Online political discussions are often dominated by a small group of active users, while most remain silent. This visibility gap can distort perceptions of public opinion and fuel polarization. Using a collective field experiment on Reddit, we examined factors predicting self-selection into silent "lurker" and active "power-user" roles and tested whether participation differentials can be reduced with norm- or incentive-based interventions. We recruited 520 United States participants, randomly assigned them to conditions in six private communities, and asked them to discuss 20 political issues over 4 weeks while completing weekly surveys. Lurking (posting nothing) was most common among users who perceived discussions as toxic, disrespectful, or unconstructive; these same perceptions also predicted power usership (more posting, conditional on not lurking). Experimentally, financial incentives for commenting reduced participation differentials, whereas we did not find effects from a civility norm treatment. These findings support preference- and incentive-based accounts of participation but suggest that light-touch interventions are unlikely to bridge participation gaps, let alone polarization.