Great email subject lines: This is what 9,673 tests have taught me

Email subject line loving cat

Great email subject lines are like big red boxes on Christmas morning with giant white ribbons tied around them in bows — they beg to be opened. Of course, with every marketer, media company, and business contact leaving scores of these giant, colorful, be-ribboned boxes on your digital front porch every day, it can be difficult for anyone to stand out.

Which subject lines will get opened? And which will get tossed?

Of course, there’s no “right” answer. The answers will be different for newsletter subject lines, marketing subject lines, transactional subject lines, subject lines for sales, etc.

But there are some strategic best practices — and tactical tips and tricks — that can help you do your best to stand out and get your email opened.

The art and science of the best email subject lines

So, here’s some art and some science — backed by thousands of emails worth of experience — to give you some direction on email subject line best practices.

Over the course of years overseeing testing of subject lines for clients, I’ve overseen something like 9,673 email subject line tests.

Is the number exactly 9,673? Probably not exactly — but it’s a catchier estimate than 10,000. Odd, or more “specific,” numbers generally outperform even and round ones. (So, there’s one email subject line trick right there!)

While I won’t claim here to be giving a quantitative data analysis, these are the qualitative things I’ve learned about the science and art of writing email subject lines that work — drawn from observing the results of thousands of email subject line A/B tests run on audiences of millions of email newsletter subscribers.

These two magic words will help you write subject lines that work

Would you like to know the secret?

These two words will get anyone to open your email…

“Curiosity gap.”

Here’s a quick curiosity gap definition: The space between what we do know and what we want — or even need — to know.

If I’d told you the two words in the subheading up above, would you have read the rest of the text? Maybe. Maybe not. But by leaving the curiosity gap open, I massively increased the chances you would read on.

The curiosity gap has gotten a bad reputation in the last decade or so. Viral news site Upworthy built a short-lived empire on it. Publishers’ Facebook headlines, in particular, abused this practice for years before the Almighty Algorithm decided to crack down.

But it was over-used for a reason — it is deadly effective.

In years of AB testing, I have never seen a curiosity gap subject line lose to a subject line that gave away the key information upfront.

Now, this is not to say that every subject line you write should use the curiosity gap — far from it. Overused, it will annoy your readers. If the gaps aren’t sufficiently closed by the information the reader gets when he or she clicks through, you may see unsubscribes.

But this is an extremely powerful tool. Perhaps the most powerful tool you have when it comes to writing email subject lines that get opened.

Use it wisely.

Examples of curiosity-gap email subject lines:

  • This surprising thing killed the cat (hint: It wasn’t curiosity!)
  • Science finds this is the perfect time to wake up
  • Jeff Bezos just lost a ton of money. Like, you’ll never believe how much…

You’re probably making this huge mistake in your email newsletter

OK, maybe you are; maybe you aren’t. But that twinge of anxiety you just felt will make you click to open.


The close cousin of the curiosity gap (often used as part of the curiosity gap), an anxiety inducing subject line will make a recipient question whether they’re doing something wrong, whether something worrying is about to happen, whether there’s something important that they’re missing out on.

As with the curiosity gap subject line, exercise caution.

But know that anxiety is a powerful motivator to click.

Examples of anxiety-based email subject lines:

  • Stop using these 5 words in your emails right now
  • These 23 misused words might cost you the sale
  • You’re not ready for what’s coming in January

Smart people already know this about great email subject lines

And you’re smart, right?


Narcissism is another powerful driver of email opens. Everyone wants to be reassured that they’re brilliant, attractive, charismatic, etc.

Subject lines that tie into this desire have a powerful pull.

Examples of narcissism-based email subject lines:

  • Smart people always go to bed at exactly this time
  • People with high emotional intelligence all have one thing in common
  • 5 reasons attractive people have a hard time getting hired

This is the most important thing to know about email subject lines that get opened


Take these two subject lines:

A: These are the worst cities to retire in
B: This is the worst city to retire in

Can you guess which one of these will win an AB test every single time?

You can bet the farm on B.

“These are” promises a list. It could be interesting… Maybe my city’s on it? Maybe the city where my parents live is on it? Meh.

“This is” creates a curiosity gap. There is one city out there that is just so awful, so expensive, so inconvenient, so dirty, so loathsome that it is literally the worst possible city to retire in!


Examples of specificity-based email subject lines:

  • This was voted the single best laptop of the year
  • Poll names the most at-risk district in tomorrow’s midterms
  • One CEO has outperformed all others this year

5 secrets for using numbers

Of course, lists are still great. If you’re giving people a list, you’re going to sell it — at least in part — with the number you choose.

Based on thousands of email subject line AB tests, here are some of the numbers that tend to do better.

4, 5, 8, 10, 11

The teens are a wasteland.

The 20s are solid. (25? Great!)

Past that, you’re in no-man’s land. (Big numbers can work wonderfully, but they’re less common and hard to get data on.)

(While my recommendations here are based on one set of — extensive — testing, you’ll find that similar results were landed on in an analysis of BuzzFeed list posts’ success and an analysis of a broader sample of blog posts.)

Other general notes about numbers…

Always use the numeral (10), not the word (ten). The numeral will always perform better; it’s just easier to read quickly.

And, again, odd numbers and more specific-feeling numbers grab attention better than round ones. (Generally speaking.)

Examples of numerical email subject lines:

  • 23 things every freshman learns at college
  • These are the 99 questions HR gets most often
  • 4 tricks that will make your emails sing

Should you ask questions in email subject lines?


People want answers.

We have enough questions in life.

Emoji for email subject lines in a word

In a word: Meh.

In my own experience, I’ve never seen an emoji move an open rate vs. a control (the same subject line without an emoji) more than 5%. That’s five percent, not five percentage points. So, a 25% open rate might become a 26.25% open rate with an emoji thrown in. Not nothing; but not that exciting, either.

That said, results may vary.

Large-scale tests by email marketing companies have generally found small but positive effects from emoji. But they’ve also found diminishing effects as more and more brands and media companies try this trick.

Other email subject line “hacks” or “tricks” that only work a little:

  • Loud punctuation!!!
  • Personalization for you, [customer name]
  • Emoticons : )

Any of these techniques may be right for the voice of your newsletter, media company, or brand. If you think your audience will respond, definitely test them.

But if you’re fiddling with little tricks, expect to see small results.

The one true best practice for getting people to open your emails: Rigorous testing

Email subject lines are important because… well, it’s obvious right? If your reader doesn’t open, nothing else in your email matters at all.

If you’re in news or media, publisher website traffic can make or break you.

Some people will open more than others, and that’s OK. But to keep up a healthy list, and to get people to see the great work you’re doing, great email subject lines are key.

These are my key lessons from a lot of email subject line testing. But audiences tastes and behaviors are always changing. And your audience is unique.

Always be trying new things. Always be testing. And always be structuring and recording those tests in a way where you learn what works and what doesn’t work for you.

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