Abstract
Despite growing concerns about how social media use can affect people's beliefs and behaviour, estimating its causal impact is difficult. Here we present a novel method: a bespoke social media platform built on an isolated Mastodon server. Participants interacted much as they would on typical social media, but we controlled who could use the platform and measured their beliefs and attitudes before and after. This approximates the ecological validity of real social media data while retaining experimental control. Participants interacted over several days in three between-subject conditions (total N=311): a Control condition with no external influence, and two "troll" conditions Left and Right) where we covertly posted typical talking points from one political side. We observed extensive engagement in all conditions demonstrating the broader feasibility of our approach, and our minimal influence campaigns had statistically credible effects on engagement, beliefs, perceptions of others, and experience on the platform.