Field Experiment (16,880 Emails): AI 'Playful' Rewrites Boost Engagement — But Only Through Emotional Tone, Not Directly

arXiv·medium signal

In a randomized crossover field experiment, 121 employees across six companies sent work emails over three weeks under three conditions: unaided, GPT-5 playful-tone rewrite, and GPT-5 professional-tone rewrite. Across 16,880 emails, playful editing raised emotional positivity (B=+0.068, p<0.001) and professional editing lowered it (B=−0.041), yet neither directly changed open, reply, or response times. The effect was fully indirect: within-sender positivity strongly predicted opening (OR=2.05) and replying (OR=3.32) — meaning AI writing shapes engagement through the tone it produces, not merely through being used.