Observed Signal · Jul 2, 2026 · Research Study · Source: https://martech.org/feed/ · Impact: 2/5 · Sentiment: Neutral
Study: Common Email Subject Line Rules Often Hurt Open Rates
A University of Helsinki study analyzed 31,812 marketing email subject lines that were sent a combined 4.6 billion times to test long-standing copywriting rules at scale. Results challenge common advice: so-called power words (e.g., “free,” “exclusive,” “save”) were associated with lower open rates, fully capitalized subject lines reduced opens by about 3.3%, and shorter subject lines performed better overall. Conversely, a single exclamation point correlated with nearly a 4% increase in opens, and modest personalization produced small gains. The researchers recommend treating copywriting rules as hypotheses to be A/B-tested for each audience. The article also warns that AI tools trained on conventional best-practice prompts can automate outdated habits; marketers should use AI to generate variations and then empirically test them.
Large-scale empirical study on email copywriting provides actionable evidence for marketers and may shift A/B testing priorities, but it is not platform-level or regulatory news and does not fundamentally change core AdTech infrastructure.
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Key Takeaways & Evidence Grounding
- Researchers at the University of Helsinki analyzed 31,812 marketing email subject lines sent a combined 4.6 billion times.
- Subject lines containing common 'power words' like 'free', 'exclusive', 'today', 'flash', and 'save' produced significantly lower open rates.
- Fully capitalized words in subject lines reduced open rates by about 3.3%.
- Subject lines containing a single exclamation point increased open rates by nearly 4%.
- Shorter subject lines consistently performed better; each additional character had a small negative effect on open rates.
Connected Companies & Entities
1 Entity mapped“MarTech is owned by Semrush Inc. (page content written by either an employee or a paid contractor of Semrush Inc.)....”
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