I. Introduction
Equipped with intelligent systems, a virtual agent produces natural conversations using text or speech and imitates nonverbal behaviors of humans [1]. The importance of social interaction with virtual agents has been highlighted in various fields such as health, learning, commerce, therapy, video games, and military systems [2]. In each field, the agents help users by providing information or services based on users' needs. These socially intelligent agents are regarded as a humanlike interactant, as it is expected to have social features to support human tasks. According to the Computers-Are-Social-Actors (CASA) paradigm [3], humans tend to apply social rules mindlessly in human-computer interaction. Therefore, a social rule people perceive appropriate and thus conform to in social situations is also reflected in the human-agent interaction.