Participants (N=100) will be recruited through direct outreach to University of Washington (UW) students via the UW Registrar's directory list, as well as community flyers distributed at colleges in the local area, online advertisements, and word of mouth. Participants will complete screening measures online. Eligible participants will be stratified on sex, age, and past 30-day peak alcohol use and randomly assigned to one of two conditions: close friends (N=50), where participants will recruit two of their existing friends, or new peers (N=50), where participants will drink with two strangers. To ensure at least two eligible friends, those in the close friend group will be asked to provide email and phone numbers of five to seven close friends. Close friends will have to meet the same inclusion and exclusion criteria as target participants and will go through the same procedures as target participants during alcohol administration. For each pair of participants matched on the three strata (age, sex, alcohol use), strangers in the new peer group will be matched to the sex of the close friends of the other matched participant's group.

Participants will be asked not to travel to the bar together to ensure they are not exposed to their peer group before entry to the bar, and must get to the lab without operating any vehicle with wheels (e.g. car, bike). Upon arriving to the lab, all participants and peers will review and sign the consent form, and provide a baseline breath alcohol content (BrAC) reading. Female participants and friends will also be asked to take a urine pregnancy test. Participants and friends will complete baseline measures and the Balloon Analogue Risk Task (BART) individually prior to entry to the bar. Then they will be taught the procedures for the daily diary portion of the study (see below). Participants will join their close friends or new peers in the bar and be given 20 minutes to relax while the researcher prepares the materials. After entering groups but prior to receiving alcohol, participants and peers will complete pre-alcohol measures and the BART. They will then be allowed to order anything from the bar menu for the following hour. Participants and close friends will be allowed to drink freely within limits (BrAC = .12%), for safety. During ad-lib, estimated BAC levels based on age, sex, and weight will be calculated prior to each drink served to ensure the safety of consuming each drink without interrupting social interaction and making the participant's BAC more salient by taking BrAC readings regularly. Participants or close friends whose BAC may exceed .12% will only be served placebo beverages for the remainder of the hour. BrAC readings will be taken at the end of ad-lib, every 10 minutes until peak BrAC is reached and readings are descending, and every 30 minutes thereafter. After ad-lib, participants will complete post-alcohol measures and the BART, then provided entertainment until they reach a BAC below .03%. The full protocol will take approximately 2.5 hours. On average, alcohol metabolizes at a rate of .015% per hour, so a participant who reaches .12% will be in the lab on average 8.5 hours. Once sober, participants and friends will be taken home in a taxi or ride share service (e.g. Uber, Lyft). Close friends will not complete the following daily diary protocols.

Daily diary measures will start on the Thursday following each participant's alcohol administration appointment and be collected for potential drinking events on Thursday, Friday, and Saturday over the following four consecutive weeks. Each day, participants will be notified of surveys by text message at 4 pm and the following morning at 11am (Friday, Saturday, and Sunday) for a total of 24 assessments. Surveys will be available for 2 hours and reminders sent via text 30 minutes before closing. Afternoon assessments take approximately 5 minutes and measure BSM, motives to drink, motives to refrain from drinking, evening plans, and what type of peers they plan to spend the evening with (e.g. close friends, new peers, family, alone). For the daily diary, BSM items have been modified to select experiences which may vary daily (e.g. "making social plans got me excited and energized today) and drinking motives have been modified into a brief prospective measure consistent with modifications used in other studies15. At morning assessments, participants will retrospectively report amount of drinks and other substances consumed, time spent drinking, and consequences experienced while drinking or currently (morning after drinking). Participants will be asked how many times their social group changed during the event defined as the majority of their drinking companions being of a different peer type than before. For instance, a participant may be at a house party with primarily close friends, but someone invites several people the participant does not know well and they are now drinking with mostly new peers. For each peer group reported (up to five groups), participants will report the following characteristics of the group: size (i.e. number of people they had direct contact with), number of drinking companions of each peer type (e.g. close friends, new peers, family), gender composition of the group (same-sex vs. mixed-sex), time spent with and drinks consumed with each group. On non-drinking evenings, participants will report similar information regarding their social groups and other activities. This survey will take approximately 5-7 minutes.