How to Build Consensus in a Health-Oriented Online Community: Modeling a "Pro-Ana" Forum

International audienceThis article presents an agent-based model of a health-related Internet forum. If recent literature demonstrates the relevance of network approaches to gain insight into consensus-building within online groups of peers, the dynamic process of mutual adjustment of participants' health orientations has been seldom explored. Our model is informed by qualitative data collected via semi-structured interviews with Internet users living with eating dis orders—often stigmatized due to the controversies surrounding "pro-ana" (anorexia) websites. The discussion threads that unfold in the forum, expressing a range of health orientations from extreme "pro-pathology" to "pro-recovery" ones, initiate a mix of conflicting and supportive reactions that can trigger change in members' orientations over time. We develop a computer simulation of message exchanges i n a forum, describing micro-behaviors through a simple mechanism of influence. We then complexify the macro-setting, considering the effects of turnover (the possibility of exiting and/or entering the forum), and different rates of active participation of members to discussions. Our model shows that under empirically plausible conditions, moderate pro-recovery orientations are more likely to emerge than radical ones refusing medical mediation. These results lead to policy recommendations to design successful health information campaigns, and advocate against access restrictions or filtering of these online communities