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Email marketers have spent years following the same advice: use powerful words, personalize subject lines, avoid ALL CAPS, and keep punctuation to a minimum. A new study suggests that some of these rules could harm performance instead of improving it.
Researchers at the University of Helsinki analyzed 31,812 marketing email subject lines sent a total of 4.6 billion times. Their goal was to determine which long-standing writing conventions hold up when tested on a large scale. The results challenge several familiar “best practices” and reinforce the value of testing hypotheses instead of relying on conventional wisdom.
Promotional language is not always convincing
Perhaps the study’s biggest surprise lies in its findings on so-called power words.
Terms like free, exclusive, Today, flashAnd to safeguard have been popping up in email marketing advice for years. They are often touted as reliable ways to increase openings. 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, simple language can attract more attention than predictable marketing phrases.
ALL CAPS also played against marketers. Subject lines containing words in all caps reduced open rates by about 3.3%, suggesting that subscribers continue to respond negatively to what sounds like a shout-out.
Research also confirmed that shorter subject lines consistently performed better. Each additional character had a slight negative effect on open rates, reinforcing the value of concise writing.
Some rules are worth testing
Some conclusions were less definitive, but nonetheless useful.
Personalization produced a slight improvement in open rates. The researchers also noted that previous studies reached conflicting conclusions depending on the audience and stage of the purchasing journey. Their recommendation is to test whether personalization improves results for your subscribers rather than assuming it always will.
The punctuation also produced an unexpected result. Subject lines containing a single exclamation point increased open rates by almost 4%. The results support using emphasis sparingly while avoiding excessive punctuation commonly associated with spam.
The researchers also found that slight deviations from conventional writing, particularly in punctuation or creative formatting, were associated with higher open rates. Carefully breaking conventions 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 tips for teams using AI to write email campaigns.
Many AI writing tools generate subject lines based on these writing 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 best to use AI to generate variations and then test the assumptions behind them. Compare a simple subject line to one filled with powerful words. Test the customization against a generic version. Try concise copy versus something longer. The goal is to learn which writing principles are valid for your audience.
Marketing folklore is not the same as proof
The biggest 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 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 audiences, and measure the results have a stronger basis for deciding which practices to keep in their strategy playbook.
The delightfully named 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.
The position FREE! Email subject rules that don’t hold up! appeared first on MarTech.
