Abstract
The rapid integration of artificial intelligence (AI) writing tools into online platforms raises critical questions about their impact on content production and outcomes. We leverage a unique natural experiment on Change.org, a leading social advocacy platform, to causally investigate the effects of an in-platform 'write with AI' tool. To understand the impact of the AI integration, we collected 1.5 million petitions and used a difference-in-differences analysis. Here we show that, at a platform level, in-platform AI access substantially altered the lexical features of petitions and increased petition homogeneity but did not improve petition outcomes. We confirmed that the results were consistent in a separate analysis of repeat petition writers who wrote petitions before and after introduction of the AI tool. The results suggest that while AI writing tools can profoundly reshape online content, their practical utility for improving desired outcomes may be less beneficial than anticipated and introduce unintended consequences such as content homogenization.