The Impact of the Menstrual Cycle on Bargaining Behavior

The study consists of two phases. The first phase involves tracking participants’ menstrual cycles over a three-month period. Participants are recruited three months before the online bargaining session, during which they report the start date of their last menstruation. Using this information, along with their average cycle length over the tracking period, we estimate the start of their next menstrual cycle. A three-month tracking period is necessary, as medical literature suggests this is the minimum duration required for accurate cycle measurements.

In the second phase, participants take part in an online bargaining experiment. They are randomly assigned to anonymous pairs and engage in a bargaining game over ten rounds. Each participant is given a unique anonymous ID to ensure confidentiality throughout the study.

We recruit respondents from Prolific who reside in the EU and US and meet the following eligibility criteria: women aged 18 to 42 who are fluent in English, do not use contraceptives, and have regular menstrual cycles. To ensure eligibility, an initial pre-screening survey will be conducted on Prolific to collect general information on participants’ menstrual cycles, contraceptive use, and hormonal intake. Only individuals who meet the inclusion criteria will be invited to participate in the online experiment and the menstrual cycle tracking phase.

2025-03-24

2025-07-25

Measures of the bargaining process and outcomes:

For the bargaining process, we analyze informed players' initial offers, uninformed players' initial demands, and both players' concessions during bargaining. Initial offers and demands capture an individual's bargaining attitudes before they are affected by the negotiation partner's behavior. To investigate the interactive nature of bargaining, we look at concessions during the simultaneous bargaining stage.

Bargaining outcomes are measures of negotiation success. We analyze negotiators' final payoffs, deal rates, and payoffs conditional on reaching an agreement.

Initial offers and demands are the proposals made by informed and uninformed players in the initial bargaining phase, which are made in private and before they start interacting with the other player.

Concessions are changes in offers and demands, respectively, that show a negotiator's willingness to give up part of their own stake to reach an agreement and make the bargaining partner better off.

Conditional payoffs are the payoffs conditional on reaching an agreement. Deal rates refer to the frequency of agreements, and Final payoffs are the payoffs including disagreement.

We collect data on (i) demographics (e.g., age, education, and work status) and (ii) risk and social preferences. These variables will be used as controls to assess the robustness of our analysis.

As a robustness check, we will also adjust the window frame for ovulation. This approach will help assess the sensitivity of our findings and further reinforce the robustness of our study's conclusions. Furthermore, to ensure the reliability and validity of our findings, we will incorporate a self-reported measure of bargaining to complement the results obtained from the behavioral measure.

Finally, as an additional robustness check, we will perform the entire analysis separately for informed and uninformed players, using bootstrapped cluster-robust standard errors with 1,000 repetitions.

The study consists of two phases. The first phase involves tracking participants’ menstrual cycles over a three-month period. Participants are recruited three months before the online bargaining session, during which they report the start date of their last menstruation. Using this information, along with their average cycle length over the tracking period, we estimate the start of their next menstrual cycle. A three-month tracking period is necessary, as medical literature suggests this is the minimum duration required for accurate cycle measurements.

In the second phase, participants take part in an online bargaining experiment. They are randomly assigned to anonymous pairs and engage in a bargaining game over ten rounds. Each participant is given a unique anonymous ID to ensure confidentiality throughout the study.

I. Menstrual cycle tracking phase: Participants will monitor their menstrual cycles for three months. Each month, they will report the start date of their period, the duration of their cycle in days, and any irregularities they experience, such as irregular bleeding, missed periods, or the initiation of contraceptive use. This information enables us to accurately determine each participant’s cycle length and menstrual phase at the time of the experimental online survey (as in \cite{Buser2012}). To maintain anonymity, participants will submit their cycle data through an online Qualtrics survey using their anonymous Prolific ID. This ID allows us to link data from the tracking phase to the online survey while ensuring confidentiality.

Participants will receive tracking reminders via Prolific’s anonymous messaging tool, which allows experimenters to communicate securely with registered participants. This ensures consistency in data collection throughout the study. A few weeks before the three-month tracking period ends, participants will receive suggested time slots for scheduling their online experiment session. This step is crucial to maintaining a balanced sample across the three main menstrual phases (Ovulation, Premenstrual, and other phases).


II. Online Experiment: Following the tracking phase, participants will take part in an experimental online survey that includes a bargaining game designed to simulate real-world economic scenarios, such as salary negotiations. In this game, two female participants negotiate the division of a fixed sum of money (the "pie size"). While the total amount is known to both players, only one participant—the informed player—knows the actual value of the pie, whereas the other—the uninformed player—does not. Participants are randomly assigned to one of these roles and retain the same role throughout the experiment. The negotiation process focuses on determining the uninformed player's share of the pie.

The game consists of ten rounds, with participants being randomly paired in each round using a random-stranger matching protocol. A bargaining round concludes when the uninformed player's request matches the informed player's offer within a time limit, resulting in a deal. If an agreement is reached, the uninformed player receives the agreed share, while the informed player's earnings equal the remaining portion of the pie. If no agreement is reached within the fixed time frame, both players receive nothing.
At the end of the experimental online survey, we also assess participants’ risk and social preferences, which are controlled for in the analysis.


Additional measures and questionnaire: At the end of the experiment, we elicit demographics (age, nationality, education, and income). Additionally, we will obtain information about their current menstrual cycle to verify the data obtained from their prior menstrual cycle tracking. Lastly, we also ask hypothetical questions about their bargaining and risk attitudes.

Participants are randomly assigned to the roles of informed and uninformed players in the bargaining game and maintain the same role throughout the 10 bargaining periods. In addition, they are grouped into matching groups of approximately six players (depending on session attendance) and are re-randomized into new pairs at the start of each bargaining period.

Matching groups of approximately six players (depending on session attendance)

No

120 matching groups

700 (350 informed players and 350 uninformed players)

120

Our objective is to recruit approximately 360 informed and 360 uninformed players, evenly distributed across three menstrual phases: Ovulation, Pre-menstruation, and a combined category for Menstrual and Post-menstrual phases. We aim to detect an effect size of approximately 0.2, with a targeted statistical power of 95% at a significance level of 0.05 to rigorously test our primary hypotheses related to the bargaining process and outcomes