Psychology as a Social Science
Scientific disciplines are usually classified into two: the Physical Sciences and the Social Sciences. When you search for a Psychology book in the library, you will most likely end up in the Social Sciences section, perhaps even near to Philosophy. This is because the field of Psychology has its historical roots in Philosophy. Old-age philosophical inquiries to Being and Personality are studied psychologically.
Psychology is considered a social science because psychological principles affect social life - people and institutions alike. Oftentimes, the purpose of any psychological research is to make the world a better place to live in, that is, socially.