Put like-minded people in a room to debate, and they leave more extreme, not less
In 1969, Serge Moscovici and Marisa Zavalloni had French students discuss their attitudes toward their president and toward Americans, then compared opinions before and after. Discussion didn't average the group toward the middle — it pushed everyone further in whichever direction they already leaned. Building on James Stoner's earlier 'risky shift' finding, the effect is now called group polarization.