Marshall Lecture 2025: Political Information and Network Effects

Why do political campaigns so often yield unexpected results? We address this question by separately estimating the direct effect of a campaign on targeted voters and the indirect effect on others in the same social environment. Partnering with a local NGO during Argentina's 2023 presidential election, we randomized the distribution of leaflets providing an expert assessment of the likely consequences of certain proposals by the outsider candidate Javier Milei. Exploiting Argentina's unique sub-precinct election reporting system, we show that the campaign reduced Milei's support among directly treated voters, as expected, but increased his support among untreated voters in treated precincts, producing a backfiring, net-positive effect for Milei. A pre-registered replication confirmed these opposite-signed effects. Using theory and a survey experiment, we show that the minority of voters who disbelieved the campaign were more motivated to discuss it with peers, convincing them to support Milei. This mobilization effect appears especially likely when campaigns criticize outsider candidates. Our results highlight how campaigns aimed at anti-elite candidates can unintentionally mobilize support for them.