SECTION 7: Behavior Design Models: Why People Do (and Don't) Act

Every product team wants users to take action — sign up, form a habit, share, upgrade. Every self-improvement effort wants a person to take action — exercise, save money, study. But most of the time the action never happens.

Why? Not usually because people lack desire. More often it is because the conditions for action were never right at the same moment.

Four frameworks — each built by researchers and practitioners on real evidence — give you a map for engineering those conditions. This section covers all four, compares them, and shows how to apply each to products and personal life.

1. The Fogg Behavior Model: B = MAP

Dr. BJ Fogg is a researcher at Stanford University's Behavior Design Lab. His central insight is beautifully simple:

  B = M · A · P

  Behavior happens when Motivation, Ability, and
  a Prompt all occur at the same moment.

  Remove any one of the three → no behavior.

This is not just a formula. It is a diagnostic tool. If a behavior is not happening, one of the three elements is missing or too weak.

Motivation (M)

Motivation is how much you want to do a thing right now. Fogg is careful to say motivation goes up and down like a wave — it is not a stable personality trait. A person might be highly motivated to exercise at New Year and unmotivated by February. Fogg calls these fluctuations the Motivation Wave. Waiting for high motivation is unreliable. It is a mistake to design systems that only work when motivation is sky-high.

Ability (A)

Ability means how easy the behavior is to do at this moment. It is not just skill — it includes time, money, physical effort, cognitive load, and social deviation ("will people think I'm weird for doing this?"). Fogg says the most reliable lever is making the behavior tinier, not trying to boost motivation. A behavior that takes 2 seconds requires almost no motivation to trigger.

Prompt (P)

A Prompt (Fogg's updated word for "trigger") is the cue that says do it now. Without a prompt, even a motivated, capable person often fails to act. Fogg identifies three types based on where they come from:

• Spark: A prompt that boosts motivation (used when ability is high but motivation is low). Example: an emotional video ad right before a "Donate" button.
• Facilitator: A prompt that makes the action easier (used when motivation is high but ability is low). Example: a one-click checkout button for a user who already wants to buy.
• Signal: A simple reminder (used when both motivation and ability are already high). Example: a calendar notification.

The Action Line

Imagine a graph: Motivation on the vertical axis (low to high), Ability on the horizontal axis (hard to easy, left to right). There is a curved line running across the graph — the Action Line.

  Motivation
  HIGH |  *                        *  *
       |     *                  *
       |        *  [Action Line] *
       |             *        *
  LOW  |                 *  *
       +------------------------------------------
            HARD        EASY         Ability

Any behavior that lands above the Action Line will happen when prompted. Below the line — it will not. You can get above the line two ways: boost motivation OR increase ability (make it easier). Increasing ability is more reliable and longer-lasting than chasing motivation spikes.

Hot vs. Cold Triggers

A hot trigger is one a motivated person can act on immediately — a link they can click right now, a sign-up form in front of them. A cold trigger is a cue that arrives when the person cannot act — a TV ad for a product when they have no phone, a message arriving while they are in a meeting. Fogg's principle: put hot triggers in the path of motivated people. Cold triggers waste the motivation wave.

2. The COM-B Model: A Diagnostic for Behavior Change

The COM-B model was created by Susan Michie, Maartje van Stralen, and Robert West at University College London, published in 2011 in the journal Implementation Science. It has over 15,000 academic citations, making it one of the most widely used behavior change frameworks in health, policy, and organizational design.

COM-B stands for:

  Capability + Opportunity + Motivation → Behavior

Where Fogg's model tells you when a behavior fires, COM-B is a diagnostic — it tells you why a behavior is not happening and which of six possible levers to pull.

Capability (C)

Capability is what a person is able to do, both physically and mentally. COM-B splits it into two types:

• Physical Capability: The body's ability to carry out the action. Example: a person with arthritis may lack the physical capability to open a child-proof pill bottle, even if they want to take their medication.
• Psychological Capability: The mental knowledge, skills, and self-regulation to perform the behavior. Example: a patient who does not understand their diagnosis lacks the psychological capability to follow a treatment plan.

Opportunity (O)

Opportunity is everything outside the person that makes the behavior possible or impossible. It is not about ability — it is about the environment.

• Physical Opportunity: Time, place, resources, and access. Example: a worker who wants to exercise but has no gym near the office lacks physical opportunity.
• Social Opportunity: Cultural norms and social cues. Example: in a workplace where everyone eats at their desks, the social norm removes the opportunity to take a proper lunch break — even if management says it is allowed.

Motivation (M)

Motivation in COM-B is broader than Fogg's version. It includes two types:

• Reflective Motivation: Conscious thinking — plans, intentions, beliefs about whether the behavior is good. Example: "I believe flossing prevents gum disease, so I intend to do it."
• Automatic Motivation: Impulses, habits, emotions — things that happen without deliberate thought. Example: reaching for your phone the moment you are bored is automatic motivation, not reflective.

3. Nir Eyal's Hooked Model: Building Product Habits

Nir Eyal is the author of Hooked: How to Build Habit-Forming Products (2014). His model is specifically designed for product teams who want users to return to their product without being prompted by advertising every time. The goal is to move users from paid acquisition to autonomous habit.

The Hook is a four-step loop:

  ┌─────────────┐
  │   TRIGGER   │◄─────────────────────────────┐
  └──────┬──────┘                               │
         │                               (loads next trigger)
         ▼                                      │
  ┌─────────────┐                        ┌──────┴──────┐
  │   ACTION    │                        │ INVESTMENT  │
  └──────┬──────┘                        └─────────────┘
         │                                      ▲
         ▼                                      │
  ┌─────────────────┐                           │
  │ VARIABLE REWARD │───────────────────────────┘
  └─────────────────┘

Step 1: Trigger

A trigger is the cue that starts the behavior. There are two kinds:

• External triggers come from outside — a notification, an email, a banner ad. These are expensive to sustain and only work while you keep paying for them.
• Internal triggers are feelings and emotions that automatically cue the behavior — boredom, loneliness, uncertainty, fear of missing out. Internal triggers are the goal. When a product is associated with an emotion ("when I feel bored, I open Instagram"), no external push is needed.

Step 2: Action

The action is the simplest behavior the product wants you to do — open the app, scroll, search, tap. Eyal draws on Fogg here: the action must be as easy as possible. High-friction actions break the loop. The rule: the simpler the action, the more likely it is to happen even at low motivation.

Step 3: Variable Reward

This is the most psychologically powerful step. The reward must be variable — unpredictable — not guaranteed. A fixed reward you can predict loses its pull. An uncertain reward creates anticipation and a dopamine spike, the same mechanism behind slot machines.

Eyal identifies three types of variable reward:

• Rewards of the Tribe (social): Connection, acceptance, social validation. Example: the number of likes on an Instagram post is unpredictable — you do not know if this one will get 3 or 300. That uncertainty keeps you posting and checking.
• Rewards of the Hunt (resources/information): Searching for useful material or resources. Example: scrolling a LinkedIn or Twitter feed is a hunt — you do not know if the next post will be valuable or worthless, but the possibility keeps you scrolling.
• Rewards of the Self (mastery/completion): Personal achievement, competence, progress. Example: clearing all unread emails (Inbox Zero), unlocking a new badge on Stack Overflow, finishing a language lesson streak on Duolingo.

Step 4: Investment

The investment phase is what separates Hooked from other models. After receiving the reward, the user puts something of value into the product — data, content, relationships, time. This has two effects:

1. It increases the user's future motivation (we value things we have put effort into — a principle called the IKEA effect).
2. It loads the next trigger. When you follow people on Twitter, the system now has enough data to send you personalized notifications — the next external trigger.

4. Nudge Theory and Choice Architecture

Richard Thaler (Nobel Prize in Economics, 2017) and Cass Sunstein published Nudge: Improving Decisions About Health, Wealth, and Happiness in 2008. Their core idea: people do not make decisions in a vacuum. Every choice is presented inside a choice architecture — a context that shapes the decision, often without the decision-maker realizing it.

A nudge is any small change to that context that predictably steers behavior in a beneficial direction, without removing anyone's freedom to choose differently. Thaler and Sunstein call this approach libertarian paternalism — guide people toward better outcomes while fully preserving free choice.

The Main Nudge Tools

Defaults

A default is what happens if you do nothing. It is the single most powerful nudge known to behavioral scientists, because most people never change a default.

The mechanism works through three forces operating at once:

• Status quo bias: People prefer not to change things. The current state feels safe.
• Implied endorsement: Users interpret the default as an expert recommendation — "whoever set this up must have decided it is the right choice for most people."
• Loss aversion: Changing a default means giving up the current state, which feels like a loss even if the new option is objectively better.

Friction

Adding steps, clicks, or delays to an action reduces how often people take it. Removing steps increases it. This is choice architecture through effort cost.

• Adding friction to bad choices: A confirmation dialog ("Are you sure you want to delete all data?") reduces accidental deletions. A cooling-off period on large financial decisions reduces impulsive regret.
• Removing friction from good choices: One-click re-order on Amazon. Pre-filled forms. Face ID for payment. Each removed step increases conversion significantly.

Framing

The same fact, described two different ways, produces different decisions. This is the framing effect — a well-established finding from Kahneman and Tversky's research in the 1970s and 1980s that Thaler and Sunstein built into nudge design.

Salience

People pay attention to what is vivid, prominent, and novel. Choice architecture can make the better option more salient — place fruit at eye level in a cafeteria and consumption rises without banning anything. Place the most popular pricing plan in a highlighted box, and it gets more sign-ups.

Comparing the Four Frameworks

When to use which

• Use Fogg B=MAP when you need someone to take a specific action once — sign up, click, try a feature. Diagnose which of M, A, or P is the weak link.
• Use COM-B when a behavior is failing and you do not know why. Run through all six sub-components like a checklist.
• Use Hooked when you want users to return on their own, repeatedly, without paid re-acquisition. Design the full loop, not just the first visit.
• Use Nudge Theory when you are designing any choice environment — a form, a pricing page, a settings panel — and want to steer people toward the better outcome without removing choice.

Personal Application: Using These Models on Yourself

These frameworks are not only for product teams. They apply to personal habit design.

• Fogg in personal life: Want to exercise daily? Make it tiny (two push-ups), anchor it to an existing routine (after brushing teeth = a hot trigger), and remove all friction (leave shoes by the door).
• COM-B in personal life: Not saving money? Capability gap: do you know how to set up an auto-transfer? Opportunity gap: does your bank make it easy? Motivation gap: do you believe it matters right now?
• Hooked in personal life: Notice which of your own habits are being engineered by apps. Awareness breaks the automatic loop. You can then design better internal triggers for habits you actually want (connect "feeling stressed" to a 5-minute walk, not to opening social media).
• Nudge in personal life: Change your defaults. Put healthy food at eye level in the fridge. Set up automatic savings transfers. Place your book on the pillow; place your phone charger in another room. You are your own choice architect.