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: