The study is a randomized, placebo-controlled design with the opioid antagonist, oral naltrexone. Following random assignment, participants will take 50mg of naltrexone or placebo once a day for 7 days. On days 1 - 7, participants complete reports of their feelings of social connection and mood in order to assess more naturalistic feelings in response to opportunities for social connection outside of the laboratory setting. Additionally, at the end of each day, they complete a physical symptoms questionnaire. On the 7th day, participants will come to the SDSU MRI scanning facility to complete tasks designed to elicit feelings of social connection in the fMRI scanner. After the scan, feelings in response to the scanner tasks will be collected.

Inclusion Criteria:

• good health
• English fluency
• willing to provide contact information for 4-6 close others
• willing to provide digital photographs of 2 close others
• own a smartphone

Exclusion Criteria:

• presence of medical devices, implants, or other metal objects in or on the body that cannot be removed
• tattooed eyeliner
• a body habitus prohibiting MRI scanning
• claustrophobia
• self-reported chronic mental or physical illness
• current and regular use of prescription medication
• previous history of having difficulty taking pills
• current use of opioid analgesics
• depressive symptoms above a 9 on Patient Health Questionnaire
• excessive alcohol use
• positive urine drug test
• body mass index (BMI) greater than 35
• pregnancy or plans to become pregnant in next 6 months
• positive urine pregnancy test

Data and resources from the current study will be deposited in the Open Science Framework (OSF) and OpenNEURO.org, in accordance with the NIH Data Sharing Policy. In addition, results will be posted on the ClinicalTrials.gov website within 1 year of the study's completion date.

After publication of the primary findings.

Researchers in both private and public sector