# Electrophysiological Hyperscanning of Negotiation During Group-Oriented Decision-Making
## 2 Materials and Methods
### Experimental Procedure  
The experiment was conducted in a quiet darkened room, where participants, after providing informed consent forms, were seated comfortably next to one another to promote direct interaction for all the duration of the experimental procedure, which took approximately 30 min. Before starting the negotiation task, an EEG eyes-open resting-state baseline of 120 s was detected by the participants using the EEG hyperscanning paradigm. EEG activity was also recorded simultaneously by the participants during the negotiation task, which lasted approximately 3 min (Figure 1A). Specifically, the experimental procedure followed three main steps: (i) the decision-making orientation assessment (step 1), (ii) the negotiation task (step 2), and (iii) the perception of negotiation exchange (step 3). _Step 1: Decision-making orientation assessment._  
To verify the decision-making orientation (individual versus group orientation), a real-life decision-making script based on a common group problem was administered to the members through the Qualtrics XM platform (Qualtrics LLC, Provo, UT, USA) before the negotiation interaction. Specifically, this script was presented to take advantage of a hypothetical scenario with high ecological validity. After instructing the participants to identify with the situation that will be presented to them, the following realistic decision-making scenario and instructions were shown to participants:  
_\"A member of a workgroup does not appear to be in line with the ideals and style of the group. What does the group do? Think about your real work group._  
_Now rate with a number from 1 to 5 (where 1 stands for \"not at all\" and 5 for \"very much\") how close the following decisions are to how your real group would behave.\"_  
Subsequently, eight items about different ways in which the real group would face the situation were presented. The eight items were presented in a random order, with four of them representing a behavior characterized by an individual decision-making orientation (1-4 item, e.g., _\"The decision follows the perspective of the most innovative member of the group\"_) and the other four a group decision-making orientation (5-8 item, e.g., _The group adopts the decision endorsed by the majority_). The individual orientation score (Individual\({}_{\text{orientation}}\)) and the group orientation score (Group\({}_{\text{orientation}}\)) were calculated by averaging the four individual and four group orientation items. _Step 2: Negotiation task._  
After the decision-making orientation assessment, the participants were introduced to the negotiation task, in which they had to negotiate and decide together how to deal with the realistic situation previously proposed in the first step. Specifically, the following instructions were provided:  
_\"This study aims to investigate how individuals negotiate their opinions. You will therefore be asked to discuss the previously proposed topic and share your perspectives. Specifically, as a first step, we ask you to individually choose from the previous sentences the one that is closest to your decision and the way your real group would behave.\"_After receiving these instructions, the participants had the chance to read again the eight previous statements, and the experimenter recorded each participant's chosen statement as _Member 1's initial choice_ and _Member 2's initial choice_. Following this decision, the two participants engaged in a three-minute timed negotiation exchange, during which they were required to collaboratively decide on a single statement that best reflected their shared perception of their real workgroup, following this instruction:  
_\"Now, reflecting on the same scenario you have just read, you must work together to agree on a single statement that represents what your real group means to both of you. Therefore, the statement must be mutually accepted and should best reflect how to act within the group and for the group.