This study aims to explore the link between public pledges for future generosity and happiness using functional magnetic resonance imaging fMRI. The experimental group publicly declared to spend money on others, while the control group announced they would spend money on themselves. They found that people in the experimental group made more generous decisions in the following tasks than those in the control and showed a stronger increase in their self-reported happiness scores. The fMRI revealed that the generous decisions activated the temporo-parietal junction (TPJ) more strongly in the experimental group vs. the control, and the connectivity between TPJ and ventral striatum varied between groups. This striatal activity can be directly related to variations in happiness.