which one does not belong to

3. 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).

4. Kandinsky groupKlee group Minimal groups A B A B A B A A A B B B 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)?

5. 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.

6. 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”?

7. 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

8. Common Knowledge? Private Knowledge? Common Knowledge Ps My partner is from the same (or different) group. Participants could expect cooperation from their ingroup partner. → ingroup bias occurred

9. 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

10. 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 2 nd day presentation; Yamagishi & Mifune, 2008)

11. 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)

12. 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?

13. Experiment: Replication of Jin & Yamagishi Klee Kandinsky Only males

14. Experiment: Replication of Jin & Yamagishi Klee Kandinsky Only females When we replicate Jin & Yamagishi (1997) using same-sex groups, can we find any sex difference in ingroup bias?

15. Cooperation levels <.01 Result of Jin & Yamagishi (1997)

16. Cooperation levels relative to the control condition ns. <.01 Aggression or discrimination against outgroup members did not occur 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)

17. 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 Comm on In- group Out- group 0.34 p = 0.007 Male Private In- group Out- group 0.21 p = 0.078 Comm on In- group Out- group 0.10 p = 0.430

18. 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 Comm on In- group Out- group 0.34 p = 0.007 Male Private In- group Out- group 0.21 p = 0.078 Comm on 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)

19. 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

20. 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.

21. 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.

22. 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.

23. 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.).

24. 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.