Profanity in Email Subject Lines: A Split Testing Experiment

  • April 1, 2016

By Parry Malm

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Worldwide, 205 Billion emails were sent every day in 2015.

In 2016, that number is expected to be even higher.


With that volume of traffic, the problem of making an email marketing campaign stand out from the crowd is becoming an increasingly difficult one to navigate.

Here at Phrasee, our goal is to use learning machines and algorithms to identify the subject line language that gets people to respond to your emails.

This is what we do.

We keep most of our intellectual property close to our chest, for obvious reasons.


See, we have split tested literally millions of email subject lines, millions of combinations of words, phrases and sentiments to understand what elicits the best responses the most consistently.

From our split testing, one data set emerged that we decided to share with the world.

We were hugely and consistently surprised by the strong statistical performance of:

Profanity in email subject lines


In these digital days and times, so few words are still capable of evoking an emotional response the way they used to.

Profanity, however, still has the ability to shock us.

Image credit: Marvel

It still stands out.

They are taboo words that we use whilst among our nearest and dearest, but that we all instinctively believe are entirely inappropriate in a business context.

But what would happen if those taboo words suddenly appeared in a context where we are simply not used to seeing them, like, say, an email marketing subject line?

Really f**king interesting statistics, that’s what!

On April 1st 2016, we ran a small-scale experiment using our subject line optimisation technology to generate subject lines containing the following words:

  1. Sh*t

  2. F**k

  3. F**king

  4. A**hole

  5. D**khead

The results were STAGGERING.

Topping the list, the word “f**king” increased open rates by a full 61.8%

Close on its heels was the word “f**k“, which saw open rates jump 53.2%

Sh*t” pushed open rates up almost 45%, “a**hole” produced a 39.6% increase, and “d**khead” finished with a very respectable 33%.

In fact, some of the email subject lines this experiment generated produced unf**kingbelievable open rates.

A few of the highest performing ones we tracked were:

  • “Open this goddamn email or f**k off.” – Which was opened by 73.4% of recipients.
  • “You won’t f**king believe the sh*t inside this email!”  Which piqued the interest of a full 80.6% of recipients (and, oddly, also had a 49% Click Through Rate).
  • “Hey D**khead! Don’t miss out on this, you lobotomised sh*tlark.” – Opened by 83.8% of recipients.

But the king of them all, the email subject line that caused us to pause and seriously ponder the possibility that adding profanity to email subject lines might actually be the most profitable course of action moving forward, was a simple, three word message, that incited a 96% open rate. It was:

“Go f**k yourself.”

For some reason, almost no one who received an email with this 3-word subject line was able to ignore it.

We suppose that is understandable, really. If we saw that in our inbox we would probably be curious too.

The main lesson: start swearing in your f**king subject lines and a**holes will open those sh*tbags. Got it?

(Editor’s note: before using these strategies, scroll up and check what day this study was conducted on …)

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