All 104 principles, in one place.

Narrow the list by the page you’re working on, the kind of psychology involved, or how well the research has held up.

Cognitive load and fluency Aesthetic-usability effect Attractive designs tend to be judged easier to use before people have used them, and that first impression forms in a fraction of a second. Mainly Homepage, also Sitewide, Product Tested in shopping-like settings Moderate Emotion and memory Affect heuristic Feelings about something steer judgements of its risks and benefits. Mainly Homepage, also Product Not tested in shopping Strong Trust and risk reduction Ambiguity aversion People tend to avoid options where the chances of a good or bad outcome are unknown, preferring ones where they know what to expect. Mainly Checkout, also Product, Cart Tested in shopping-like settings Moderate Pricing and value perception Anchoring The first number people see biases their judgement of later numbers. Mainly Product, also Homepage, Category, Cart Tested on real shoppers Strong Loss and scarcity Anticipated regret People choose in ways that avoid regret they expect to feel later. Mainly Product, also Cart, Checkout, Post-purchase Tested on real shoppers Moderate Emotion and memory Anticipation and savouring People get pleasure from looking forward to something good, so a wait can add value if it is framed as anticipation. Mainly Post-purchase, also Product, Checkout Tested in shopping-like settings Moderate Cognitive load and fluency Attribute alignability Differences on a shared dimension (4kW vs 6kW) are easy to compare and drive choice; unique features on each product are hard to trade off and get ignored. Mainly Search, also Category, Product Tested in shopping-like settings Moderate Social influence Authority People defer to credible experts and institutions. Mainly Product, also Homepage, Sitewide, Checkout Tested on real shoppers Moderate Attention and perception Autocomplete position bias Searchers overwhelmingly look at and pick the top few query suggestions, so their order shapes what gets searched. Mainly Search, also Sitewide Tested in shopping-like settings Moderate Social influence Bandwagon effect Demand for something rises when people see others choosing it. Mainly Category, also Homepage, Product, Search Tested on real shoppers Strong Attention and perception Banner blindness People tend to overlook content that looks like an advert or sits where adverts usually appear. Mainly Homepage, also Sitewide, Category Tested in shopping-like settings Moderate Cognitive load and fluency Berrypicking Real searches evolve: people gather useful bits across a series of reworded queries, not one query and one answer. Mainly Search, also Sitewide, Category Tested on real shoppers Framework Social influence Blemishing effect A small negative alongside strong positives can make the overall message more persuasive. Mainly Product, also Category Tested on real shoppers Moderate Choice architecture Centre-stage effect In a small row of similar items shown side by side, the one in the centre gets more attention and is chosen more often. Mainly Category, also Product, Homepage Tested on real shoppers Moderate Choice architecture Choice overload Too many options can reduce the likelihood of buying and satisfaction afterwards, but only under certain conditions. Mainly Category, also Homepage, Product, Sitewide, Search Tested on real shoppers Contested Cognitive load and fluency Chunking (Miller's law) People hold only a few chunks of information in mind at once. Mainly Checkout, also Sitewide, Product Tested in shopping-like settings Strong Cognitive load and fluency Cognitive load Working memory is limited, so unnecessary demands on it make tasks slower and more error-prone. Mainly Checkout, also Sitewide, Category, Product Tested in shopping-like settings Moderate Motivation and commitment Commitment and consistency After agreeing to a small request, people are somewhat more likely to agree to a larger related one. Mainly Checkout, also Homepage, Product Not tested in shopping Contested Motivation and commitment Communication frequency ideal point Post-purchase contact helps up to an ideal volume, then triggers pushback and lower repurchase. Mainly Post-purchase, also Sitewide Tested on real shoppers Moderate Choice architecture Compromise effect Given a range, people tend to avoid the extremes and pick a middle option. Mainly Product, also Category Tested on real shoppers Strong Choice architecture Consideration set formation Shoppers first narrow to a small set they will seriously evaluate, adding an option only if it seems worth the effort of evaluating. Mainly Search, also Homepage, Category, Product Tested in shopping-like settings Framework Emotion and memory Consumption rituals (unboxing) Performing a small structured ritual before consuming something increases involvement and enjoyment. Mainly Post-purchase, also Product Tested in shopping-like settings Moderate Motivation and commitment Curiosity gap A gap between what people know and want to know motivates them to close it. Mainly Homepage, also Product, Post-purchase Tested in shopping-like settings Moderate Emotion and memory Customer delight (surprise) An unexpectedly positive experience produces delight, which is distinct from satisfaction. Mainly Post-purchase, also Sitewide, Checkout Tested in shopping-like settings Moderate Emotion and memory Customer gratitude When customers see a seller investing in them out of goodwill, the gratitude drives reciprocal buying beyond what trust alone explains. Mainly Post-purchase, also Sitewide Tested on real shoppers Moderate Choice architecture Decoy effect Adding a clearly worse option makes a similar, better option more attractive. Mainly Product, also Category Tested on real shoppers Contested Choice architecture Default effect People tend to stick with whatever is pre-selected. Mainly Checkout, also Product, Cart Tested on real shoppers Strong Pricing and value perception Discount framing (% vs £) The same discount feels bigger as a percentage on cheap items and as a cash amount on expensive ones. Mainly Product, also Homepage, Category, Cart Tested on real shoppers Moderate Attention and perception Display orientation (list vs grid) Vertical lists are read top-down with strong position effects; grids and horizontal rows are scanned more broadly. Mainly Search, also Homepage, Category Tested in shopping-like settings Moderate Choice architecture Elimination by aspects Faced with many options, people take one attribute at a time and discard everything that fails it. Mainly Search, also Category Tested on real shoppers Framework Motivation and commitment Endowed progress effect A head start towards a goal makes people more likely to finish. Mainly Checkout, also Cart, Post-purchase Tested on real shoppers Moderate Loss and scarcity Endowment effect People value things more once they feel they own them. Mainly Product, also Cart, Post-purchase Tested in shopping-like settings Moderate Pricing and value perception Evaluability (joint vs separate evaluation) Attributes that are hard to judge in isolation only influence choice when options are seen side by side. Mainly Search, also Category, Product Tested in shopping-like settings Moderate Emotion and memory Expectation-disconfirmation Satisfaction depends on performance relative to what was promised, so beating a modest promise satisfies more than missing an ambitious one. Mainly Post-purchase, also Sitewide, Product, Cart, Checkout Tested on real shoppers Strong Cognitive load and fluency Fitts's law The time to hit a target depends on its size and distance. Mainly Sitewide, also Product, Cart, Checkout Not tested in shopping Strong Motivation and commitment Fogg behaviour model A behaviour happens when motivation, ability and a prompt come together at the same moment. Mainly Sitewide, also Homepage, Category, Product, Cart, Checkout Not tested in shopping Framework Choice architecture Framing effect The same information leads to different choices depending on how it is presented. Mainly Product, also Homepage, Category, Cart Tested in shopping-like settings Strong Attention and perception Gaze cueing People automatically look where others are looking. Mainly Homepage, also Product Tested in shopping-like settings Moderate Attention and perception Gestalt grouping Elements that are close, similar or enclosed together are seen as belonging together. Mainly Sitewide, also Category, Product, Checkout Not tested in shopping Strong Motivation and commitment Goal-gradient effect Effort increases as people get closer to a goal. Mainly Cart, also Checkout, Post-purchase Tested on real shoppers Moderate Motivation and commitment Habit formation Behaviour repeated in a stable context becomes cued by that context and continues with little deliberation. Mainly Post-purchase, also Sitewide, Homepage, Cart Tested on real shoppers Framework Emotion and memory Halo effect One strong positive impression colours judgement of other, unrelated qualities. Mainly Homepage, also Sitewide, Product Not tested in shopping Strong Choice architecture Hick's law The time taken to choose rises with the number of options, but only with its logarithm, so each extra option adds less time than the last. Mainly Sitewide, also Homepage, Category, Checkout Not tested in shopping Strong Motivation and commitment IKEA effect People value things more when they helped create them. Mainly Product, also Cart Tested in shopping-like settings Strong Attention and perception Information scent (information foraging) People follow cues that predict whether a path is worth the effort, and abandon it when the 'scent' weakens. Mainly Search, also Sitewide, Homepage, Category, Product Not tested in shopping Framework Choice architecture Interactive decision aids Tools that screen options and lay finalists side by side cut effort and improve decisions, and also steer which attributes matter. Mainly Search, also Homepage, Category, Product Tested on real shoppers Moderate Motivation and commitment Labour illusion Showing the work being done on someone's behalf raises perceived value, even when it means waiting. Mainly Search, also Product Tested in shopping-like settings Moderate Pricing and value perception Left-digit effect £19.99 feels meaningfully cheaper than £20 because the left-most digit dominates. Mainly Product, also Category Tested on real shoppers Moderate Social influence Liking and similarity We are more persuaded by people who are like us. Mainly Product, also Homepage Tested on real shoppers Moderate Loss and scarcity Loss aversion Losing something hurts more than gaining the same thing pleases. Mainly Cart, also Product, Checkout Tested on real shoppers Strong Pricing and value perception Mental accounting People sort money into mental budgets and judge spending within them. Mainly Cart, also Product, Checkout Tested on real shoppers Strong Attention and perception Mental simulation Content that helps people imagine using a product can raise purchase intent. Mainly Product, also Homepage Tested on real shoppers Moderate Choice architecture Mere categorisation effect Splitting a range into more categories makes it feel more varied and leaves shoppers more satisfied, especially unfamiliar ones. Mainly Category, also Sitewide, Search, Homepage Tested in shopping-like settings Moderate Emotion and memory Mere exposure effect Repeated exposure to something tends to increase liking for it, up to a point. Mainly Sitewide, also Homepage, Post-purchase Tested in shopping-like settings Strong Emotion and memory Narrative transportation Being absorbed in a story reduces counter-arguing and shifts attitudes. Mainly Product, also Homepage Tested in shopping-like settings Strong Trust and risk reduction Online trust formation Willingness to buy online depends on beliefs about the seller's competence and integrity, plus structural assurances. Mainly Checkout, also Homepage, Sitewide, Product Tested on real shoppers Strong Choice architecture Over-search in ranked lists When results are already sorted best-first, looking further down exposes people to worse options and can lower the quality of what they choose. Mainly Search, also Category Tested in shopping-like settings Moderate Pricing and value perception Pain of paying Paying registers as a loss, and the more visible the payment, the more it hurts. Mainly Checkout, also Cart Tested on real shoppers Moderate Pricing and value perception Partitioned pricing Splitting a price into a base price plus surcharges changes how the total is perceived, usually making it seem lower because the surcharges get less attention. Mainly Cart, also Category, Product, Checkout Tested on real shoppers Strong Emotion and memory Peak-end rule Experiences are judged by their most intense moment and their ending. Mainly Post-purchase, also Checkout Tested in shopping-like settings Strong Trust and risk reduction Perceived justice in complaint handling Complainants judge a resolution on three things: the outcome, the process, and how they were treated. Mainly Post-purchase, also Sitewide Tested in shopping-like settings Strong Trust and risk reduction Perceived risk Buying is held back by perceived financial, performance and social risk. Mainly Product, also Cart, Checkout Tested on real shoppers Strong Attention and perception Picture superiority effect Images are remembered better than words. Mainly Product, also Homepage, Category, Search Tested on real shoppers Strong Motivation and commitment Points pressure and medium maximisation People increase effort to reach a reward threshold and chase the points themselves, even when the points do not improve the outcome. Mainly Post-purchase, also Sitewide, Cart, Checkout Tested on real shoppers Moderate Choice architecture Position (rank) bias Items higher in a ranked list get disproportionately more attention and clicks, whatever their true relevance. Mainly Search, also Category, Homepage Tested on real shoppers Strong Trust and risk reduction Post-purchase dissonance After a hard-to-reverse choice people feel doubt, and reduce it by seeking reassurance and rating their choice more highly. Mainly Post-purchase, also Product, Cart, Checkout Not tested in shopping Moderate Trust and risk reduction Precise-number credibility Specific numbers read as more factual than round ones. Mainly Product, also Homepage Tested in shopping-like settings Moderate Motivation and commitment Present bias People overvalue immediate rewards relative to later ones. Mainly Product, also Cart, Checkout Tested on real shoppers Moderate Pricing and value perception Price-quality heuristic When quality is hard to judge, people read a higher price as higher quality. Mainly Product, also Category Tested on real shoppers Moderate Cognitive load and fluency Processing fluency Things that are easy to read and take in tend to be liked more, and choices between them feel easier to make. Mainly Sitewide, also Homepage, Category, Product Tested in shopping-like settings Moderate Social influence Prosocial referral rewards People refer more effectively when the reward goes to their friend, because referring then looks generous and the friend has a reason to act. Mainly Post-purchase, also Sitewide, Homepage Tested on real shoppers Moderate Cognitive load and fluency Prototypicality (Jakob's law) People prefer sites that look and work like the ones they already know. Mainly Sitewide, also Homepage, Category, Checkout Tested in shopping-like settings Moderate Motivation and commitment Psychological reactance People push back when they feel their freedom to choose is being taken away. Mainly Sitewide, also Homepage, Checkout Tested in shopping-like settings Moderate Attention and perception Psychology of waiting Waits that are uncertain, unexplained or empty feel longer and anger people more than known, explained or filled waits. Mainly Post-purchase, also Sitewide, Checkout Tested in shopping-like settings Moderate Motivation and commitment Question-behaviour effect Asking people about their intentions or satisfaction can change what they later do. Mainly Post-purchase, also Sitewide Tested on real shoppers Contested Social influence Reciprocity People feel obliged to return a favour. Mainly Cart, also Homepage, Product, Post-purchase Tested on real shoppers Moderate Cognitive load and fluency Recognition over recall Recognising something is easier than recalling it from memory. Mainly Search, also Sitewide, Category, Cart, Checkout Tested in shopping-like settings Strong Cognitive load and fluency Response-time thresholds (Doherty) Delays beyond roughly 0.1s, 1s and 10s break the feeling of directness, flow and attention in turn. Mainly Sitewide, also Homepage, Category, Search, Product, Cart, Checkout Tested on real shoppers Moderate Trust and risk reduction Return-fee fairness Who pays for a return strongly shapes later spending, because customers treat a return fee as unfair. Mainly Post-purchase, also Product, Cart, Checkout Tested on real shoppers Moderate Motivation and commitment Review-request timing and incentives A review request works as a memory cue but can feel like pressure, so asking too soon backfires. Money buys more reviews; social norms buy longer ones. Mainly Post-purchase, also Product Tested on real shoppers Moderate Choice architecture Satisficing Most people stop at the first option that is good enough rather than hunting for the best. Mainly Search, also Homepage, Category, Product Not tested in shopping Framework Attention and perception Scanning patterns (F-pattern) People scan web pages rather than read them, and on text with little formatting their attention concentrates at the top and left. Mainly Category, also Homepage, Product, Search Tested in shopping-like settings Moderate Loss and scarcity Scarcity Limited availability raises perceived value and desire. Mainly Product, also Category, Cart Tested on real shoppers Strong Pricing and value perception Search costs and price sensitivity What is easy to search on becomes what people choose on: easy quality information lowers price sensitivity, easy price comparison raises it. Mainly Search, also Category, Product Tested on real shoppers Moderate Emotion and memory Self-reference effect Information that relates to oneself is processed more deeply and remembered better. Mainly Product, also Homepage Tested in shopping-like settings Strong Emotion and memory Serial position effect The first and last items in a sequence are remembered best. Mainly Sitewide, also Category, Product Tested in shopping-like settings Strong Emotion and memory Service recovery paradox A customer whose problem is fixed very well can end up more satisfied than one who never had a problem. Mainly Post-purchase, also Sitewide Tested in shopping-like settings Contested Choice architecture Shopping momentum A first purchase shifts people into a buying mindset that makes further, even unrelated, purchases more likely. Mainly Cart, also Product Tested on real shoppers Moderate Trust and risk reduction Signalling theory Costly commitments signal quality that shoppers cannot observe directly. Mainly Product, also Homepage, Sitewide Tested on real shoppers Framework Choice architecture Single-option aversion People hesitate to buy when shown only one option, even one they like, and go off to search. Mainly Product, also Cart Tested in shopping-like settings Moderate Social influence Social identity People favour brands and groups that reflect who they think they are. Mainly Homepage, also Product, Post-purchase Tested on real shoppers Moderate Social influence Social proof When unsure, people look to what others have done and follow it. Mainly Product, also Homepage, Category, Cart, Checkout Tested on real shoppers Strong Pricing and value perception Sort-order effect The direction a list is sorted in changes what people choose: descending price or quality order pushes choice towards higher-priced options. Mainly Search, also Category Tested in shopping-like settings Moderate Choice architecture Status quo bias People prefer things to stay as they are. Mainly Cart, also Product, Checkout, Post-purchase Tested on real shoppers Strong Loss and scarcity Sunk cost effect Having already put money, and less reliably time or effort, into something, people are more likely to carry on with it. Mainly Checkout, also Product, Cart, Post-purchase Tested in shopping-like settings Moderate Pricing and value perception Temporal reframing (pennies-a-day) A cost feels smaller when broken into a small daily or per-use amount. Mainly Product, also Cart Tested on real shoppers Moderate Trust and risk reduction Tracking transparency Customers judge fulfilment by the activity they can see, and visible progress late in the wait counts for more than early progress. Mainly Post-purchase, also Product, Checkout Tested on real shoppers Moderate Loss and scarcity Urgency (time limits) Deadlines can speed up decisions, and restrictions can signal that a deal is good. Mainly Product, also Homepage, Cart, Checkout Tested on real shoppers Contested Attention and perception Visual preference heuristic Shoppers prefer image-led assortments and see more variety in them, but large image-led sets are scanned less carefully and feel more overwhelming than text. Mainly Search, also Homepage, Category Tested in shopping-like settings Moderate Cognitive load and fluency Vocabulary problem People use many different words for the same thing, so any single label will miss most people's wording. Mainly Search, also Sitewide, Category Tested in shopping-like settings Moderate Attention and perception Von Restorff (isolation) effect The item that differs from its surroundings is the one noticed and remembered. Mainly Product, also Sitewide, Category, Cart Tested in shopping-like settings Strong Social influence Word-of-mouth triggers Interesting products get talked about straight away; products that are frequently cued and publicly visible keep getting talked about. Mainly Post-purchase, also Sitewide, Product Tested in shopping-like settings Moderate Motivation and commitment Zeigarnik / Ovsiankina effect Unfinished tasks create a pull to return and complete them. Mainly Cart, also Homepage, Checkout Not tested in shopping Contested Pricing and value perception Zero-price effect 'Free' is disproportionately more attractive than a very low price. Mainly Cart, also Homepage, Product, Checkout Tested on real shoppers Moderate