Oxytocin promotes coordinated out-group attack during intergroup conflict in humans
- Beijing Normal University, China
- Leiden University, The Netherlands
- University of Amsterdam, The Netherlands
Figures
Figure 1
General experimental procedure.
https://doi.org/10.7554/eLife.40698.003
Figure 2
Illustration of one round of the IADC game in the simultaneous and sequential decision-making blocks, respectively.
https://doi.org/10.7554/eLife.40698.004
Figure 3
Oxytocin modulates contributions to group fighting.
(A) Attackers contribute less than defenders, especially in early rounds (range 0–20). Curves were smoothed with a moving average window of three investment rounds. (B) Giving individuals oxytocin rather than placebo increases the number of non-contributing members in attacker groups especially under simultaneous decision-making (with 0–3 members per round across 15 rounds; range 0–45; displayed M ± 1 SE). Connectors indicate significant difference, with *p<0.05.
Figure 4
Response time for decisions to (not) contribute.
Oxytocin increased the speed with which attackers made their decisions to not contribute. Connectors indicate significant difference, with *p<0.05.
Figure 5 with 1 supplement
Oxytocin modulates within-group coordination.
(A) Giving individuals oxytocin rather than placebo enables better coordination (lower within-group variance) in attacker groups, especially in early rounds. Curves were smoothed with a moving average window of three investment rounds. (B/C) Giving attackers oxytocin rather than placebo increases their leftovers when not winning the conflict (B) and spoils from winning conflicts (C) (N = 76 because four attacker groups never won). (D) Bootstrapping illustration of the oxytocin shifts on the contribution and payment. Bivariate distributions of 1000 bootstrapped sample means for each condition (Treatment x Procedure) plotted against the contribution and payment. (E) Oxytocin increased non-contributing attackers only in failed attacks but not in successful attacks. Connectors indicate significant difference, with † p < 0.10; *p < 0.05; **p < 0.01.
Figure 5—figure supplement 1
Oxytocin increases attacker group’s within-group coordination especially in the simultaneous decision-making block. Connectors indicate significant difference, with *p<0.05, *** p< 0.001.
https://doi.org/10.7554/eLife.40698.009
Figure 6 with 1 supplement
Oxytocin enables a track-and-attack strategy (strength of attack increases when defender groups are vulnerable rather than strong, as indicated by α → −1).
(A) When attacker groups are given oxytocin investments regress negatively on α (the rival’s historical investments to defense), especially during simultaneous than sequential decision-making. (B) Stronger negative regression of attack on rival’s defense history (α → −1) among attacker groups associates with better coordination (i.e. lower within-group variance). (C) Better coordination (i.e. lower within-group variance) associates with higher spoils when winning the conflict. (D) Oxytocin’s effect on spoils from successful attacks is mediated by treatment effects on tracking α (more strategic when α → −1) and better within-group coordination. † p < 0.10; * p < 0.05; ** p < 0.010).
Figure 6—figure supplement 1
Oxytocin influences payment through its effects on strategic tracking and better within-group coordination.
(A) Better coordination (i.e. lower within-group variance) associates with higher spoils and leftovers. (B) Oxytocin’s effect on attacker groups’ spoils from successful attacks and leftovers from attack failure is mediated by treatment effects on tracking α and better within-group coordination. Significant pathways were highlighted in bold. (* p < 0.05; ** p < 0.01).
Tables
Table 1
Payoff matrix of one-round IADC.
https://doi.org/10.7554/eLife.40698.005| Role | Participant | Initial endowment (MU) | Individual contribution (Ii) | Group pool (G) | Payment | ||
|---|---|---|---|---|---|---|---|
| Attackers lose GAttacker ≥ GDefender | Attackers win GAttacker > GDefender | ||||||
| Leftover | Leftover | Spoil | |||||
| Attack | Attacker-1 | 20 | IAttacker-1 | GAttacker | 20 − IAttacker-1 | 20 − IAttacker-1 | (60 − GDefender)/3 |
| Attacker-2 | 20 | IAttacker-2 | 20 − IAttacker-2 | 20 − IAttacker-2 | (60 − GDefender)/3 | ||
| Attacker-3 | 20 | IAttacker-3 | 20 − IAttacker-3 | 20 − IAttacker-3 | (60 − GDefender)/3 | ||
| Defend | Defender-1 | 20 | IDefender-1 | GDefender | 20 − IDefender-1 | 0 | 0 |
| Defender-2 | 20 | IDefender-2 | 20 − IDefender-2 | 0 | 0 | ||
| Defender-3 | 20 | IDefender-3 | 20 − IDefender-3 | 0 | 0 | ||
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Table note: For each round, each individual received an initial endowment of 20 MUs (Monetary Units).
Each individual decided the amount (Ii , 0 ≤ Ii ≤ 20) to the group’s pool G (0 ≤ G ≤ 60, GAttacker = IAttacker-1+IAttacker-2+IAttacker-3, GDefender = IDefender-1+IDefender-2+IDefender-3). When GAttacker ≤GDefender, attackers failed and defenders survived and all six individuals kept their remaining endowment (leftovers, 20 – Ii). When GAttacker >GDefender, defenders failed and left with 0. The attackers won and took away defenders’ remaining MU (spoils from winning, 60 – GDefender), which were divided equally among attacker group members (each attacker: (60 – GDefender)/3) and added to their remaining endowments (20 – IAttacker-i).
Additional files
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Supplementary file 1
Supplementary Table 1.
(A) Table 1A. Match demographic information and prosocial-related traits. (B) Table 1B. Mood changes from pre-experiment to post-experiment. (C) Table 1C. Point estimates for indirect effects and bootstrapped 95% bias-corrected confidence intervals for multiple mediational analysis in which attacker group’s tracking (strategic tracking when α→ −1) and within-group variance (variance) were represented as mediators in the association between Treatment and spoils from winning a conflict during simultaneous decision-making. (D) Table 1D. Point estimates for indirect effects and bootstrapped 95% bias-corrected confidence intervals for multiple mediational analysis in which attacker group’s tracking (strategic tracking when α→ −1) and within-group variance (variance) were represented as mediators in the association between Treatment and spoils and leftovers during simultaneous decision-making.
- https://doi.org/10.7554/eLife.40698.012
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Transparent reporting form
- https://doi.org/10.7554/eLife.40698.013
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Oxytocin promotes coordinated out-group attack during intergroup conflict in humans
eLife 8:e40698.
https://doi.org/10.7554/eLife.40698
