This study will identify which areas of the brain mediate the interaction between people involved in economic decision-making.

Healthy, right-handed native English speakers between 21 and 55 years of age may be eligible for this study. Candidates are screened with a telephone interview, a questionnaire to determine handedness, and a neurological examination if one has not been performed within the last year by an NIH physician.

Participants bargain for money by playing two-person trust games, while undergoing functional magnetic resonance imaging (fMRI). MRI uses a strong magnetic field and radio waves to obtain images of body organs and tissues. The MRI scanner is a metal cylinder surrounded by a strong magnetic field. During the procedure, the subject lies on a stable that can slide in and out of the scanner, wearing earplugs to muffle loud knocking sounds that occur with electrical switching of the magnetic fields. Functional MRI involves taking MRI scans while the subject performs a task-this to learn about how the brain regions are involved in performing the task. For this test, the subject plays or watches two-person games involving trust. The game may be played against a stranger or a friend. The winner receives a cash payoff.

The subject performs one of the following two tasks during MRI scanning:

• Playing the trust games and indicating as quickly as possible by pressing a right or left button whether he or she wants to stop or continue playing, or
• Observing games that others have played and predicting as quickly as possible by pressing a right or left button whether the players wanted to stop or continue the games.

The test takes about 90 minutes. After the scan, participants complete written questionnaires about their experience in the scanner and their views on issues related to participating in competitive games.