Emerging evidence shows that writing with AI assistance can change both the views people express and the opinions they hold. Yet, we lack a substantive understanding of behavioral and process-level changes in co-writing with AI that underlie the opinion-shaping power of these tools. We conducted a mixed-methods study, combining retrospective interviews with 19 participants about their co-writing experience with quantitative analysis tracing idea engagement in 1,291 AI co-writing sessions. Our analysis shows that engaging with the AI's suggestions---reading them and deciding whether to accept them---becomes a central activity, taking away from more traditional processes of ideation and language generation. As writers often do not complete their own ideation before engaging with suggestions, the suggested ideas and opinions seeded directions that writers then elaborated on. At the same time, writers did not notice the AI's influence and felt in control, as they---in principle---could always edit the final text. We term this shift Reactive Writing: an evaluation-first, suggestion-led writing practice that departs substantially from conventional composing in the presence of AI assistance and is highly vulnerable to AI-induced biases and opinion shifts.
ACM CHI Conference on Human Factors in Computing Systems