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Subject lines: why language diversity matters

by Stu Elmes

Choosing just the right email subject line for the email marketing campaign you’ve just finished building might be the most important decision you make today. 


There are several good reasons, but here are a few of the best ones: 

1. Open rates – As many as 47% of email subscribers decide whether or not to open an email based on the subject line alone. 

2. Spam reports – 69% of email recipients report having marked a marketing email as “spam” based solely on its subject line.    

3. ROI – The quality of the subject lines you use can have a MASSIVE impact on the revenue your emails generate.  

In today’s crowded inbox, finding ways to capture subscriber attention and get that all important open is more crucial than ever beforeEffective email subject lines, optimized for the tastes and preferences of a brand’s unique audience, are the most effective tool to help you accomplish exactly that. 

Increasing subject line efficacy through better use of language

There are many factors that separate the “good” email subject lines – the ones that engage subscribers and generate revenue – from the “bad” email subject lines – the ones that do little more than annoy subscribers and damage your brand. Understanding these factors, and building email subject lines that reflect this understanding, can (and often does!) mean the difference between email marketing success and email marketing failure. 

As a company that has made understanding exactly what makes a subject line work the very foundation of our business, the team here at Phrasee has analyzed the performance of millions of email subject lines across dozens of industries over the years. This exhaustive process taught us a lot about what makes an effective email subject line – and what doesn’t.  

The story all that performance data told us was one of rapidly evolving subscriber preferences, shifting audience tastes, and an urgent need for ongoing, rigorous subject line split testing to keep brands ahead of the curve.   

But there’s another part of the story all those email sends had to tell that surprised us a bit. A lesson that took an awesome team of data scientists and the world’s most advanced marketing language generation/optimization AI platform to unearth; when it comes to email subject line performance and effective split testing, language diversity matters. 

Language diversity and email marketing performance

Phrasee’s data science team analyzed over 420 million subscriber interactions with email subject lines and identified a clear correlation between language diversity and email subject line performance. 

Recycling the same old words, phrases, and themes again and again simply doesn’t cut the mustard anymore. It takes fresh, engaging subject line language to capture and maintain subscriber interest these days. Keep plugging the same language that worked a month ago into your subject line split tests, and all you’ll get is a poorly-performing subject line that tested better than a few other poorly-performing subject lines. 

And what good is that?   

Incorporating more linguistic variance into the subject lines you test on your audience is the only effective way to find out what’s going to maximize your subscriber engagement today. And that’s important, because maximized subscriber engagement means more opens, more click-throughs, and more revenue for you and your brand. 

This fact is just as true regardless of which brand is sending the campaign, regardless of which industry that brand operates in, and regardless of the demographic makeup of its audience. Hence, upgrading the linguistic diversity of your brand’s subject lines is an ideal way to differentiate your brand in the inbox and gain a subscriber engagement advantage over the competition.  

Measuring language diversity

The trouble is, measuring subject line language diversity is hard. It took our awesome team of data scientists quite some time to figure out how to do it effectively and consistently, even with the help of the most advanced AI tech in the email biz! 

But figure it out we did. 

We’ve built an easytouse tool that lets you measure the linguistic diversity of your brand’s subject lines and visualize how your subject line diversity measures up against that of retail competitors, allowing you to maximize your email marketing success.  

It’s free, and it’s awesome. 

Here it is: 

The Phrasee Language Diversity Calculator

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