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Research Design This laboratory-in-the-field experiment is built around three interventions: (1) viewing of a prerecorded expert appeal about the benefits of intersectarian cooperation, (2) participation in a small-group discussion about intersectarianism after seeing the expert appeal, and (3) being offered money to vote for a co-sectarian in the subsequent election game. [...] Each group is made up of three Shia and three Sunni participants [...].

Study Population Participants are drawn from across Beirut, with a view to assembling a representative sample of Shia and Sunni residents of Lebanon’s capital. [...] The average age of participants is 37.3. On average, participants have 12 years of schooling. They come from households with monthly incomes of around $2,100. Men and women, and Shia and Sunni Muslims, are all equally represented. [...]

Setting On arrival, individuals were randomly assigned to six-member groups. Each group was balanced with regard to sect (three Shia and three Sunni), gender (three men and three women), and age (three participants aged 18–40 and three aged 41–64). [...] Participants were asked to keep their names secret throughout the study because names can be a marker of sectarian affiliation. Our assistants acted as group moderators: They informed participants that there were three Shia and three Sunni Muslims in the group without revealing which specific participants belonged to which sect or introducing them [...].  [...] They were randomly assigned to groups, but in the analyses that follow, we include a control for group moderator effects. [...]

Interventions [...] There are three of them: baseline, expert appeal, and expert appeal plus participant discussion. This structure is replicated twice—once in the absence of a clientelistic intervention (conditions 1–3) and once in the presence of clientelism (conditions 4–6). The total number of participants in the study is 360, the total of 60 six-member teams. Ten groups (i.e., 60 individuals) were randomly assigned to each intervention. [...] The expert appeal intervention is set up to resemble a political talk show. In the video, four experts—two Shia and two Sunni male journalists in their 50s—discuss pressing problems in contemporary Lebanese politics, the promise of intersectarian cooperation, and the coming together around a common national identity in solving the problems. [...] The participant discussion intervention is designed to follow on from experts’ appeal in order to ensure that participants talk specifically about intersectarian cooperation. [...] In this instance, a written prompt is replaced by a prerecorded message from experts. [...] The question then is not just whether our interventions can sway members of different sects to cooperate across the sectarian divide, but, more realistically, whether these interventions can be effective in the presence of clientelism. To test this proposition, we assigned 180 participants in their six-person groups to a clientelism intervention within which the three experimental conditions were replicated. The clientelistic appeal was delivered by our confederates who acted as “election brokers.” [...] Participants were called to a private room to meet with our confederate one-on-one. We used two confederates: one Shia and one Sunni, both male and in their mid-20s. Participants always met with the confederate who was their co-sectarian. Our confederates verified participants’ contact information and then briefly described the first game, a simulated election with multiple rounds in which participants would be asked to vote for a Shia or Sunni candidate proposing to divide a fixed amount of money in different ways across different sects, and that the Shia or Sunni candidates were actors. Then those not randomized into the clientelism intervention were asked to return to the group, and those in the clientelism treatment were made the clientelistic offer. The confederate explained that unbeknownst to researchers, a good friend and co-sectarian of the confederate (and therefore also a co-sectarian of the subject) was the actor delivering messages in the simulated elections. The broker then offered the subject $10 “in secret” to vote for the candidate of their sect across all the elections irrespective of the candidate’s policy platform. [...] Subjects were free to accept or reject the money. [...]

Outcome Measures The first game, designed to measure unconditional cooperation, was a simulated election and took place over the course of four rounds. In each round, two candidates— one Shia, one Sunni; both besuited men in their late 50s of similar physical appearance—appeared in person before the six-person group to deliver competing policy proposals with regard to how to divide a certain amount of money between Shia and Sunni Muslims in the group. Prior to this, all participants were endowed with 40 tokens each [...] and 20 tokens were collected from every participant by way of what was described to participants as a tax. The resultant 120 tokens ($60) were placed at the center of the table; this amount was to be divided according to the proposal made by the winning candidate. Over the course of four elections, the candidates delivered competing proposals for how to divide “the tax” between the two sects. Votes were cast in writing by secret ballot. Election winners were determined at the very end of the study. [...] The order of elections was fixed to facilitate the implementation of the study, and the two candidates alternated in delivering speeches. In every election, one of the candidates proposed equal distribution of tokens—20 per person—to both Shia and Sunni participants. The other candidate proposed some form of unequal distribution favoring his own sect by suggesting that either all or most of the tokens should go to his co-sectarians. [...] The second game is the other–other allocation game, which is commonly used as a measure of unconditional cooperation and ingroup favoritism in an intergroup context. In this game, participants were given 10 tokens that they had to spend and were asked to distribute these between an anonymous co-sectarian and an anonymous person from the opposite sect. Participants did not know which specific individuals in the group were affected. [...] In the other–other allocation game, participants cannot benefit personally from their allocation decisions, unlike in the simulated election where in the context of iterated votes, it might be in participants’ self-interest to support a candidate from the opposite sect. In short, the other–other allocation game provides a clean measure of altruistic regard toward the outgroup and, consequently, a clearer test of the hypothesized psychological channel behind increased cooperation. The third and final game was the standard public goods game, which was used to measure the strength of conditional cooperation within groups. In this task, participants had to decide how many of 10 tokens to keep for themselves and how many to surrender into the common pool. At the end of every round, all tokens in the common pool were multiplied by two, and the resultant sum was shared equally across all group members. [...]