When it comes to writing subject-lines, many marketers follow the famous KISS mantra.

Which, if you don’t know, means to “keep it simple, stupid.”

(No offence.)

However, new research by Touchstone has discovered that blind dedication to this cause could be the reason why many recipients are failing to read your emails.

Using its new technology to test on virtual recipients instead of real life subscribers, Touchstone actually found that the greater the language complexity, the better the click and open rates.

First, a bit more information...


For its study, Touchstone used two methodologies.

The first was the Coleman–Liau index, which relies on the number of characters in a word instead of syllables.

The second was the Automatic Readability index, which like Coleman-Liau, primarily uses the number of characters to gauge the understandability of a piece of text. 

The text is then classified by the US grade system, ranging from being understandable by a child in kindergarten through to the level of an undergraduate university student. 

Other methods might use the number of syllables in a word to define complexity, but the Touchstone algorithm is not currently programmed to think in terms of syllables.

Examples of subject lines:

Grade 2-3

  • Rewards Coupons, Fri. and Sat.
  • Big flight savings
  • Don't miss these awesome deals

Grade 6-7

  • You qualify! Because you're an email subscriber: awesome savings in top destinations
  • Tired of always looking exhausted?


  • Easy, flameless, effective. Cute odor-neutralizing Fragrance Spheres. Just $5.49
  • Designs with Character (Literally!)
  • Budget-Friendly Swimsuits, Embarrassing Prom Moments, and More

What it found

Touchstone’s study involved analysing 675,000 subject lines and the results of 41bn sent emails. 

First, all subject lines in the database were categorised according to understandability using the two chosen methodologies, before determining whether the language complexity had any impact on open rate, clickthrough rate or click-to-open rate. 

The grey bars in the charts below also represent how many emails of each type was sent.

With best practice guidelines for subject lines recommending marketers to keep subject lines as simple as possible, many emails are sent with subject lines with an understandability level aimed at people aged 9-14.

However, as the above graphs show, the average open, click and click-to-open rates all tend to improve the more complex the language in the subject line.

In fact, the subject lines that performed the best were those with the vocabulary of a 16-to-18-year-old.

What can we learn?

Not only does this study suggest that complex language leads to greater email engagement, but it also once again proves the value of testing and optimisation.

It's easy for marketers to assume that they know what language is most appealing to their customers.

But to get the best results from email, it's worth making use of one of the many available email marketing tools to scale up testing and optimise email messages.

Not only will this save time and resources, but result in far better engagement from consumers.

Finally, for more on this topic check out these other studies looking at how to create a great email subject line:

Nikki Gilliland

Published 10 August, 2016 by Nikki Gilliland @ Econsultancy

Nikki is a Writer at Econsultancy. You can follow her on Twitter or connect via LinkedIn.

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Comments (5)

Pete Austin

Pete Austin, Founder and Author at Fresh Relevance

I'm going to suggest an alternative interpretation - these results show that marketers are putting too much work into their subject lines compared to their preheaders (typically the very first text from the body of the email).

When subscribers see your email in their in-box, they see a line of text made up from two things: the subject line on the left and the preheader on the right, filling up a constant amount the space. They decide whether to open the email based on all this text, not just the subject line.

A longer subject line means more subject and less preheader and vice-versa. And if marketers are putting more work into the subject line, than into the preheader, then a longer subject line means better average quality for the combination of both - producing more opens.

Based on the article above, I think the researchers didn't correct for subject line length in their analysis, and the real driver of the results was how much preheader text filled up the space left after the subject. As in my experience pre-headers are often very bad, this is not a surprise.

The conclusion should be: leave your subject lines alone and put more work into your preheaders.

almost 2 years ago

Tracy Sestili

Tracy Sestili, Owner at Social Strand Media

Actually, I disagree with Pete's comment above. I think that people also look at the 'From' primarily, subject lines second. The preheader is tertiary by comparison. And also, this report just proves what people have been saying for years - talk in 6th grade English so that people know what you're trying to say. Often I see marketers try to be too clever with their writing or their subject lines and it gets lost on the recipient.

almost 2 years ago

Pete Austin

Pete Austin, Founder and Author at Fresh Relevance

@Tracey: The article and I didn't mention From. I think we all agree that From has the biggest effect. Then Subject and finally Preheader. This report shows that long and complicated subjects work worse. It does not separate these factors. So we do not know if the problem is with long subjects/short preheaders. And it doesn't look at causation, so it's not proof.

By the way it's really hard to stick to grade 6 English. This comment is grade 5.8, but your comment is grade 7.8. So telling marketers to stick to grade 6 may be a bad idea.

almost 2 years ago


Dan Hamilton, Head of Client Solutions at Econsultancy Guest Access TRAININGSmall Business Multi-user

Your theories about pre-headers are very interesting so I would be very interested to see any empirical data that you have that lead you to your conclusions

almost 2 years ago


Leon Klatt, Consultant at Alchemy Worx

Interesting reading the article - the report shows more complex works better, so not sure how Pete comes to the opposite conclusion

7 months ago

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