Table 3.

Main theories and effects of visible embodied conversational agents.

Embodied conversational agent (ECA) theory	Explanation	Source
Theory of social inhibition/facilitation	When in the presence of others, people perform learned tasks better and novel tasks worse. Empirical results have demonstrated that this principle also applies for the presence of ECAs.	[65]
Social agency theory	By adding a visible ECA as a screen tutor, the social interaction schema is primed, which will cause the learner to try to understand and deeply process the computer-delivered instructions.	[61]
Social modeling/social learning theory	Humans derive their knowledge, attitudes, behavior, and goals by observing and imitating the surrounding social agents.	[18,24]
Situational dependency	Pedagogical agents are helpful when there is a need to increase companionship and decrease complexity.	[57]
Social exchange theory	People prefer equitable relationships in which the contribution of rewards and costs are roughly equal. This equity principle also applies to human-computer relationships.	[58]
Persona effect	The presence of a lifelike character in an interactive learning environment—even one that is not expressive—can have a strong positive effect on a student’s perception of his or her learning experience.	[59]
Image principle	The image of an ECA is not a key factor for learning; instead, the level of animation of the ECA is the key factor for learning.	[61]