Ingroup/outgroup membership modulates fairness consideration: neural signatures from ERPs and EEG oscillations

Abstract

Previous studies have shown that ingroup/outgroup membership influences individual's fairness considerations. However, it is not clear yet how group membership influences brain activity when a recipient evaluates the fairness of asset distribution. In this study, subjects participated as recipients in an Ultimatum Game with alleged members of both an experimentally induced ingroup and outgroup. They either received extremely unequal, moderately unequal, or equal offers from proposers while electroencephalogram was recorded. Behavioral results showed that the acceptance rates for unequal offers were higher when interacting with ingroup partners than with outgroup partners. Analyses of event related potentials revealed that proposers' group membership modulated offer evaluation at earlier processing stages. Feedback-related negativity was more negative for extremely and moderately unequal offers compared to equal offers in the ingroup interaction whereas it did not show differential responses to different offers in the outgroup interaction. Analyses of event related oscillations revealed that the theta power (4-6 Hz) was larger for moderately unequal offers than equal offers in the ingroup interaction whereas it did not show differential responses to different offers in the outgroup interaction. Thus, early mechanisms of fairness evaluation are strongly modulated by the ingroup/outgroup membership of the interaction partner.

Figure 1. The acceptance rates and reaction times in the UG as a function of the offer type.

Bars indicate standard error. Asterisks indicate significant effects (*p < 0.05, **p < 0.01).

Figure 2. ERP responses and topograhic maps.

(a) ERP responses time-locked to the onset of different offers at the midline Fz, FCz, Cz, CPz and Pz. The shaded 250–350, 400–600 ms time window was used for the calculation of the peak amplitudes of the FRN and P3 respectively. (b) Topographic maps for the FRN effects in the 250–350 ms time window. (c) Topographic maps for the P3 effects in the 400–600 ms time window.

Figure 3. PCA results for different offers from ingroup/outgroup partner, as well as the corresponding scalp topographies.

The waveforms of the PCA components were derived from the corresponding peak channel.

Figure 4. ERSP results for different offers from ingroup/outgroup partner at Fz.

Dark rectangles mark the time/frequency window used in the statistical analysis and the corresponding scalp topographies.

Figure 5. Timeline of a single trial in the Ultimatum game.