Why Do Online Conspiracy Theories Matter?

This chapter explains why online conspiracy theories matter by demonstrating that they are widespread, growing, and socially consequential. Drawing on evidence from elections, public health crises, and social movements, it shows that conspiracy beliefs are not fringe phenomena but are held by substantial segments of the population and increasingly shape political behavior, governance, and everyday decision-making. The chapter documents how social media platforms amplify conspiracy theories through public visibility, anonymity, emotional contagion, and identity-signaling mechanisms such as hashtags, while automated bots further accelerate diffusion and sentiment polarization. It reviews evidence linking conspiracy beliefs to harmful outcomes, including reduced compliance with public health measures, intergroup hostility, political violence, and large-scale loss of life. Conspiracy theories are shown to persist over time, adapt to new crises, and circulate globally, with a common anti-government, anti-media, and anti-science orientation that aligns them with populist and authoritarian politics. Together, these dynamics make online conspiracy theorizing a central threat to public trust, collective safety, and democratic governance.