For years, email marketers have adhered to certain guidelines: incorporate power words, personalize subject lines, avoid using ALL CAPS, and minimize punctuation. However, a recent study suggests that some of these rules may not actually improve performance as expected.
A research team at the University of Helsinki examined 31,812 marketing email subject lines that were collectively sent 4.6 billion times. Their objective was to assess the effectiveness of long-standing copywriting conventions when tested on a large scale. The outcomes challenge several traditional “best practices” and emphasize the importance of testing assumptions rather than relying solely on conventional wisdom.
One of the most surprising findings of the study pertains to the use of “power words.”
Words like free, exclusive, today, flash, and save have been recommended in email marketing advice for a long time as effective ways to boost open rates. However, subject lines containing these words in the study actually resulted in lower open rates compared to those without them.
It is speculated that these words have become so common that they no longer capture attention. In a crowded inbox, plain language may be more engaging than predictable marketing phrases.
Using ALL CAPS also proved to be detrimental for marketers. Subject lines with fully capitalized words saw a decrease in open rates by approximately 3.3%, indicating that subscribers still react negatively to what seems like shouting.
Furthermore, the research reaffirmed that shorter subject lines consistently outperformed longer ones. Each additional character had a slight negative impact on open rates, highlighting the significance of concise writing.
Some rules deserve testing
While some findings were inconclusive, they still offer valuable insights.
Personalization resulted in a modest enhancement in open rates. The researchers also observed conflicting conclusions in previous studies depending on the audience and the stage of the purchasing journey. They recommend testing the impact of personalization on your subscribers to determine its effectiveness, rather than assuming it always works.
Unexpectedly, punctuation had an interesting effect. Subject lines with a single exclamation point saw a nearly 4% increase in open rates. This finding suggests that using emphasis sparingly, while avoiding excessive punctuation commonly linked to spam, can be beneficial.
The researchers also discovered that minor deviations from traditional writing styles, such as creative punctuation or formatting, were linked to higher open rates. Intentionally breaking away from conventions may help an email stand out in a crowded inbox, as long as readability and professionalism are maintained.
What marketers should test
The findings also provide practical guidance for teams utilizing AI to craft email campaigns.
Many AI writing tools generate subject lines based on established copywriting best practices. Consequently, requesting AI to create subject lines filled with urgency and promotional language could perpetuate outdated habits.
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It is advisable to use AI to generate variations and then test the underlying assumptions. Compare a simple subject line with one containing power words. Evaluate personalized content versus a generic approach. Experiment with concise copy versus longer text. The goal is to identify which writing principles resonate best with your audience.
Marketing folklore isn’t the same as evidence
The key takeaway extends beyond just email marketing.
Over the years, marketing has accumulated numerous rules, formulas, and best practices that are often reiterated in conferences, agency presentations, and online guides. Some endure because they consistently yield positive results, while others persist simply due to repetition and acceptance as conventional wisdom.
Marketers who treat established advice as a hypothesis, test it with their target audience, and analyze the outcomes have a more solid basis for determining which practices to retain in their strategy.
The study, amusingly titled “Do You Want $150 for FREE? Measuring the effect of language on marketing email open rates,” was published in the journal Ampersand and is available for download here.
