Why We Open Some Emails Instantly and Others Never

In the morning, your inbox shows 37 new messages. You skim the subject lines, delete most of them unread, and end up opening maybe three emails.


You make this selection in a matter of seconds, usually without thinking about it. Those who enjoy puzzles are still very familiar with the mechanism behind it. It's the same one that keeps you stuck on a matchstick puzzle until you've found the solution.

When a knowledge gap starts to itch

Behavioral researcher George Loewenstein of Carnegie Mellon University published a still widely cited explanation for curiosity in 1994. In his essay The Psychology of Curiosity, he describes it as a kind of withdrawal feeling that arises as soon as we notice a gap between what we know and what we would like to know. Curiosity, according to this view, feels like an itch that only goes away once the gap is closed. Loewenstein already pointed out that the advertising industry is familiar with this principle: in so-called mystery ads, you only find out at the very end what product it's actually about.

What's interesting is the size of the gap. If you know absolutely nothing about a topic, curiosity usually doesn't arise because you don't notice the missing information at all. On the other hand, if you feel like you're already very close to the answer, the need for resolution becomes particularly strong. That's why a subject line like "Newsletter Issue 47" leaves you cold, while "Why almost everyone gets this simple puzzle wrong" immediately gnaws at you.

What a Berlin waiter has to do with your inbox

A second building block comes from Berlin in the 1920s. Psychologist Kurt Lewin reportedly observed in a restaurant that a waiter remembered open orders with remarkable accuracy and quickly forgot them once the bill was paid. His student Bluma Zeigarnik investigated the phenomenon in a series of experiments in 1927. Her test subjects remembered interrupted tasks significantly better than completed ones, and this is exactly what is known today as the Zeigarnik effect.

Television series have been using this effect for decades with the cliffhanger at the end of each episode. A newsletter can do the same thing on a smaller scale. If each issue ends with an open question whose answer only follows in the next mailing, your readers have a genuine reason to wait for the next email rather than letting it get lost in the stream of other messages. A "puzzle of the week" with the solution in the following issue is a simple example of this, one that also fits a small side project or a niche website.

Suspense needs a good framework

The best idea, however, doesn't help much if the execution is bumpy. If you want to create a newsletter that sparks curiosity, it helps to have a tool that takes most of the technical work off your hands. With a drag-and-drop editor, you can design each issue without any design knowledge, and through A/B tests you can find out which of two subject lines actually gets opened more often. Segmentation ensures that puzzle fans get their brain teasers and bargain hunters get their deals. For the German market, it's also worth checking whether the provider implements the GDPR properly.

It's also important that the email actually delivers on the promise of the subject line. A gap filled with hot air will create distrust the next time around. Anyone who has been disappointed twice will often not open the third email at all, or will go straight to the unsubscribe link. Especially with small projects that have a manageable list, every single unsubscribe makes a noticeable difference. Good subject lines therefore work like a fair puzzle, where the solution surprises you yet was right in front of you the whole time.

For your own inbox, the whole thing also makes for a fun little experiment. Over the next few days, pay attention to which subject lines make you click, and ask yourself what gap they open up in you. And if you're currently wondering why you read this text all the way to the last paragraph: the answer is in the first section.

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