(individual) When Mother Teresa accepted the Nobel Prize for Peace in October 1979 and decided to use the $190,000 award to construct a leprosarium, was she acting in her own interest? Was she behaving selfishly?
This depends on how one defines "selfish." By the generally used sense of the word, Mother Teresa was not acting selfishly. She was acting altruistically. Her actions benefited others and not herself. Depending on the organization of her leprosarium, it's conceivable she might have received some salary or other tangible resource as administrator of the leprosarium. But I doubt that remuneration was what motivated her to use the money thusly.
Under a more expansive definition of "selfish" though, she was certainly acting in self-interest. Her economic interests are not limited to money or resources that can be directly exchanged for money. I consider the interests of my family and friends when I make my choices, as the emotional well-being of my family influences my happiness. This emotional stability is an interest of mine that cannot be purchased directly with dollars, but influencing it by influencing the well-being of my family members could be considered selfish. Similarly, Mother Teresa derived a sense of satisfaction through the well-being of others. She could not exchange resources directly for that sense of satisfaction. One cannot simply measure and hand over pieces of emotion to another. Nevertheless, such exchanges do happen and people do make individual choices to maximize their emotional profit. Measuring the value of such exchanges though isn't easy to do. But we could consider the satisfaction Mother Teresa received from seeing leprosy patients helped as an equal exchange. At least she did at the time, though she might not have characterized it as such an exchange.