Friday, March 04, 2022

"Hooked" - Book Review and Summary

The book "Hooked," by Nir Eyal, applies psychology to website/app design to explain how successful products create unprompted user engagement - aka habits - where users visit a website/app repeatedly on their own.

(I'll simply use "app" from now on for "website/app.")

If you find that you regularly open an app - like FacebookYoutube, or Email - out of habit, that app has successfully "hooked" you. Why does that happen? If you're creating a product to be regularly used or want to use an app less, this book will help you.

Eyal identified a 4-step process, called the Hooked Model, that apps use to encourage users to repeatedly return. The book describes each step and why they work. I enjoy books that explain behavior with psychology. Plus, I'm building a website, so I was interested in seeing what parts I could incorporate.

It also helped me see when other apps use the Hooked Model. For example, I use a journaling app called Day One, and they do all of these steps, which has helped me journal regularly. It also explains why some apps, like Duolingo, feel pushy (I go on a mini-rant in the summary section).

One of Eyal's suggestions I'll use right away is to not just look at how people use my website (through usability testing). But to also create a user narrative that clearly describes the person's desires, emotions, and context when they start using the website. And it's not just making it up on my own, but using good, ol' fashion marketing research where you interview people, asking "why" at least 5 times to get to an emotional motivation.

Again, I highly recommend the book to anyone building an app.

The rest of this post contains my notes and quotes from the book. I recommend reading/listening to the book first and using the summary below for future reference. Or, read below as a teaser to see if you'd like it.

The Hooked Model

The Hooked Model consists of four parts:

  1. A trigger to visit the app
  2. An action to take within the app
  3. A variable reward for taking action
  4. An investment of time/data with the app

The more frequently a user goes through the Hooked Model, the sooner they'll create a habit of using the app. "[T]his framework is for creating habit-forming products, not one-time-use goods." (p. 167). 

"Ultimately, though, the [Hooked Model] should be 'used to help nudge people to make better choices (as judged by themselves).' Accordingly, this book teaches innovators how to build products to help people do the things they already want to do but, for lack of a well-designed solution, don't do." (p. 12).

In other words, don't use the Hooked Model to create an addictive app; only use it to help your users.


In many ways, Hooked is an extension of the book "Don't Make Me Think" by Steve Krug. Krug's book is all about usability: how to make your app as easy to use as possible. It's not that you treat your users as dumb, but it's respecting their time and meeting them in their real-life experience. YOU, the innovator, invest the extra time optimizing your app so that your USERS don't need to invest the time themselves figuring out how to use a poorly designed app.

And if you created an app that helps people (Eyal uses the Bible app and Fitbod for a couple of case studies), you want to help them form those healthy habits with good design.

It's an act of service.

And of course, some companies inappropriately harness the Hooked Model to create bad habits and addictive behavior. But once you understand the model, you can fight back to break a habit.

Next, Eyal dives into each part of the Hooked Model.


The Hooked Model starts with a trigger. Something happens which prompts you to take action. The trigger can be external or internal.

External Triggers

Examples of external triggers are push notifications or getting an email. "External triggers are embedded with information, which tells the user what to do next." (p. 41). Following usability principles, effective external triggers have a single clear call to action. "Reducing the thinking required to take the next action increases the likelihood of the desired behavior occurring with little thought." (p. 43).

Here are the different types of external triggers:

Paid Triggers

Think online advertisements where you see an ad telling you to download an app. "Habit-forming companies tend not to rely on paid triggers for very long, if at all." (p. 44). They might use them to acquire new users, but they use other triggers to bring them back.

Earned Triggers

These can't be bought, but often require an investment of time. Think favorable press mentions, guest blog posts, podcast interviews, or video collaborations. It can be unpredictable but highly effective in creating traffic spikes.

Relationship Triggers

"[R]elationship triggers drive growth because people love to tell one another about a wonderful offer." (p. 45). Word-of-mouth product referrals are perhaps the most effective triggers. This is what often drives viral growth.

"Proper use of relationship triggers requires building an engaged user base that is enthusiastic about sharing the benefits of the product with others." (p. 46). And this is why you see so many "share on XYZ" requests online.

Owned Triggers

"Owned triggers consume a piece of real estate in the user's environment. They consistently show up in daily life and it is ultimately up to the user to opt in to allowing these triggers to appear. For example, an app icon on the user's phone screen, an email newsletter to which the user subscribes, or an app update notification only appears if the user wants it there." (p. 46). Email lists are considered one of the most valuable assets a business owns because of how powerful owned triggers are. 

"While paid, earned, and relationship triggers drive new user acquisition, owned triggers prompt repeat engagement until a habit is formed." (p. 47).

External to Internal Triggers

"The ultimate goal of all external triggers is to propel users into and through the Hooked Model so that, after successive cycles, they do not need further prompting from external triggers. When users form habits, they are cued by a different kind of trigger: internal ones." (p. 47).

Sophisticated ones will only send you a message if you stop engaging with the product to restart the Hooked Model.

Internal Triggers

"Internal triggers manifest automatically in your mind. Connecting internal triggers with a product is the brass ring of habit-forming technology." (p. 48).

When using an app provides psychological relief, it can form a habit as you form positive associations with that app over time. The emotion - frequently negative - could be boredom (Youtube), loneliness (Facebook), stress (Headspace), uncertainty (Google), validating importance (email), or many others.

If you've ever tried to lose weight, you'll notice the similarity to over-eating. We don't over-eat because we're hungry, but to provide relief from something else. A complete aside: if you're hungry, ask yourself if you'd eat a raw vegetable, like carrots. If not, then you're probably not physically hungry. Pause, and try to identify the internal emotional trigger that led you to want to eat. Then experiment with trying something different to meet the emotional need. The same pause works for over-used apps.

Unlike an external trigger, the internal's call to action of what to do next is encoded in your memory.

Building for Triggers

"The ultimate goal of a habit-forming product is to solve the user's pain by creating an association so that the user identifies the company's product or service as the source of relief." (p. 52).

So, step one is to "identify the particular frustration or pain point in emotional terms, rather than product features." (p. 52). What might a user be feeling right before they use your app?

This is a leveling up of usability principles: don't just look at how people use your app (through usability testing), but also create a user narrative that clearly describes the user's desires, emotion, and context when they start using the app (through interviews).

"One method is to try asking the question 'Why?' as many times as it takes to get to an emotion. Usually, this will happen by the fifth why. This is a technique adapted from the Toyota Production System, described by Taiichi Ohno as the '5 Whys Method.' " (p. 54).

Once you come up with three internal triggers, you can use those as part of the product requirements. Again, investing the time to understand your users and their needs - not building something solely based on what YOU think is needed - is an act of service.


The next step in the Hooked Model is taking action. A habit is something done with little or no conscious thought, so the first action needs to also require little to no conscious thought.

"[Dr. B. J.] Fogg posits that there are three ingredients required to initiate any and all behaviors: (1) the user must have sufficient motivation; (2) the user must have the ability to complete the desired action; and (3) a trigger must be present to activate the behavior. The Fogg Behavior Model is represented in the formula B = MAT, which represents that a given behavior will occur when motivation, ability, and a trigger are present at the same time and in sufficient degrees." (p. 62).

We already talked about the trigger, so here's the motivation and ability parts:


What type of motivation is your user seeking or avoiding? "Fogg states that all humans are motivated to seek pleasure and avoid pain; to seek hope and avoid fear; and finally, to seek social acceptance and avoid rejection." (p. 63).

Remember, "What motivates some people will not motivate others, a fact that provides all the more reason to understand the needs of your particular target audience." (pp. 64-65). This is why interviewing your users is so important.

So, you want to create a simple action your user can take that connects to their motivation. The trigger will have a call-to-action for the simple action. Ideally, the trigger occurs when they're highly motivated. I have a notification right now from a research app that says, "There is one task available when you're ready."

"However, even with the right trigger enabled and motivation running high, product designers often find users still don't behave the way they want them to. What's missing in this equation? Usability—or rather, the ability of the user to take action easily." (p. 67).

Let's look at ability next.


"[Denis J.] Hauptly states, understand the reason people use a product or service. Next, lay out the steps the customer must take to get the job done. Finally, once the series of tasks from intention to outcome is understood, simply start removing steps until you reach the simplest possible process. Consequently, any technology or product that significantly reduces the steps to complete a task will enjoy high adoption rates by the people it assists." (p. 67).

Hauptly's advice is golden. The most popular products in history are simple and removed steps. Do you want to communicate with a friend? Travel -> Mail (less physical effort and asynchronous) -> Email (free, instant delivery and links) -> Texting (no subject lines or proper punctuation)

6 elements influence the difficulty of a task (pp 71-72 emphases are mine):

  1. "Time—how long it takes to complete an action. 
  2. Money—the fiscal cost of taking an action. 
  3. Physical effort—the amount of labor involved in taking the action. 
  4. Brain cycles—the level of mental effort and focus required to take an action. 
  5. Social deviance—how accepted the behavior is by others. 
  6. Non-routine—according to Fogg, 'How much the action matches or disrupts existing routines.' "

The drive to consolidate steps led to app inventions like: logging in with Facebook/Apple/Google (instead of an email/password), share buttons, 1-Click purchases, smartphone cameras buttons on home screens, and infinite scrolling.

Ask: what features does your product have within one of those six elements that are 10x better than how users are currently solving their problem (connected to an emotion)? Again, we're taking usability to the next level.

There are also four preception effects worth knowing about:

  1. Scarcity effect: "The appearance of scarcity affected their perception of value." (p. 85). Classic examples are limited stocked items.
  2. Framing effect: "The mind takes shortcuts informed by our surroundings to make quick and sometimes erroneous judgments." (p. 87). For example, a professionally designed website builds trust.
  3. Anchoring effect: "People often anchor to one piece of information when making a decision." (p. 88).
  4. Endowed progress effect: "a phenomenon that increases motivation as people believe they are nearing a goal." (p. 89). We used to say you're "smelling the barn."

Here's one incredible piece of research on the endowed progress effect:

"Two groups of customers were given punch cards awarding a free car wash once the cards were fully punched. One group was given a blank punch card with eight squares; the other was given a punch card with ten squares that came with two free punches. Both groups still had to purchase eight car washes to receive a free wash; however, the second group of customers—those that were given two free punches—had a staggering 82 percent higher completion rate." (p. 89).

This is why LinkedIn shows you a "profile is X% complete" with a specific call to action for what you can do next to "improve your profile strength."

To recap: you want to create a simple action for your user to take that's tied to their motivation. The simple action should do a better job of satisfying their emotional desire than any alternative. This usually takes the form of a simple task: liking a post, reading a message, logging a meal, answering a journal prompt, etc.

Variable Reward

The next part of the Hooked Model is a variable reward. The user received a trigger and took a simple action. What happens next?


Simply offering a reward is not good enough; it needs to be variable. "Without variability we are like children in that once we figure out what will happen next, we become less excited by the experience. The same rules that apply to puppies also apply to products. To hold our attention, products must have an ongoing degree of novelty." (p. 98).

As a throw-back example, watching "The Sixth Sense" is rewarding the first time. It also provides an entirely different reward the second time. But, by the third viewing, it's no longer novel, so we move on to another movie.


There are three types of rewards: the tribe, the hunt, and the self.

1. Rewards of the Tribe

"Our brains are adapted to seek rewards that make us feel accepted, attractive, important, and included." (p. 100). Social acceptance and connection are huge rewards. We were created for relationships, so any product that helps us connect (even if it's not the best long-term way of connecting) does very well.

"It is no surprise that social media has exploded in popularity. Facebook, TwitterPinterest, and several other sites collectively provide over a billion people with powerful social rewards on a variable schedule. With every post, tweet, or pin, users anticipate social validation. Rewards of the tribe keep users coming back, wanting more." (p. 100).

Another example: "Stack Overflow devotees write responses in anticipation of rewards of the tribe. Each time a user submits an answer, other members have the opportunity to vote the response up or down." (p. 102). 

2. Rewards of the Hunt

"The need to acquire physical objects, such as food and other supplies that aid our survival, is part of our brain's operating system." (p. 107). My wife loves puzzles, and I like Legos. Some people are bargain shoppers or garage sale junkies. I regularly have conversations where friends boast about a great deal they found for a product (OK... I do it too).

"The Twitter timeline, for example, is filled with a mix of both mundane and relevant content. This variety creates an enticingly unpredictable user experience. On occasion a user might find a particularly interesting piece of news, while other times she won't. To keep hunting for more information, all that is needed is a flick of the finger or scroll of a mouse. Users scroll and scroll and scroll to search for variable rewards in the form of relevant tweets." (p. 108).

3. Rewards of the Self

"We are driven to conquer obstacles, even if just for the satisfaction of doing so. Pursuing a task to completion can influence people to continue all sorts of behaviors." (pp. 110-111). This coincides with the endowed progress effect. When walking on a there-and-back route, I HAVE to touch something - like a pole or wall - before turning around to mark its completion. It shouldn't be highly satisfying, but it is.

"The rewards of the self are fueled by 'intrinsic motivation' as highlighted by the work of Edward Deci and Richard Ryan. Their self-determination theory espouses that people desire, among other things, to gain a sense of competency. Adding an element of mystery to this goal makes the pursuit all the more enticing." (p. 111). The mystery might be when you achieve competency or the trials experienced along the way.

As someone hooked on email's reward of self, I liked this quote: "Users can give up when they sense the struggle to get their in-boxes under control is hopeless. To combat the problem and give users a sense of progress, Google created 'Priority Inbox.' Using this feature, Gmail cleverly segments emails into sorted folders to increase the frequency of users achieving 'in-box zero'—a near-mystical state of having no unread emails." (p. 113).

That's the variability and reward components. There are some caveats, though.

Not All Reward Systems Work

You can't just gamify an app and call it a day.

Not Relevant

"Rewards must fit into the narrative of why the product is used and align with the user's internal triggers and motivations. They must ultimately improve the user's life." (p. 118).

I've tried using Duolingo to learn a language. They employ a bunch of Hooked ideas but don't quite nail it. For example, you earn XP after each lesson, which earns you a spot on a leader board. Every 6-ish days you move up into a new leaderboard, stay the same, or drop down to a leader board. But it has nothing to do with learning a language! They also track streaks, constantly ask you to share, reward badges earned XP, and send notifications of friend accomplishments. I get it, but none of these inherently help me learn the language. But perhaps my motivation is different than their core users, which is why it doesn't work for me.

Loss of Autonomy (underline is mine)

"the most successful consumer technologies—those that have altered the daily behaviors of billions of people—are the ones that nobody makes us use. Perhaps part of the appeal of sneaking in a few minutes on Instagram or checking scores on is our access to a moment of pure autonomy—an escape from being told what to do by bosses and coworkers. Unfortunately, too many companies build their products betting users will do what they make them do instead of letting them do what they want to do. Companies fail to change user behaviors because they do not make their services enjoyable for its own sake, often asking users to learn new, unfamiliar actions instead of making old routines easier. Companies that successfully change behaviors present users with an implicit choice between their old way of doing things and a new, more convenient way to fulfill existing needs. By maintaining the users' freedom to choose, products can facilitate the adoption of new habits and change behavior for good." (p. 123).

Usability testing helps with this. I know my problem is it's designed for how I use it, but that's not necessarily how others think about it.

Finite Variability

"Experiences with finite variability become less engaging because they eventually become predictable." (p. 127). It seems like most of the Facebook games I've played have this problem. Or, reaching the next reward takes exponentially more time, which effectively removes the reward.

"[G]ames played to completion offer finite variability, while those played with other people have higher degrees of infinite variability because the players themselves alter the gameplay throughout." (pp. 127-128). This explains why multiplayer games are so popular! Are there any ways users can interact with each other in your product?

Inherent Variability

I found this interesting. If there's inherent variability, you don't need to artificially add more. Instead, you want to give users a way to understand the variables:

"Whether the product is an enterprise-focused service helping customers get a grip on the effectiveness of their marketing spend, a financial information portal, a health tracking app, or a corporate dashboard, all sorts of products operate in conditions of inherent variability. Companies building these sorts of products and services need not necessarily add more uncertainty, but rather give the user a greater sense of agency and control over inherently variable circumstances." (p. 130).

Next Steps

"Fundamentally, variable reward systems must satisfy users' needs while leaving them wanting to reengage." (p. 130). Again, this is for apps that are used regularly. I personally like rewards of the self the most, but that can be hit or miss, whereas social and hunt rewards seem to work for a lot more people.

Do this: "Speak with five of your customers in an open-ended interview to identify what they find enjoyable or encouraging about using your product. Are there any moments of delight or surprise? Is there anything they find particularly satisfying about using the product?" (p. 133). Similar to usability testing, it doesn't require a vast panel of people to identify the major areas of improvement. All of these interviews can be done in a single day.

"Brainstorm three ways your product might heighten users' search for variable rewards" (p. 133).


"The more users invest time and effort into a product or service, the more they value it. In fact, there is ample evidence to suggest that our labor leads to love." (p. 136). Here are three reasons why:

1. We irrationally value our efforts

IKEA is a perfect example of this. Not only did they figure out a way to save manufacturing and shipping costs by selling deconstructed furniture, but people love their furniture because they helped build it.

2. We seek to be consistent with our past behaviors

"Little investments... can lead to big changes in future behaviors." (p. 139). In numerous studies, once you commit to a small thing, you're willing to do more and more to stay consistent. I love this one. It means doing small things, like flossing each day or making your bed every morning, can lead you to do other things: cleaning your room, house, and workspace.

3. We avoid cognitive dissonance

"To avoid the cognitive dissonance of not liking something that others seem to take so much pleasure in, we slowly change our perception of the thing we once did not enjoy." (p. 140). Everyone who enjoys black coffee or stout beers knows about this one.

"Together, the three tendencies just described influence our future actions: The more effort we put into something, the more likely we are to value it; we are more likely to be consistent with our past behaviors; and finally, we change our preferences to avoid cognitive dissonance." (p. 140).

Jessi Schell helped explain why people spend money on Zynga's games. It's a cycle that relies on all three tendencies: "Combine that with the psychological idea . . . of rationalization, that anything you spend time on, you start to believe, 'This must be worthwhile. Why? Because I've spent time on it!' And therefore it must be worth me kicking in twenty dollars because look at the time I've spent on it. And now that I've kicked in twenty dollars, it must be valuable because only an idiot would kick in twenty dollars if it wasn't." (pp. 141-142).

How Investment Fits Into The Hooked Model

"To form the associations needed to create unprompted user engagement, something more than the three-step feedback loop is required. The last step of the Hooked Model is the investment phase, the point at which users are asked to do a bit of work. Here, users are prompted to put something of value into the system, which increases the likelihood of their using the product and of successive passes through the cycle. Unlike in the action phase of the Hook... investments are about the anticipation of longer-term rewards, not immediate gratification." (p. 143).

This took me a little while to get straight. The action is simple/fast and has an immediate variable reward. The investment, however, takes a little bit more work, and it improves the long-term/future use of the app.

"In the investment phase, however, asking users to do a bit of work comes after users have received variable rewards, not before. The timing of asking for user investment is critically important. By asking for the investment after the reward, the company has an opportunity to leverage a central trait of human behavior." (p. 144). That trait is reciprocation, where you feel compelled to do something in return for something done for you. For example, if someone gets you a present, you often feel obligated to give them one.

In general, reciprocity is good and helps society function (if I'm nice to you, you're nice in return). The same is true of the apps we use: "The big idea behind the investment phase is to leverage the user's understanding that the service will get better with use (and personal investment). Like a good friendship, the more effort people put in, the more both parties benefit." (p. 144).

Storing Value

There are 5 types of value that a product can store that enhance the user's experience. Here's each one and how they can make an app better.

1. Content: As users consume content, the product can make personalized recommendations based on past behavior. Pandora was one of the first apps that explicitly worked this way.

2. Data: "[T]he more information users invested in the site, the more committed they became to it." (p. 147). Think: adding your resume to LinkedIn or your spending data to Mint.

3. Followers: "Investing in following the right people increases the value of the product by displaying more relevant and interesting content" (p. 149). For a creator, switching services means abandoning a following you've worked to acquire and nurture.

4. Reputation: "Reputation is a form of stored value users can literally take to the bank. On online marketplaces such as eBayUpworkYelp, and Airbnb, people with negative scores are treated very differently from those with good reputations." (p. 151).

5. Skill: "Investing time and effort into learning to use a product is a form of investment and stored value." (p. 152). Remember, "non-routine is a factor of simplicity, and the more familiar a behavior is, the more likely the user is to do it." (p. 152). Quickbooks is an example of an app that takes time to learn because, despite accounting being nothing more than arithmetic, it's complicated. But once you do (and experience the joy of a reconciled set of books), the app becomes better to use.

Also, remember that the investment needs to be relevant, and the user needs to have sufficient motivation and ability to engage. So, creating a detailed profile page that asks for info that's not required doesn't count.

"I recommend that you progressively stage the investment you want from users into small chunks of work, starting with small, easy tasks and building up to harder tasks during successive cycles through the Hooked Model." (p. 153). This one can be tricky because you know the app experience will be significantly better once the investment is made, but that can be too much at once. Try to find meaningful milestones that can be rewarded.

Using the Hooked Model

In summary (pp. 163-164): 

"[A]sk yourself these fundamental questions for building effective hooks:

  1. What do users really want? What pain is your product relieving? (Internal trigger) 
  2. What brings users to your service? (External trigger) 
  3. What is the simplest action users take in anticipation of reward, and how can you simplify your product to make this action easier? (Action) 
  4. Are users fulfilled by the reward yet left wanting more? (Variable reward) 
  5. What "bit of work" do users invest in your product? Does it load the next trigger and store value to improve the product with use? (Investment)"

Avoid manipulation

By this point, you might feel a little uneasy about the Hooked Model. It can feel like it's exploiting our psychological natures to get us to do things we wouldn't normally do. Eyal adds two additional questions you need to answer:

"[T]he maker needs to ask two questions. First, "Would I use the product myself?" and second, "Will the product help users materially improve their lives?" (p. 167).

"If you find yourself squirming as you ask yourself these questions or needing to qualify or justify your answers, stop! You failed." (p. 168).

Yes, it's a bit of self-policing. Eyal also points out that he studied what the larger apps are already doing to create the Hooked Model. So it's not like they'll read this book and take their manipulation to the next level.

Habit Testing

The final section addresses user testing. Research is one of those practices that everyone agrees is good, but few do (this sounds like an app opportunity). One reason why it's so valuable is that creators have the curse of knowledge: once you know how an app is supposed to work, it's hard to imagine what it's like to use the app for the first time.

So, whenever you come up with an idea, the only way to know if it will work is to test it with actual users. Here's the three-step process:

1. Identify: "define what it means to be a devoted user. How often "should" one use your product?" (p. 201). Be realistic and honest.

2. Codify: "codify the steps they took using your product to understand what hooked them." (p. 202). "You are looking for a Habit Path—a series of similar actions shared by your most loyal users." (p. 203).

3. Modify: "revisit your product and identify ways to nudge new users down the same Habit Path taken by devotees." (p. 203). And test it with usability testing.

"Habit Testing is a continual process you can implement with every new feature and product iteration." (p. 204).

Final Thoughts

It's a lot of notes, but that's because it's a highly actionable book. I like Eyal's writing style ("Indistractable" is similarly well written). If you do any product development for an app designed for regular use, I highly recommend reading the book.

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