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Oxytocin promotes coordinated out-group attack during intergroup conflict in humans

  1. Hejing Zhang
  2. Jörg Gross
  3. Carsten De Dreu
  4. Yina Ma  Is a corresponding author
  1. Beijing Normal University, China
  2. Leiden University, The Netherlands
  3. University of Amsterdam, The Netherlands
Research Communication
Cite this article as: eLife 2019;8:e40698 doi: 10.7554/eLife.40698
6 figures, 1 table, 1 data set and 2 additional files

Figures

General experimental procedure.
https://doi.org/10.7554/eLife.40698.003
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
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.

https://doi.org/10.7554/eLife.40698.006
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.

https://doi.org/10.7554/eLife.40698.007
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.

https://doi.org/10.7554/eLife.40698.008
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).

https://doi.org/10.7554/eLife.40698.010
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).

https://doi.org/10.7554/eLife.40698.011

Tables

Table 1
Payoff matrix of one-round IADC.
https://doi.org/10.7554/eLife.40698.005
RoleParticipantInitial endowment
(MU)
Individual contribution
(Ii)
Group pool (G)Payment
Attackers lose
GAttacker 
 ≤ GDefender
Attackers win
GAttacker > GDefender
LeftoverLeftoverSpoil
AttackAttacker-120IAttacker-1

GAttacker

20IAttacker-120 − IAttacker-1(60 − GDefender)/3
Attacker-220IAttacker-220IAttacker-220 IAttacker-2(60 − GDefender)/3
Attacker-320IAttacker-320IAttacker-320 − IAttacker-3(60 − GDefender)/3
DefendDefender-120IDefender-1

GDefender

20IDefender-100
Defender-220IDefender-220IDefender-200
Defender-320IDefender-320IDefender-300
  1. 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).

Data availability

The group-level statistics are plotted in Figures 3-6, Figure 5-figure supplement 1 and Figure 6-figure supplement 1.The individual data is plotted in Figures 6 and Figure 6-figure supplement 1. The source data for Figures 3-6 is publicly available on the Open Science Framework at https://osf.io/7jeas/ (doi: 10.17605/OSF.IO/7JEAS).

The following data sets were generated
  1. 1
    Open Science Framework
    1. SANP_Neuroscience
    2. Zhang Hejing
    3. Ma Yina
    (2018)
    Oxytocin promotes coordinated out-group attack during intergroup conflict in humans.
    https://doi.org/10.17605/OSF.IO/7JEAS

Additional files

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
Transparent reporting form
https://doi.org/10.7554/eLife.40698.013

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