The em dash has long been a stylistic choice for writers, but new data suggests it has become a digital fingerprint of AI-assisted text. Dmytro, founder of KissMyPost, analyzed 15.8 million long posts and 241.5 million comments across 100 major English-language subreddits from January 2016 to September 2026. The findings are stark: the share of long posts containing an em dash surged from 0.70% in early 2021 to 6.53% in Q2 2025, representing a more than nine-fold increase in prevalence.

The August 2024 Inflection Point

Contrary to popular belief, the spike did not coincide with the November 2022 launch of ChatGPT. While usage crept up to 1.19% over the following 18 months, the dramatic jump occurred in August 2024. In July of that year, only 1.28% of long posts used an em dash; by August, that figure hit 1.95%, and by December, it reached 3.12%. This timing aligns closely with the release of GPT-4o, suggesting that as model capabilities and accessibility improved, so did the adoption of its specific punctuation quirks in user-generated content.

Distinguishing AI From Autocorrect

Critics might argue that keyboard software changes are responsible for the shift, but the data refutes this. When Apple introduced Smart Punctuation in iOS 11 in 2017, em dash usage rose in both posts and comments simultaneously. In contrast, the 2024-2025 trend shows a massive divergence: long posts gained 5.35 percentage points in em dash usage, while commentsβ€”which are typically typed directly into the interfaceβ€”only gained 0.67 points. This discrepancy supports the hypothesis that users are pasting AI-drafted text into posts, rather than typing it natively.

The Post-GPT-5 Correction

Interestingly, the trend reversed slightly after OpenAI updated its default model. The peak of em dash usage in posts occurred in July 2025 at 6.83%. Following the rollout of GPT-5 as the default model in August 2025, the share dropped to 5.70%. OpenAI CEO Sam Altman later noted that ChatGPT now follows custom instructions to avoid em dashes, and indeed, December 2025 saw the lowest usage since the peak. However, the baseline remains elevated, with 2026 figures hovering between 5.09% and 6.26%, still seven to nine times higher than pre-AI levels.

Key Takeaways

  • Em dash usage in Reddit posts increased nine-fold from 2021 to 2025, peaking in July 2025.
  • The spike began in August 2024, correlating with GPT-4o's release, not ChatGPT's initial launch.
  • Posts saw a 5.35 percentage point increase in em dash usage, while comments rose only 0.67 points, indicating pasted AI text.
  • The introduction of GPT-5 and custom instruction updates led to a partial decline in em dash frequency, though usage remains historically high.

The Bottom Line

While the em dash is no longer a definitive detector for AI text, its statistical dominance in long-form posts serves as a clear marker of the era of large language models. The data proves that AI hasn't just changed what we write, but how we write it, embedding specific stylistic tics into the fabric of online discourse.