External triggers are cues from outside the user — push notifications, emails, ads, CTAs — that prompt action. Effective triggers are timely, relevant, and actionable. Generic triggers are ignored or actively resented.
In 2014, Nir Eyal published Hooked, a framework for building habit-forming products. The model has four stages: Trigger → Action → Variable Reward → Investment. The trigger is where everything starts. Without a trigger — something that prompts the user to act — the rest of the model never gets a chance to run.
Triggers come in two types. An internal trigger is a feeling or thought that already lives inside the user — boredom, loneliness, anxiety, curiosity. When you open Instagram because you feel bored, boredom is the internal trigger. An external trigger is something in the environment — a notification, a message, an email subject line, a red badge on an app icon. External triggers are what designers build. They are the knocks on the door that invite users back in.
The goal of every external trigger is to bridge the gap until an internal trigger takes over. At first, users need to be reminded to use a product. Over time — if the product delivers enough value — the internal trigger (the feeling of wanting to use it) develops on its own. The external trigger becomes less necessary. A product that has achieved internal triggers has achieved habits. External triggers are the scaffolding that gets users there.
“External triggers tell users what to do next. The best ones make the next action so obvious that thinking isn't required.”
— Nir Eyal, Hooked, 2014
Push notifications are the most direct owned trigger available to a product. Done well, they bring users back at exactly the right moment with exactly the right information. Done badly, they train users to turn off notifications entirely — permanently destroying the channel.
The two lock screens below show the same app sending notifications. The bad version sends high-volume, generic, time-insensitive messages. The good version sends fewer, more specific, contextually relevant ones. Both are “push notifications.” The difference in how they're received is enormous.
Generic, repeated, and promotional. Each one slightly devalues the channel until the user turns them off.
One location-triggered, one milestone-based. Both feel like the app is paying attention to you, not shouting at you.
The quality of an external trigger is measured by how much friction exists between seeing it and acting on it. A notification that says “Time to move!” requires the user to interpret what that means, decide whether they're ready, and figure out what to do when they open the app. A notification that says “You're at the gym — start your session?” requires none of that. The trigger is so specific and well-timed that the next action is obvious. That's the standard to design toward.
Email is one of the oldest and most effective owned external triggers. But an email only works as a trigger if it gets opened — and the entire decision of whether to open it happens in a single glance at the subject line and preview text in an inbox. The email's content is irrelevant if the trigger doesn't fire.
Below are two versions of the same re-engagement email campaign. Same product, same offer, different trigger quality. The first is generic and could have been sent by any app to any user. The second uses the user's actual behaviour — what they were doing before they lapsed — to make the trigger feel personal and relevant.
“We miss you! Come back” — this subject line has been sent by every app that has ever existed. It carries no information, no specificity, and no urgency.
Same product, same offer — but the behaviour-triggered subject line tells you exactly what was left behind and why to return.
The difference between these two emails isn't the copywriting — it's the data. The good subject line could only exist if the product tracked that the user had a streak, how long it was, when it ended, and when they usually trained. External triggers are powered by internal data. A product with no user-level behavioural data can only send generic triggers. A product that tracks the right things can send triggers that feel like the app knows you.
Here's the counterintuitive truth about external triggers: the best outcome is when your users no longer need them. A product that has been used consistently enough, and delivered enough value, develops internal triggers — the user thinks of it on their own. The feeling of boredom triggers opening Twitter. The anxiety before a presentation triggers opening Notion. The desire to track a run triggers opening Strava. The app is never not on their mind.
External triggers are what you use until internal triggers develop. They bring users back until the habit is established. The goal isn't to keep sending notifications forever — it's to use those notifications to create enough repeated use that the product becomes something users reach for on their own. Once that happens, the external trigger has done its job.
Eyal, N. (2014). Hooked: How to Build Habit-Forming Products. Portfolio/Penguin. · Fogg, B. J. (2009). A behavior model for persuasive design. Persuasive '09. · Laja, P. (2016). Push notification strategy. CXL Institute.