We discuss issues in designing virtual humans for applications which require long-term voluntary use, and the problem of maintaining engagement with users over time. Concepts and theories related to engagement from a variety of disciplines are reviewed. We describe a platform for conducting studies into long-term interactions between humans and virtual agents, and present the results of two longitudinal randomized controlled experiments in which the effect of manipulations of agent behavior on user engagement was assessed.

Abstract. We discuss the ethical and practical issues involved in developing virtual humans that relate personal, fictitious, human autobiographical stories ("back stories") to their users. We describe a virtual human exercise counselor that interacts with users daily to promote exercise, and the integration of a dynamic social storytelling engine used to maintain user engagement with the agent and retention in the intervention. A longitudinal randomized controlled experiment tested user attitudes towards the agent when it presented the stories in first person (as its own history) compared to third person (as happening to humans that it knew). Participants in the first person condition reported enjoying their interactions with the agent significantly more and completed more conversations with the agent, compared to participants in the third person condition, while ratings of agent dishonesty were not significantly different between the groups.

In this paper we describe a prototype of a serious game designed to improve hospital patients' confidence in managing their hospital stay. We present findings of an evaluation of our visual novel computer game by comparing it with traditional approaches to increasing patient empowerment and patient self-efficacy. The results indicated a significant effect of the type of intervention on improvement in empowerment, but only for participants with gaming experience or who achieved a high level of engagement in the game. This study provides evidence that games may outperform other types of health intervention under certain conditions.

Abstract. We present an empirical study on the impact of linguistic and cultural tailoring of a conversational agent on its ability to change user attitudes. We designed two bilingual (English and Spanish) conversational agents to resemble members of two distinct cultures (Anglo-American and Latino) and conducted the study with participants from the two corresponding populations. Our results show that cultural tailoring and participants' personality traits have a significant interaction effect on the agent's persuasiveness and perceived trustworthiness.