The present study analyses decision-making and argumentation by high school students in a debate situation on a socioscientific issue, the use of embryonic stem cells in research and therapy. We tested the influence on the debates of two different contexts. Adolescent students at the high school level in the same grade (mean age 16.4 years) from rural and urban zones of Provence, France participated in three debate sessions. During the first session students listed the background questions they want to ask the expert(s). They were also required to identify one or two major issues that would serve as an outline for the future debate. They then discussed these with the expert(s) during the second session and took note of the answers. During this session, control groups met with a neuroscientist whereas the experimental "contextualized" group met with the same neuroscientist together with a representative of an association of patients suffering from a neurodegenerative disease. Analysis of the students' arguments and decision-making revealed that contextualization introduced dynamism in the students' exchanges: they paid more attention to their peers' arguments and were more motivated to argue their own opinion. However, this type of contextualization may contribute to reinforcing ideology in scientific progress. Keywords: debate, socioscientific issue, argumentation, contextualization, expertise, decision-making, human embryonic stem cells.