Hooked Book Summary: A Founder’s Guide to Building Habit-Forming Products
Ever wonder why you instinctively tap open Instagram or feel compelled to check Slack without anyone reminding you? It’s not just because those apps have cool features – it’s because they’ve become habits. For a first-time founder, building a product that people use out of habit can be a game-changer. Products that become part of users’ daily routines enjoy higher engagement and retention. In fact, many breakout successes (Instagram, Slack, TikTok, Duolingo) didn’t win only on technology, they won on psychology. They keep users coming back through habit-forming design, not expensive marketing. As Nir Eyal – entrepreneur and author of Hooked: How to Build Habit-Forming Products – puts it, “Great retention… comes from habit”. His book dissects how the most addictive apps hook us, and how you can ethically use those techniques to increase your own product’s stickiness.
The Hook Model: a Habit Loop
At the heart of Hooked is the Hook Model, a four-step cycle that many successful products use to create habits. The idea is that by guiding users through these steps repeatedly, you “manufacture desire” for your product in their lives. The four stages of the Hook Model are: Trigger, Action, Variable Reward, and Investment. Let’s break down each step and see how you can apply them, with real examples along the way.
The Hook Model visualized as an infinite loop of four stages: Trigger, Action, Variable Reward, and Investment. Users cycle through these steps repeatedly, each pass building a stronger habit.
1. Trigger: Sparking the Habit
Every habit starts with a Trigger: the spark plug that cues the user to take an action. Triggers come in two flavors: external triggers and internal triggers.
External triggers are cues in the user’s environment that tell them what to do next. Think of a phone notification, a “Sign up now!” button, or an email reminder. Anything outside the user’s mind. For example, when Duolingo sends you a push notification at 7PM saying “Time for your Spanish lesson!”, that’s an external trigger nudging you to open the app.
Internal triggers are cues from within the user’s own brain, emotions or routines that automatically lead them to your product. Often, these are negative feelings or uncomfortable emotions that the product promises to relieve. Feelings like boredom, loneliness, confusion, or FOMO (fear of missing out) are powerful internal triggers that drive us to certain apps. For instance, feeling bored might cue you to open TikTok or Instagram for entertainment. Feeling anxious about work updates can prompt you to check Slack (“You fear you’re missing out on important company updates? You’ll go check your Slack messages.”) In a habit-forming product, over time you want users to form an association like: “Whenever I feel ___ [bored, lonely, etc.], I instinctively open ___ [your app]”.
For first-time founders, the key is to identify the core need or emotion your product addresses. Ask yourself: What itch does my product scratch, and when/where does that itch happen? Once you know that, you can choose effective external triggers (notifications, emails, etc.) to prompt users at the right times. But a trigger alone isn’t enough, it must be followed by an easy, rewarding action.
2. Action: Simplest Step at the Right Time
A trigger sets the stage, but Action is the next step. It’s the simple behavior the user does in anticipation of a reward. In the Hook Model, an action should be easy to do and tied directly to the trigger. Nir Eyal draws on psychologist B.J. Fogg’s behavior model: for any behavior to happen, a user needs sufficient motivation and the ability to do it with minimal friction. In practical terms: the more you reduce the effort required, the more likely the user will act. “All technology is about shortening the distance between the recognition of the need and the reward,” as Eyal says.
Think about the tiny, quick actions you already do in apps: tapping a notification to open a message, scrolling through a feed, clicking a “Play” button. These are low-effort actions that satisfy an immediate urge. The simpler and quicker the action, the more likely users will do it without hesitation, and do it often. For example, Instagram made the action of giving feedback as easy as a double-tap on a photo to “like” it, which instantly shows a heart icon, a little burst of satisfaction. On TikTok, the action is literally a flick of your thumb to swipe to the next video. Slack’s action might be something as effortless as glancing at a channel or sending a short reply emoji. All these are tiny actions done in seconds.
As a founder, design your product’s key action to be as frictionless as possible. Can the user perform the core action in one or two taps, or a few seconds? Remove unnecessary steps and make it intuitive. The goal is that when the trigger strikes, the user can seamlessly perform the action without thinking twice. Once they do, you reward them, and that’s where the magic really kicks in.
3. Variable Reward: Keeping Users Coming Back for More
After the user takes the action, they must get a Reward – some payoff that satisfies them, at least partly. But here’s the twist: the most habit-forming products don’t give the exact same reward every time; they give variable rewards. It’s the variability, the element of surprise or uncertainty, that keeps users hooked and coming back. Psychologists have known since B.F. Skinner’s famous experiments with pigeons that rewards on a variable schedule (i.e. unpredictable rewards) can ramp up our desire dramatically. In one example, pigeons pressed a lever more obsessively when they sometimes got food versus when they always got it. Humans aren’t so different. This is exactly why slot machines are addictive (you never know if the next pull will hit the jackpot), and why we compulsively check apps like social media. There’s always a chance of something new or something gratifying appearing.
In product terms, a variable reward means the user gets what they came for, but with some variability or novelty to keep it interesting. There are a few types of variable rewards you can use:
Rewards of content. New, interesting information or media. Scrolling an Instagram or TikTok feed is compelling because each new post or video could be funny, exciting, touching, or none of the above. You don’t know until you look. That mystery is its own reward. Similarly, checking Twitter or LinkedIn brings unpredictable updates. Users keep scrolling in anticipation that the next piece of content might be great.
Social rewards. Feedback and recognition from others. For instance, on Instagram or TikTok, after you post content, you might get variable social rewards: one day you get 5 likes, another day 50 likes, sometimes a new follower or comment. The amount and timing of the positive feedback varies, which keeps you checking the app to see what you got. Slack provides social rewards too, sometimes you open it and see praise from a colleague or a funny meme in the team channel, other times just routine info. The chance of a gratifying social interaction makes it habit-forming.
Achievement rewards. Progress or points that feel satisfying. Gamified products like Duolingo give points, badges, or streaks for completing lessons. Interestingly, even these have variable elements: Duolingo might surprise you with a new badge or a fun animation when you hit a milestone. The next lesson might introduce a new challenge or story, keeping the experience from feeling stale.
The common thread is that your product should reward the user for their action, but also leave them with a bit of anticipation for the next time. As Eyal explains, you want to “scratch the user’s itch, but leave them wanting more.” If the reward is too predictable or too fully satisfying every time, users have no reason to return. But if it’s fulfilling and has an element of surprise or incompleteness, users will be inclined to repeat the cycle. So, ask: What itch does my product scratch, and how can I reward users in a way that feels good but isn’t identical every time? It could be fresh content, a sense of community, or incremental progress. Get creative, even a small novelty or randomness can work wonders for engagement.
4. Investment: Getting Users to Put Skin in the Game
The final step of the Hook Model is Investment. This is where the user does a bit of work or puts something of value into the product, making it better and priming them for another round of the Hook cycle. In other words, after receiving the immediate reward, the product prompts the user to invest in some way, which in turn increases the likelihood of them using the product again in the future. This stage is often subtle, but it’s incredibly powerful. It’s about getting users to “put skin in the game” so that they own their experience and have a reason to come back. Investment can take many forms, for example:
Content creation. The user adds content or data to the product. Every time a user posts a photo on Instagram, writes a tweet, or even saves songs to a Spotify playlist, they are investing in the service. They’re essentially storing value in the product (in the form of their content, profile, or personalization). This increases the product’s personal value to them and also sets up future triggers. For instance, posting on Instagram is an investment – not only do you expend effort, but you’re now waiting for likes and comments, which will serve as external triggers (notifications) to pull you back in. When you send someone a message on WhatsApp, you don’t get an immediate reward right that second, but you’ve loaded the next trigger – the reply you’re anticipating is an external reward that will bring you back later. In Slack, writing a message or organizing a channel is an investment, you contribute to the conversation, and you’ll come back to see responses.
Building a following or network. When users invest by adding friends, followers, or connections, they embed themselves deeper into the product’s ecosystem. For a new app, encouraging users to invite a couple of teammates or integrate their contacts is a classic investment. It gives the user more reasons to return (because their friends or colleagues are there), and it sets up future triggers (your friend tags you – you get a notification to come back).
Personalization. Any time a user customizes their account or preferences, or the system adapts to them, that’s an investment that increases value for the next round. TikTok is a great example: when a user likes, comments, or follows creators, the algorithm learns their preferences. The user is training their feed to be more relevant. Next time they’re bored, the content will be even more tailored and rewarding, and those likes/follows also serve as an investment that makes the app stickier for them. In Duolingo, the more time you invest in learning, the higher your streak count goes, and nobody wants to lose a hard-earned streak. Users have effectively stored value in the form of a streak or level, which motivates them to come back tomorrow to protect that investment.
Crucially, investment steps increase the likelihood of another trigger down the line and deepen the user’s attachment. The more effort, data, or reputation a user pours into a product, the more it would hurt to abandon it. This is how habits take hold: each successive loop makes the product more valuable to the user, either practically or psychologically. The energy invested in the product creates new triggers for the user, creating a retention loop, a hook. Over time, users might start to trigger themselves internally. For example, a user thinks, “I’ve got a 50-day Duolingo streak going; I can’t stop now,” or “I wonder if anyone liked the photo I posted?” – and bam, they’re back in the app without any external prompt. That’s when you know the habit loop is working.
For your product, consider what small ask you can have at the right moment. After you’ve delivered the immediate reward, is there a way for the user to invest just a little bit more? It could be as simple as prompting them to hit a Like button, add something to a wishlist, create a profile, or invite a friend. It should be quick and not feel like a chore. Remember, they just got a reward, so they’re in a positive frame of mind. Even a tiny investment is effective if it makes the product better for the user next time.
How Top Startups Keep Users Hooked
By now, you’ve probably recognized the Hook Model in many of your favorite apps. Let’s look at well-known startups and how they apply Trigger → Action → Reward → Investment in practice:
TikTok: Trigger: That pang of boredom when you have a free moment is a huge internal trigger for TikTok’s core audience. Users often open TikTok almost reflexively when they want quick entertainment. (Of course, TikTok’s push notifications or trending alerts serve as external triggers too.) Action: Open the app and boom – a video starts playing immediately. The action is as simple as it gets: just watch (and swipe for the next video when ready). No login dance or menu navigation; TikTok puts the reward right in front of you with minimal effort. Variable Reward: TikTok’s feed is the epitome of variable reward. The next video could be hilarious, awe-inspiring, weird, or exactly what you didn’t know you wanted to see. Or it could be a dud – but if it is, you swipe again and surprise! maybe the next one hits the spot. This unpredictability creates a dopamine-fueled loop of “just one more video.” You’re basically playing a slot machine of short videos, where the payout is a clip that makes you laugh or go “wow.” Investment: TikTok makes it easy to invest subtly: when you find content you enjoy, you might tap Like, leave a comment, or follow the creator. Each of those actions teaches TikTok’s algorithm about your tastes – effectively investing data that will make your future feed even more tailored (meaning future rewards feel more rewarding). You might also create videos yourself (a larger investment of creativity and time) which really hooks you in – once you’ve posted, you’ll return to see the views and comments roll in. Even just the small act of customizing your feed by watching and reacting is an investment that loads the next triggers (notifications, better video picks) and deepens the habit.
Instagram: Trigger: A push notification that your friend tagged you (external trigger), or simply feeling a twinge of boredom or FOMO (internal trigger) makes you tap the Instagram icon. Action: Instantly, you start scrolling your photo feed or Stories – an incredibly easy, swipe-able action. Variable Reward: You see a fresh stream of content: maybe a beautiful travel photo, a funny meme, or surprise news from a friend. You might also get the reward of social validation – new likes or comments on your own posts. You never know what delights await, which keeps you scrolling. Investment: When you hit that “Like” button or leave a comment, you’re not just interacting – you’re investing a bit of yourself. Posting your own photo or Story is an even bigger investment; you’re now waiting to see how people react. Over time, you’ve followed people, curated your feed, and built a gallery of your memories – all investments that make Instagram more valuable to you and harder to abandon.
Slack: Trigger: An unread message badge or that gentle ding (external trigger) draws you in, or perhaps an internal trigger – the anxious feeling that you haven’t checked the team channel in a while. (We’ve all felt that “I might be missing something important” itch at work.) Action: You click into Slack; in one second you’re scanning the latest messages. Variable Reward: Sometimes there’s a direct reward – you find the info you needed or see a decision made. Other times, the reward is social or emotional: a teammate shared a GIF that makes you laugh, or you spot shout-outs in #general giving you a sense of belonging. Not every check-in is gold, but every so often there’s something rewarding (a crucial update, a bit of praise, a funny exchange), and that uncertainty keeps you checking frequently. Investment: You contribute to the conversation – reply to a thread, react with an emoji, or share a file. Each action embeds you (and sometimes your colleagues) further. Maybe you create a new channel for a project or integrate a bot to improve workflow. These investments mean next time you need to use Slack (your projects and discussions live there), and you’ll get pulled back when colleagues respond. Over weeks and months, Slack becomes the central hub of your work life, a habit that’s both externally and internally reinforced. Slack’s growth came not from ads, but from users habitually engaging because the product experience itself creates the routine.
Duolingo: Trigger: Duolingo leans on external triggers like clever daily push notifications (“It’s Duo! Time for your French lesson!”) at times when you’re likely free. Internally, users have the trigger of wanting to learn a new language or not wanting to break their streak. That personal goal or aspiration is a powerful internal trigger that Duolingo ties into. Action: The app makes the action simple: tap the notification and you’re immediately in a lesson. Each lesson is bite-sized and user-friendly, so the “work” feels more like playing a game. In just a few taps and swipes (translate this phrase, tap the correct answer), you’ve completed today’s session. Variable Reward: Duolingo is masterful in using gamified rewards. Sometimes you earn a few XP points; other times you level up or unlock a funny new character. You always get a check mark or a fanfare for completing a lesson, but the specifics vary – you might get a harder challenge that teaches a new word (novelty), or a simple review session that you ace quickly (confidence boost). Duolingo also gives immediate feedback on exercises, which is satisfying, and you never quite know what silly sentence or new badge might come next. These little surprises keep the experience fresh for learners. Investment: Each lesson you finish is time invested in your learning. Duolingo cleverly visualizes your investment with a streak count and daily goal progress. The more days you’ve racked up, the more loss aversion kicks in – you don’t want to lose that streak, so you keep coming back. Users might also invest by engaging in the forums or following friends for accountability, but even just the streak and earned fluency level feels like personal capital you’ve built. The result? A habit loop where the thought of losing what you’ve built becomes an internal trigger. Duolingo’s success shows how hooks can even make education addictive in a positive way.
These examples prove that the Hook Model isn’t theoretical. It’s actively at work in the apps we use daily. As a founder, studying these can spark ideas for your own product. You don’t have to be building the next TikTok or Duolingo; hooks apply to everything from productivity tools to e-commerce, if you understand your users’ psychology.
Applying the Hook Model to Your Startup
Reading about these big names is inspiring, but how can you start putting the Hook Model into practice for your product? It begins with asking the right questions about your user experience. Here’s a quick checklist to brainstorm around the Hook Model for your startup:
Trigger: What’s the itch that your product will consistently solve? Identify a situation or emotion when users would crave what you offer. Then plan an external trigger to catch them at that moment, e.g., a timely notification or email. Example question: “When might my user feel a need for my product, and how can I remind them at that time?”
Action: What’s the simplest action the user can take to get a bit of value? Reduce it to as few steps as possible. Make it intuitive and quick. Ask yourself: “Is this action stupid-easy to do? Can a first-time user do it in seconds without confusion?” Remember, any friction here will break the loop.
Variable Reward: What reward does the user get for the action, and how can I sprinkle in some variability? You want to fulfill the user’s need and surprise or delight them. Think: “What will make users say ‘cool!’ or feel satisfied? How can I keep it fresh so it doesn’t become boring or expected?” Even small unpredictability, like new content, changing feedback, occasional bonuses can make a big difference.
Investment: In what small ways can users invest in the product during or after the reward? This could be creating content, personalizing something, or simply doing something that loads a future benefit. Consider: “What could a user do now that will make the product better or more engaging for them next time?” It should be a logical, easy step: like saving a preference, following a friend, or inputting a bit of info. This increases the chance they’ll come back again, triggered either by external prompts or their own internal motivation, having now put value into your product.
By mapping out these four steps for your user journey, you’re essentially creating a habit roadmap for your product. Try writing it down like a cycle: Trigger → Action → Reward → Investment → back to Trigger. Is each step clear and compelling? Where are the weak spots? This exercise can reveal a lot. For example, you might realize you have a decent trigger and action, but your reward isn’t rewarding enough, or it’s too predictable. Or maybe users enjoy the reward but aren’t investing anything, so they drift away after the novelty wears off. Use the Hook framework to iterate on those areas.
Final Takeaway:
Win Hearts, One Habit at a Time
Building habit-forming products is not about tricks or coercion. It’s about understanding your users deeply and aligning with their natural behaviors and desires. It’s how your product makes people feel and how it fits into their lives that creates true retention. The Hook Model is a tool to help you design with that in mind from day one. So, as you plan your startup’s next feature or growth strategy, take a step back and think in Hooks: What’s my trigger? Is the action simple? Am I rewarding the user in a satisfying, variable way? How do I encourage them to invest and come back? Answering these questions can set your product on the path from a one-time use to a lifetime habit.
By applying the Trigger → Action → Reward → Investment cycle in your own startup, you can design products that bring users back again and again. Not through force, but by fitting into the natural habit loops of everyday life. Who knows, your app could be the next one people are “addicted” to! Build responsibly, keep the user’s well-being in mind, and aim to become a positive part of your customers’ daily routines. Get out there and get your users hooked – their future selves (and your retention metrics) will thank you. Good luck, and stay creative!
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