Email marketing experts have spent years following the same advice: use powerful words, personalize the subject line, avoid ALL CAPS, and keep punctuation to a minimum. A new study suggests that some of these rules may be hurting performance rather than helping it.
Researchers at the University of Helsinki analyzed 31,812 marketing email subjects sent a total of 4.6 billion times. Their goal was to determine which long-standing copywriting conventions hold up when tested on a large scale. The findings call into question several familiar “best practices” and reinforce the value of testing hypotheses instead of relying on conventional wisdom.
The study’s biggest surprise may be the findings on so-called power words.
Terms like free, exclusive, Today, flashAND save have been appearing in email marketing recommendations for years. They are often presented as reliable methods for increasing openings. In the study, subject lines containing these words generated significantly lower open rates than those without them.
Chances are, these words are so familiar that they no longer stand out. In crowded inboxes, simple language can attract more attention than predictable marketing phrases.
ALL CAPS also worked against marketers. Subject lines containing all-caps words reduced open rates by about 3.3%, suggesting that subscribers continue to respond negatively to what sounds like yelling.
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 are worth testing
Some results were less definitive, but still useful.
Personalization produced a modest improvement in open rates. The researchers also noted that previous studies had come to conflicting conclusions depending on the audience and stage of the purchase journey. Their recommendation is to test whether personalization improves outcomes for your subscribers rather than assuming that it always will.
Punctuation also produced an unexpected result. Subject lines containing a single exclamation mark increased open rates by nearly 4%. The finding supports using emphasis sparingly while avoiding excessive punctuation commonly associated with spam.
The researchers also found that slight deviations from conventional writing, including punctuation or creative formatting, were associated with higher open rates. Carefully breaking convention can help an email stand out in a crowded inbox, as long as readability and professionalism remain intact.
What marketers should test
The findings also offer useful insights for teams using AI to write email campaigns.
Many AI-powered writing tools generate subject lines based on these copywriting best practices. As a result, asking AI to produce “high-converting” topics full of urgency and promotional language can automate outdated habits.
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It’s best to use AI to generate variations, then test the hypotheses behind them. Compare a simple object to one filled with powerful words. Test customization against a generic version. Try a concise text with something longer. The goal is to learn which writing principles are true for your audience.
Marketing folklore is not the same as evidence
The broader lesson goes 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 work constantly. Others survive because they have been repeated often enough to become accepted wisdom.
Marketers who treat established advice as a hypothesis, test it with their audience, and measure the results have a stronger basis for deciding which practices to keep in their playbook.
The delightfully titled study, “Want $150 FREE? Measuring the Effect of Language on Marketing Email Open Rates,” was published in the journal Ampersand and can be downloaded here.
