Email marketers have spent years following the same advice: use power words, personalize subject lines, avoid ALL CAPS, and keep punctuation to a minimum. A new study suggests some of those rules may be hurting performance instead of helping it.
Researchers at the University of Helsinki analyzed 31,812 marketing email subject lines sent a combined 4.6 billion times. Their goal was to determine which long-standing copywriting conventions hold up when tested at scale. The results challenge several familiar “best practices” and reinforce the value of testing assumptions instead of relying on conventional wisdom.
Promotional language isn’t always persuasive
The study’s biggest surprise may be its findings on so-called power words.
Terms like free, exclusive, today, flash, and save have appeared in email marketing advice for years. They’re often presented as reliable ways to increase opens. In the study, subject lines containing these words generated significantly lower open rates than those without them.
It’s likely that these words are so familiar that they no longer stand out. In crowded inboxes, straightforward language may earn more attention than predictable marketing phrases.
ALL CAPS also worked against marketers. Subject lines containing fully capitalized words reduced open rates by about 3.3%, suggesting subscribers continue to respond negatively to what feels like shouting.
The research also confirmed that shorter subject lines consistently performed better. Each additional character had a small negative effect on open rates, reinforcing the value of concise writing.
Some rules deserve testing
Some findings were less definitive, but still useful.
Personalization produced a modest improvement in open rates. The researchers also noted that previous studies reached conflicting conclusions depending on the audience and stage of the buying journey. Their recommendation is to test whether personalization improves results for your subscribers rather than assume it always will.
Punctuation also produced an unexpected result. Subject lines containing a single exclamation point increased open rates by nearly 4%. The finding supports using emphasis sparingly while avoiding the excessive punctuation commonly associated with spam.
The researchers also found that slight departures from conventional writing, including creative punctuation or formatting, were associated with higher open rates. Carefully breaking convention may help an email stand out in a crowded inbox, provided readability and professionalism remain intact.
What marketers should test
The findings also offer useful guidance for teams using AI to write email campaigns.
Many AI writing tools generate subject lines based on these copywriting best practices. As a result, asking AI to produce “high-converting” subject lines full of urgency and promotional language can automate outdated habits.
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It’s better to use AI to generate variations, then test the assumptions behind them. Compare a straightforward subject line against one filled with power words. Test personalization against a generic version. Try concise copy against something longer. The objective is to learn which writing principles hold true for your audience.
Marketing folklore isn’t the same as evidence
The larger lesson extends beyond email.
Marketing has accumulated decades of rules, formulas, and best practices that are repeated in conference sessions, agency presentations, and online guides. Some survive because they consistently work. Others survive because they’ve been repeated often enough to become accepted wisdom.
Marketers who treat established advice as a hypothesis, test it with their audiences, and measure the results have a stronger foundation for deciding which practices to keep in their playbook.
The delightfully named study, “Do You Want $150 for FREE? Measuring the effect of language on marketing email open rates,” was published in the journal Ampersand, and can be downloaded here.
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