Hook Model

Trigger · Action · Variable Reward · Investment.

Nir Eyal’s 2014 framework for building habit-forming products. Four phases the user runs through, repeatedly, until the product becomes a reflex. The follow-up book draws the line between hooks the user is glad you built and hooks they regret.

Origin

Nir Eyal is an author and lecturer on behavioral design who taught at Stanford’s Graduate School of Business. His 2014 book Hooked distilled habit-forming products into a four-step loop: trigger, action, variable reward, and investment.

Eyal spent years in the video-gaming and advertising industries before distilling what he saw into the Hook Model. Notably, he followed Hooked with Indistractable — a book about defending yourself against the very techniques he documented — arguing the ethics depend on whether products serve users’ own intentions.

The Model

Most habit-forming products run the same four-step loop. A trigger fires, the user takes a low-cost action, gets a variable reward, and invests something — content, data, a relationship — that re-arms the next trigger. Each turn strengthens the habit and raises the switch cost.

Coined by
Nir Eyal (2014)
Source
Hooked
In practice
Close the loop with user investment
Loop

The Four-Step Loop

An external trigger nudges; a low-friction action follows; a variable reward fires; the user invests something that re-arms the next trigger. Cycle four times and you have a habit; cycle forty and you have a daily app.

From
Nir Eyal, 2014
Pattern
Self-reinforcing
External

Triggers Start Outside

The first triggers come from outside the user: a notification, an email, an ad, a friend sharing. The product cannot yet rely on memory. Each external trigger is one attempt to start the loop; most fail. The ones that complete the action teach the next trigger to be cheaper.

Examples
Push · email · ad
Watch for
Numbing the channel
Internal

Triggers Move Inside

The product wins when the trigger comes from inside the user. Boredom triggers Instagram, anxiety triggers Twitter, loneliness triggers Tinder. No notification needed; the emotion is the cue. Internal triggers separate apps people use from apps people reach for.

Tactic
Map the feeling
Examples
Boredom · anxiety
Action

Make It Trivially Easy

The action has to be smaller than the motivation. Open the app, pull down, tap, type one word. Anything above the friction line breaks the loop. Stories, swipes, double-taps, infinite scroll — all engineered to lower action cost below whatever the trigger could supply.

Rule
Action ≪ motivation
Pairs with
Fogg Behavior Model
RelatedFogg Behavior Model
Reward

Variable Rewards Hook

The third step is the addictive one. If every refresh gave the same content, the loop would die. The reward is variable: usually meh, occasionally great. Slot machines, Twitter feeds, dating apps, and inboxes all share this shape. The unpredictability is the engine.

Mechanism
Variable reinforcement
Examples
Slot · feed · inbox
Invest

Make Them Invest

The fourth step is what builds the next trigger. Posting, following, customizing, uploading, rating, friending — all are user investment that loads value into the product. The more the user has invested, the more the product becomes irreplaceable. Switch cost compounds with each turn.

Examples
Posts · follows · customizations
Effect
Switch cost ↑
RelatedIKEA Effect
Ethics

Persuasion vs. Manipulation

Eyal’s follow-up book is Indistractable — he draws a sharp line between hooks for things users will be glad they did (Duolingo, language learning) and hooks for things they regret (slot apps, doom-scrolling). The model itself is neutral; the test is whether the user would thank you for the loop in a calm moment.

Test
Calm-moment thank-you
Eyal book 2
Indistractable, 2019
Daily

Auditing Your Loop

Walk through the four steps for a feature: trigger, action, reward, investment. If any step is weak, the loop will not catch — the user touches the product once and forgets. If every step is engineered for the user’s regret, you built a slot machine and should fix it.

Cost
Five minutes
Catches
Weak loops
✦

The Hook Model in the Age of AI

Models can personalize every step of the loop in real time — for good and for ill.

✦ AI Era

AI Personalizes Every Step

A model can pick the right trigger for this user at this moment, generate the action smaller, vary the reward better, and frame the investment more cleverly. The Hook Model used to be one tuned loop; now it is millions, each tuned to one person. Done with intent: phenomenal. Done without: slot machines at scale.

Power
Per-user tuning
Risk
Slot machines at scale
✦ AI Era

The Ethical Bar Goes Up

When the loop is tuned for one person, the user cannot see the version other people get; they cannot tell whether their loop was tuned for retention or for them. Transparency, opt-out, and “show me what is set for me” become moral requirements, not features.

Required
Visibility · opt-out
Required
Per-user disclosure
Further Reading

Source

Reference