Glossary
Plain-language definitions for the psychology behind the mechanics. Every term is the why under a pattern you'll meet across the library: the reason a streak holds or a price reads as a deal, and links straight to the mechanics that put it to work.
Adaptive Preference (Sour Grapes)
We quietly downgrade things we can't have, and sometimes upgrade things that are forbidden. The fox decides the unreachable grapes were probably sour anyway.
Anchoring
The first number we see biases our judgment of what follows, even when it's irrelevant. An initial price, quote, or estimate sets a reference point that later figures get pulled toward.
Attention and Salience
We notice what is novel, personally relevant, or visually distinct, and ignore the rest. If something isn't salient, for practical purposes it isn't there.
Authority
We defer to legitimate experts and figures of authority, often automatically, and to the mere symbols of authority: titles, uniforms, credentials, even when they're hollow.
Autonomy, Competence, Relatedness
The three psychological needs from self-determination theory: a sense of meaningful choice and control (autonomy), a sense of getting better and making progress (competence), and a sense of connection and belonging (relatedness).
Availability Bias
We judge how likely or common something is by how easily examples come to mind, which vivid or recent events distort. People overestimate dramatic risks and underestimate quiet ones.
Ballistic Behavior
Making a decision and then not checking its consequences, firing and never watching where it lands. It preserves the feeling of competence at the cost of ever learning, and it gets worse under pressure.
Bartle Taxonomy
A classification of video game player types developed by Richard Bartle in 1996. The four types, Achievers, Explorers, Socializers, and Killers (adapted as Competitors in app contexts), describe different motivational profiles that respond to different mechanics. Understanding which type dominates a product's user base changes which mechanics to build and which ones will silently drive churn.
Bounded Rationality
People don't weigh every option perfectly; they reason as well as they can within limits of time, attention, and information, then settle. Most real decisions are 'good enough,' not optimal.
Bystander Effect
The more people present, the less likely any one of them is to help, because responsibility feels diffused and everyone waits for someone else. Naming one person directly breaks it.
Calibration
How well a person's confidence matches reality: a well-calibrated person is right about 90% of the time when they say they're 90% sure. Most people are overconfident by default, and calibration is a trainable skill.
Central-Variable Thinking
The seductive error of collapsing a complex, many-part situation into a single master cause or lever. It's cognitively cheap and emotionally reassuring, and usually wrong.
Charm Pricing
Prices ending in 9 (or just below a round number) that feel meaningfully cheaper than they are, partly because we read left-to-right and partly through learned bargain associations. $3.99 reads as closer to $3 than to $4.
Choice Architecture
The design of the environment in which people choose. There is no neutral arrangement. A cafeteria, a form, or a signup flow always orders options somehow, and that order shapes what people pick.
Choice Overload
Too many options can reduce satisfaction and make people less likely to choose at all. More is not always more.
Cognitive Dissonance
The discomfort of holding a behavior and a belief that clash, which we resolve by adjusting one of them, usually the easier one. Having worked hard for something, we talk ourselves into valuing it more.
Cognitive Load
The amount of mental effort a task demands. When a screen, form, or choice carries too much load, people disengage or make errors. Reducing it is often the highest-leverage change available.
Commitment and Consistency
Once we take a stand or make a small commitment, we feel pressure to act consistently with it, and we generate our own reasons to justify it. This is why small, voluntary first steps change later behavior.
Commitment Device
A voluntary arrangement that binds your future self, chosen now because you expect to be tempted later. Locking savings away, or letting an app forfeit your money if you skip the gym, are commitment devices.
Confirmation Bias
We seek, notice, and believe information that fits what we already think, and discount what doesn't. Two fans watching the same game each see more fouls by the other team.
Contrast Effect
We judge things relative to what we just saw, not on absolute terms. A mid-priced item looks like a bargain right after an expensive one; the same water feels hot or cold depending on what your hand touched before.
Core Loop
The fundamental repeated sequence of actions that defines the primary experience of an app or game. Mechanics work best when they reinforce the core loop rather than sitting alongside it. A fitness app's core loop is completing a workout, mechanics should reward and sustain that behaviour, not replace it.
Craving
The anticipation of a reward, triggered by the cue, that actually powers a habit. Once the brain learns to expect the reward on seeing the cue, the behavior pulls itself forward. Pepsodent toothpaste succeeded partly because users came to crave the minty tingle that signaled clean.
Curse of Knowledge
Once you know something, you can't easily imagine not knowing it, so experts routinely overestimate how clear their message is to a newcomer. It's why insiders write instructions outsiders can't follow.
Decision Fatigue
Self-control and judgment degrade as you make more decisions in a row. It's why hard choices are best made early, and why a long sequence of small decisions quietly lowers the quality of the last ones.
Declared vs. Revealed Preference
What people say they want (declared) often differs from what their behavior shows they want (revealed). Design for what people do, not for what they claim in a survey; the two frequently diverge.
Decoy Effect
Adding a third, deliberately inferior option can steer people toward a target choice. Offering a print-only magazine subscription at the same price as print-plus-digital makes the combined option look obviously better and shifts most buyers to it.
Default
The pre-set option that takes effect if a person does nothing. Because people tend to stick with defaults, they are one of the most powerful design choices available; automatic pension enrollment has raised participation dramatically wherever it has been used.
Descriptive vs. Injunctive Norms
Descriptive norms are what people actually do; injunctive norms are what people approve of. When they conflict, the descriptive norm usually wins, which is why 'most people do X' is such a strong message.
Door-in-the-Face
Opening with a large request that gets refused, then retreating to the smaller one you actually wanted, which now feels like a reasonable concession to meet.
Dunbar's Number
A rough cognitive limit, often cited as around 150, on the number of stable social relationships a person can maintain. A useful reminder that 'community' has a natural human scale.
Dunning-Kruger Effect
People with the least skill in an area tend to overestimate their ability the most, because the knowledge needed to do the task well is the same knowledge needed to see you're doing it badly.
Ego Depletion
The idea that willpower behaves like a muscle that tires with use and recovers with rest. Relying on in-the-moment willpower is fragile; designing so it isn't needed is more reliable. The strength of this effect is debated in current research.
Endowment Effect
The tendency to assign more value to things simply because we own them. Accumulated streak counts, XP, and rare items are all subject to the endowment effect, they feel more valuable because the user has them, not because of their intrinsic utility.
Feedback Loop
A cycle in which the output of a behavior feeds back to influence the next round. Good feedback is timely, specific, and actionable; without it, people can't tell whether they're improving.
Flow
Full absorption in a task pitched at just the right difficulty, hard enough not to bore, easy enough not to frustrate. As skill grows, the challenge has to rise to stay in the zone.
FOMO (Fear of Missing Out)
The anxiety that others are having rewarding experiences from which one is absent. Limited-time events exploit FOMO by making participation time-sensitive and the consequence of inaction permanent, missing an event means a permanent gap in collection or standing.
Foot-in-the-Door
Getting agreement to a small request makes a later, larger request more likely to succeed, because the first 'yes' shifts self-image. People who agreed to display a tiny sign later agreed to a large one.
Framing Effect
The same fact influences people differently depending on how it's worded. '90% fat-free' and '10% fat' describe the identical product but land very differently.
Freemium
A business model where the core product is free but enhanced features, content, or capabilities require payment. The design challenge is calibrating what is free, enough to create habit and demonstrate value, and what is paid, enough to justify the price without feeling punitive.
Fresh Start Effect
People are more willing to pursue a change at moments that feel like a new beginning: a Monday, the first of the month, a birthday, New Year. Temporal landmarks reset the sense of a clean slate.
Friction (Hassle Cost)
Small amounts of effort, an extra click, a form field, a short delay, that disproportionately determine whether a behavior happens. Removing friction encourages a behavior; adding it discourages one.
Fundamental Attribution Error
We explain other people's behavior by their character and underexplain the situation they're in. A person who doesn't stop to help may not be unkind. They may just be in a hurry.
Golden Rule of Habit Change
You rarely erase a habit; you replace it. Keep the same cue and the same reward, and insert a new routine in the middle.
Goodhart's Law
When a measure becomes a target, it stops being a good measure, because people optimize the number rather than the thing it stood for. A bounty on rat tails led people to breed rats for their tails.
Growth vs. Fixed Mindset
Believing ability can be developed (growth) versus believing it's fixed. Praising effort and process rather than the person supports the more resilient mindset.
Habit Loop
The structure of a habit: a cue triggers a routine, which delivers a reward. Repeated enough, the loop runs automatically and the brain stops fully attending to it.
Halo Effect
One strong impression colors judgment of unrelated qualities. We assume attractive or likeable people are also more competent or honest, with no evidence for it.
Heuristic
A mental shortcut that gives a fast, usually-good-enough answer without full analysis. Heuristics work well most of the time, which is exactly why the occasional misfire is hard to notice.
Hook Model
A four-step cycle for building product habits: a trigger prompts an action, which delivers a variable reward, followed by an investment that makes the next trigger more likely. The investment also raises the cost of leaving.
IKEA Effect
We value things more when we've put our own effort into them. Partly assembling or customizing something makes people rate it higher and cling to it longer.
Implementation Intention
A specific 'when X happens, I will do Y' plan made in advance, which sharply raises follow-through. 'I'll take my flu shot on Tuesday after lunch' works better than 'I'll get around to it.'
Intrinsic vs. Extrinsic Motivation
Intrinsic motivation comes from within (the activity is satisfying in itself); extrinsic motivation comes from outside rewards or pressure. Intrinsic motivation produces more durable behavior.
Isolation Effect (Von Restorff)
The item that stands out from its surroundings is the one that gets remembered. A brand that breaks its category's conventions is more memorable for it.
Keystone Habit
A single habit that sets off a chain reaction, reshaping other behaviors around it. When Alcoa's CEO made worker safety the one priority, the discipline it required cascaded into communication and quality improvements across the company.
Liking
We say yes more readily to people we like, and liking is easily built through similarity, familiarity, praise, and cooperation toward a shared goal.
Loot Box
A purchasable or earnable container that yields a randomised set of items when opened, with the specific contents unknown until the reveal. The value comes from the uncertainty of the reveal rather than the items themselves, which makes loot boxes a direct application of variable ratio reinforcement. They monetise the anticipation of a rare drop, and because outcomes are probabilistic, spend is decoupled from a guaranteed result. Several jurisdictions now regulate them as a form of gambling and require published drop-rate odds.
Loss Aversion
A cognitive bias where the pain of losing something is felt more strongly than the pleasure of gaining something of equal value. Streaks and energy systems both exploit loss aversion, the user is not motivated to gain a reward so much as to avoid losing what they have already accumulated.
Measurement as Uncertainty Reduction
A measurement is any observation that leaves you less uncertain than before, expressed as a quantity; it doesn't need to be exact to be useful. This reframing makes many supposedly unmeasurable things measurable.
Metagame
Any game layer that exists outside the core loop, collection systems, social standing, progression tracks, that gives players long-term goals beyond the session-by-session experience. Well-designed metagames extend engagement across months rather than days.
Minimum Viable Action
The smallest, simplest version of a behavior you can ask for that still tests whether the design works. Rather than 'replace all your plates with smaller ones,' start with one.
Motivation Crowding Out
Adding an external reward can weaken existing intrinsic motivation, so the behavior collapses once the reward stops. Pay people for something they used to enjoy and they may enjoy it less.
Negative Social Proof
The common mistake of publicizing how widespread an unwanted behavior is, which normalizes it further. Telling people 'many visitors steal wood from this forest' increased theft; it broadcasts that everyone's doing it.
Network Effect
A product or service becomes more valuable as more people use it. A phone, a social network, or a marketplace is worth more when your contacts are already there, which also makes such products hard to leave.
Nudge
A change to how choices are presented that steers behavior without removing options or changing incentives. Putting fruit at eye level is a nudge; banning candy is not.
Overconfidence
Most people rate themselves above average and hold their judgments with more certainty than accuracy warrants. More data often raises confidence without raising accuracy.
Pain of Payment
The more tangible a payment feels, the more it hurts and the more it restrains spending. Cards, contactless, and app payments dull that pain, so people spend more and remember spending less than with cash.
Paywall
A point in the user journey where further progress requires payment. Paywall placement and framing are among the most consequential design decisions in a freemium product. Paywalls that appear before the user understands the product's value proposition consistently underperform those that appear immediately after a moment of genuine frustration or desire.
Peak-End Rule
We judge an experience mostly by its most intense moment and its ending, not by the average or the total. A longer unpleasant procedure can be remembered as better than a shorter one if it ends more gently.
Placebo and Expectancy Effect
What we expect to experience shapes what we actually experience. Told a wine is expensive or a painkiller costly, people report and genuinely feel better results, even when the product is identical.
Planning Fallacy
We consistently underestimate how long our own projects will take, even when we know similar projects ran long. The Sydney Opera House was planned for four years and took fourteen.
Present Bias (Hyperbolic Discounting)
We overweight rewards available now and heavily discount future ones, which reverses our own plans. We choose salad for next week and cake today; the near reward looms larger simply because it's near.
Price Relativity
We can't judge value in the absolute, so we judge it against a comparison set. The same product looks cheaper next to a premium option and pricier next to a budget one; choosing the comparison changes the perception.
Primacy and Recency
We give extra weight to what comes first (primacy) and what comes last (recency) in a sequence. First impressions frame everything after; final moments dominate the memory.
Randomized Controlled Trial (A/B Test)
Splitting people at random into a group that gets the change and a group that doesn't, then comparing outcomes. It's the cleanest way to know whether an intervention actually caused a result rather than something else.
Reactance
The urge to reassert a freedom when you feel it being taken away. Heavy-handed pressure to do something can make people want to do the opposite; reaffirming their choice ('it's up to you') defuses it.
Reciprocity
The social norm of responding to a positive action with another positive action. Gifting mechanics, daily login bonuses framed as gifts from the app, and free trial extensions all use reciprocity to create goodwill and a sense of obligation.
Replicability
The extent to which a research finding holds up when the study is run again. Many well-known effects replicate weakly, so a single striking result is a reason to test, not to believe. Be skeptical, not cynical.
Satisficing
Choosing the first option that clears your bar rather than searching for the best one. Picking a film that looks fine instead of reading every review is satisficing.
Scarcity Bias
The tendency to place higher value on things that are rare or decreasing in availability. Energy systems, limited-time events, and first-purchase bonuses all create artificial scarcity to increase perceived value.
Self-Determination Theory
A well-established account of motivation holding that people thrive when three basic needs are met: autonomy, competence, and relatedness. Support these and motivation becomes self-sustaining; thwart them and it stays fragile.
Self-Efficacy
Your belief that you can actually do a specific thing. It's built by early, achievable wins, and without it people won't attempt a behavior even when they're motivated and able.
Self-Fulfilling Prophecy
A belief that causes its own confirmation through the behavior it prompts. Expecting a bank to fail, depositors withdraw their money, and the withdrawals cause the failure.
Self-Narrative (Self-Perception)
The story people tell about who they are guides their future behavior. Change how someone labels themselves, 'I'm someone who saves,' and the behavior tends to follow.
Session Pacing
The deliberate structuring of how long sessions run and how frequently users return. Energy systems pace sessions by limiting play. Daily quests shape return frequency. The goal is a sustainable engagement rhythm rather than binge-and-churn cycles.
Side Effects
In any connected system, every action ripples outward and produces consequences beyond the one intended. Interventions that solve a problem in isolation routinely create new problems elsewhere.
Social Proof
The tendency to look to others' behaviour as a guide for one's own, particularly in uncertain situations. Insight Timer's '594,622 people here today' and Strava's challenge participant counts both use social proof to validate the decision to engage.
Sunk Cost Fallacy
The tendency to continue investing in something because of prior investment, regardless of future prospects. Long streaks and accumulated XP exploit sunk cost psychology: the longer a user has played, the more costly abandonment feels.
Survivorship Bias
Drawing conclusions only from the cases that made it through, while the failures, which are invisible, quietly distort the picture. In WWII, engineers wanted to armor the bullet holes on returning planes until a statistician pointed out the holes to reinforce were the ones on the planes that never came back.
System 1 and System 2
Two modes of thinking that run in parallel: System 1 is fast, automatic, and intuitive; System 2 is slow, effortful, and deliberate. Most behavior is driven by System 1, and System 2 usually goes along with it.
Temptation Bundling
Pairing something you should do but avoid with something you enjoy, so the enjoyable part pulls the useful part along. Only letting yourself watch a favorite show while exercising is temptation bundling.
Test-Learn-Adapt
Treating each design as a hypothesis: run a small, cheap experiment, learn from the result, and adjust. It replaces 'we're confident this works' with 'let's find out.'
Trigger (Cue)
Whatever starts a behavior. External triggers live in the environment (a notification, an icon); internal triggers are learned associations, often an uncomfortable emotion like boredom or loneliness that the behavior comes to relieve.
Unintended Consequences
Outcomes of an action that weren't planned, common wherever behavior is shaped by many interacting causes. Whether a change helps or backfires often can't be known in advance; only tested.
Unity (Shared Identity)
We favor people we consider 'one of us', same family, team, or group, more strongly than people who are merely similar to us. It's belonging, not resemblance.
Value of Information
The idea that measuring something is only worth doing if reducing that uncertainty would actually change a decision, and by enough to justify the cost. Most variables aren't worth measuring; a few are worth a great deal.
Vanity Metric
A number that looks impressive and rises reliably but doesn't reflect real success or guide a decision. Page views can climb while the outcome that matters stays flat.
Variable Ratio Reinforcement
A schedule of reinforcement where rewards are delivered after an unpredictable number of responses. It produces the highest rate of response and the most resistance to extinction of any reinforcement schedule. The mechanism behind loot boxes, gacha pulls, and slot machines.
Veblen Good
A product for which a higher price can increase demand, because the price itself signals quality or status. Some luxury goods sell better when they cost more.
Zeigarnik Effect
The tendency to remember uncompleted tasks more readily than completed ones. Set collection, quest tracking, and season pass progress bars all exploit the Zeigarnik effect, the visible incomplete progress creates a cognitive pull toward completion.