Tristen’s study will examine the effects of pleasure on gratitude. Research participants will be asked to 1) take a drug (naltrexone) that blocks opioids, natural substances in the body associated with pleasure, or sugar pills (placebo); 2) come to the lab while on the study drugs to complete a gratitude task; and 3) complete daily reports on their feelings of gratitude and pro-social behavior. For the gratitude task, researchers will contact several close friends and family members to gather positive messages (example from prior study: “I’ve never met anyone as kind and charming as you”), as well as neutral messages or facts (e.g., “You have dark hair.”), and present these to the participants who will rate how grateful, positive, connected, and pleasant the messages made them feel. Participants will have the opportunity thank their friends and family for the messages, and the content of these thanks will be analyzed. Finally, participants will be asked to keep a daily diary of their general feelings of gratitude, relationship quality and satisfaction, and prosocial behaviors (e.g., “Today, I did something thoughtful for someone else, I helped a close friend with a problem”), and any specific gratitude-related events that may have occurred during the day. Tristen hypothesizes that the opioid blocker naltrexone will reduce feelings of gratitude in the lab, lead to less appreciative thanks to friends and family, and lead less gratitude, relationship satisfaction, and prosocial behavior towards others in daily life.