This document provides an abstract for a study on developing a theory of public diplomacy. It aims to test different explanations for why states practice public diplomacy, including rational, idealistic, and social trend motivations, using a novel quantitative method and statistical analysis. The study finds that rational explanations related to factors like wealth and prestige best explain public diplomacy activities. However, it also finds evidence that a social trend or norm is emerging as more states participate in public diplomacy beyond rational calculations. Idealistic motivations are found to have limited explanatory power despite concepts of "credible diplomacy." The document outlines the theoretical background and emergence of public diplomacy as a field of study.