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Subject Line Effectiveness Decay: why it’s a problem

You know what they say…


Deep stuff, that.

When Robert Frost wrote those oft-repeated words about the temporal nature of beauty all those many years ago, it’s safe to say that he probably wasn’t thinking about email subject lines, but he might as well have been.

Because the beauty of a finely-crafted email subject line is temporal too.

It’s the second immutable law of email marketing dynamics: “The Subject Line Effectiveness Decay Principle” (which we just made up). It states that any email subject line, no matter how effective, will become less effective over time.

For brands which use email marketing as a key part of their digital marketing strategy, the Subject Line Effectiveness Decay Principle can present a very real problem – the only problem that really matters – lost marketing revenue.

Here’s how…

Subject Line Effectiveness Decay: why it’s a problem

As we at Phrasee have said many, many times before; a subject line can make or break any email marketing campaign.

The subject line is the first (and often only) part of each campaign that your subscribers will see. Many will make the decision whether to open or ignore your email based on the subject line alone. Which is great if you are good at writing email subject lines (which we are).

The thing is, over the years that we’ve been tracking email marketing subject line performance at scale, we’ve noticed something: the performance of every subject line decays over time.

A subject line which generates loads of opens and clicks today will generate fewer tomorrow, and fewer again next week. The data is clear on this point (and we have a LOT of data).

Now, we are aware that it may seem obvious that using the same subject line over and over again will not produce good results for long, but there’s more to the story.


Changing a few words or phrases within a subject line does little to buck the effectiveness decay trend.

Even subject lines incorporating similar elements to those of previous lines see their results decay over time. Subject lines with low structural variance/diversity will produce diminishing returns in performance.

It is only highly varied/diverse subject line language structures which will consistently perform well over time.

Here at Phrasee, we call this “phreshness” (clever, we know).

Opens, clicksconversions and ROI will all diminish over time for brands which do not keep their subject lines fresh and on-brand.

The good news is that human language is incredibly diverse, and there are literally millions of ways to structure the language of any marketing message. For those willing to put in the time and effort, keeping a brand’s subject line content fresh is entirely within their power.

The bad news is that finding new, appealing, and on-brand ways to structure subject line language can become quite time-consuming. For those too busy or unwilling to spend their valuable time writing fresh subject line content, there’s Phrasee (which is also good news)!  

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