We've learned about rational self interest. Many people see this term as a person being "selfish" but the term refers to a person looking at how to benefit their interest to the fullest extent. Usually we can see this term in society for example when I went to Chicago this past weekend with my family there were a lot of homeless people asking for change while people were passing by. Several people said "No" that they wouldn't spare some change. The change they have could eventually add up to paying the tolls, rent, or food expense. So because there is a benefit to keeping that change, the people refusing to give out change are benefiting their self interest because the marginal cost doesn't outweigh the marginal benefit of using that change for something for their interest.