Behavioral economics & viral marketing case studies
Contrast Principle 
Details
Contrast Principle means we judge things by comparing them to what came just before. The first item sets the frame, and everything after feels bigger, smaller, cheaper, or better based on that anchor.
Think of trying on a jacket after checking a much more expensive one. The new jacket suddenly feels like a bargain, even if the price hasn’t changed. The contrast makes it look better.
In marketing this principle shapes perception fast. Showing a premium option first makes standard options feel more affordable. Placing products side by side changes how each one is valued.
Contrast Principle 
Guide
Contrast Principle 
Research
Bone (1990) showed that contrast happens only when two options are very different. If the products are similar, people don’t separate them. They “pull” the new product toward the reference and rate it more similarly (assimilation).
The study found the effect depends on how alike the items are, how well people remember the reference, and how many related features come to mind. For marketing: contrast works only when the difference is obvious and big; if things look too similar, the contrast effect disappears.
A study showed that when people judge a product right after seeing something very good or very bad, their opinion shifts in the opposite direction. The extreme example changes how they imagine the product or how they read the rating scale. In short, a product can feel better or worse just because of what came before it.
Contrast Principle 
Examples
1. HelloFresh
HelloFresh ads always show the contrast between messy grocery lists, crowded stores, random ingredients vs their neat box with pre-cut portions and a simple step-by-step recipe card.
Dollar Shave Club’s iconic ad hits you with a direct contrast of traditional razor brands that charge bloated prices for unnecessary bells and whistles, while DSC costs only a few bucks a month.
By placing the expensive, over-engineered razors next to their simple cheap ones
Decoy Effect Details
Decoy Effect means adding a third, less appealing option can push people toward the choice you want. The weaker option makes the target option look like the smartest deal.
Think of choosing between two subscription plans. When a third plan appears that’s overpriced and poorly balanced, it suddenly makes the mid-tier plan look perfect. The decoy steers the decision without saying a word.
In marketing this effect shapes pricing tables, bundles, and memberships. A well-placed decoy guides customers toward the option with the best margin or value.
Decoy Effect Guide
Decoy Effect Research
A study of an online diamond retailer found that after introducing a “decoy” option, the retailer’s gross profit rose by 21.4 % across different diamond price segments.
The study tested whether adding a decoy option makes people pick more expensive hotel and restaurant offers. Researchers ran a pilot test and a few studies with 463 adults in the USA.
1. study (fast-food menu)Options:
Results:
2. Study Hotels (cancellation policies)Options:
Results:
The famous decoy-pricing example used 3 real subscription prices from The Economist:
In Ariely’s experiment, when all 3 options were shown, 84% of people chose the combo and 16% chose web-only, while 0% picked the pointless print-only option, making it a perfect decoy.
But when the decoy (print-only) was removed, 68% picked the cheaper web-only and only 32% chose the combo.
This shows how one “useless” option can massively shift preference toward a higher-priced product, simply by changing the comparison frame.
Decoy Effect Examples
1. Jamba Juice
Jamba introduced a slightly smaller-than-large cup priced almost the same as the big one. The medium suddenly looked bad, so customers jumped to the large. The decoy wasn’t meant to sell, its job was to push people up a size.
Panera tested 3 sizes: small, medium, and a barely-bigger-than-medium large. The medium served as the decoy. It made the largest soup look like way better value for only a tiny price jump.
Pseudo-Set Framing Details
Pseudo-Set Framing means we get motivated when tasks feel like part of an incomplete set, even if the set is totally made up. Our brain hates leaving things unfinished.
Think of loyalty cards that start you off with a few stamps already filled. You suddenly feel closer to completing the set, so you push harder to finish it, even though the extra stamps were artificial.
In marketing this bias drives progress bars, starter points, checklists, and reward systems. When people see themselves as partway through a set, they’re more likely to keep going.
Pseudo-Set Framing Guide
Pseudo-Set Framing Research
Website 1
Website 2
Website 3
The HBS pseudo-set framing research was tested in the real world with the Canadian Red Cross during its 2016 holiday fundraising campaign.
Over 7,000 donors were randomly sent to one of three pages:
The effect was huge: 21% of people in the pseudo-set condition donated the entire six-item kit, compared to only 5% in the gift condition and 3% in the cash condition. That's roughly a 320% increase in full-completion behavior. Simply showing the items as a “set” made people far more likely to finish it.
Pseudo-Set Framing Examples
1. LinkedIn - Profile Strength Meter
LinkedIn breaks your profile into a set of 5-7 pieces (photo, headline, experience, skills, summary, connections, etc). These elements don’t actually need to be treated as a set, but framing them together with a progress bar creates urgency to complete the full set. This pseudo-set framing makes users finish their profiles far more often.
H&M dresses mannequins in a full outfit. Usually 5-7 items like a jacket, shirt, pants, shoes, and accessories. Even though each item is sold separately, the outfit looks like one complete set in your mind. Because of this, many shoppers try to buy the whole outfit, not just one piece.
Sunk Cost Effect Details
Sunk Cost Effect means we keep investing time, money, or energy into something just because we’ve already put a lot into it, even when quitting would be smarter. The past effort traps us.
Think of staying in a bad project just because you spent months on it, even though it doesn't earn any money. Or you're stuck in a bad relationship even though you don't love the other person anymore. The time you already invested pulls you in, not the actual value.
In marketing this effect keeps customers subscribed, committed, or loyal to things they’ve already paid for or spent effort on. The more they’ve put in, the harder it feels to walk away.
Sunk Cost Effect Guide
Sunk Cost Effect Research
In a 1985 study by Arkes and Blumer, people got theatre tickets at different prices:
54 people who paid more for a theater season pass ended up going to more shows over 6 months, just to use their pricey ticket. The cheaper the ticket, the less likely people were to use it.
Sunk Cost Effect Examples
1. Starbucks rewards
Starbucks gives you points that expire. When you have something like 70/100, you don’t want that effort to go to waste. So you buy another coffee or two just to “finish the set.” The more points you’ve collected, the stronger the pull.
People spend thousands on the Peloton bike. After that, they feel they must keep the monthly subscription active, or else that big investment feels wasted. The high upfront cost keeps them inside the system much longer.
Unit Biasy Details
Unit Bias means we feel the urge to finish a full unit of something, no matter its size. If it’s presented as one piece, one portion, or one task, our brain treats it as the natural amount to complete.
Think of eating a whole chocolate bar even when you’re not that hungry, just because it comes as one bar. If it were split into tiny pieces, you’d likely stop earlier.
In marketing this bias shapes how people consume and buy. Portion sizes, product bundles, and task steps guide behavior simply by defining what “one” looks like.
Unit Bias Guide
Unit BiasResearch
A key study by Geier, Rozin, and Doros (2006) introduced the term “unit bias.”. In the pretzel test, one group got a whole pretzel and another group got half. Those who got a whole pretzel ate the whole thing, even though it was twice as much food.
Those who got only half felt it was also enough size, because their brain treated that half as a full unit.
In a study, when people got 4 small 100-calorie snack packs, they ate about 25% fewer calories than those who got the same food in one big 400-calorie bag. They could open all four packs, but most stopped after one, because one pack felt like “one serving.”
Unit Bias Examples
1. Coca-Cola’s mini cans
By selling soda in smaller cans (90 calories vs 140+ in a standard can), Coke charges a premium per ounce, but consumers don’t mind because one can feels like a full serving.
TikTok was the first big app to use unit bias with infinite scroll for short videos. With normal pagination, one page feels like one “unit,” so people naturally stop after finishing it. TikTok removed that stopping point. Each video feels like one tiny unit, and the app instantly gives you the next one with no break. Your brain never gets a “time to stop” signal, so you keep watching longer than you planned.
Cheerleader Effect Details
Cheerleader Effect means people look more attractive when they’re seen in a group than when they’re seen alone. The brain blends faces together, smoothing out flaws and boosting the overall impression.
Think of looking at a group photo and noticing everyone seems better-looking together, but when you see each person separately, the effect fades. The group creates a stronger visual average.
Showing products, customers, or people together creates a more appealing image than showing them one by one.
Cheerleader Effect Guide
Cheerleader EffectResearch
A 2013 study by Walker and Vul tested this effect in five experiments with over 130 people. Participants rated faces shown alone and the same faces shown in a group.
The results: faces looked more attractive in group photos than alone. This was true for both men and women. The effect worked even when the “group” was made of separate faces combined together, which shows it’s all about how our brain averages faces. When we see a group, our brain blends the features, and the average looks more “normal” and symmetric, so each person gets a small beauty boost.
They also found that groups bigger than 4 people didn’t add extra effect. A small group was enough to do the trick.
The effect disappears if the group is just the same face copied, because there’s nothing to average.
Cheerleader Effect Examples
1. Zara & H&M - mannequin groups
Stores like Zara & H&M often display outfits on clusters of mannequins, not single ones.
One isolated mannequin looks fine, but a styled group creates a collective vibe trendy, cohesive, and aspirational.
Platforms like Netflix, HBO Max, and Disney+ often show ensemble cast thumbnails instead of single-character shots. A show looks more appealing when you see a group of faces.
Temptation Bundling Details
Temptation Bundling means you pair something you should do with something you want to do, so the fun part pulls you through the boring part. The reward makes the task easier to stick with.
Think of only letting yourself watch your favorite show while exercising. The workout becomes less painful because the entertainment carries you through it.
In marketing this strategy makes habits stronger. Pairing valuable tasks with enjoyable perks, like learning with rewards or chores with small treats, keeps people engaged longer.
Temptation Bundling Guide
Temptation Bundling Research
Researchers led by Katherine Milkman tested if they can nudge people to go to the gym. They gave people fun audiobooks but locked them so they could only listen at the gym.
Afterward, 61% of participants said they’d pay to keep gym-only audiobook access, showing people actually want this kind of commitment device.
Temptation Bundling Examples
1. Amazon Kindle + audiobook “Whispersync”
Amazon links the “should” (reading) with the “fun thing” (audiobook). You only get the nice audiobook switch feature after you buy the ebook. People read more because the reward is tied to the action.
Play gives you reward points every time you top up or pay your phone bill. And those points can be swapped for Netflix, HBO, Disney+ and other streaming perks. So you do the “boring” task first (paying your bill), and you unlock the fun stuff after (free shows and movies). Classic temptation bundling.
Endowment Effect Details
Endowment Effect means we value things more simply because we own them. Once something feels like ours, its worth rises in our mind.
Think of trying to sell an old item you barely use and being shocked that others won’t pay the price you think it deserves. You see more value in it because it’s yours.
In marketing this effect makes trials, personalization, and early ownership work.
Endowment Effect Guide
Endowment Effect Research
Researchers gave one group of people a coffee mug and asked how much they’d sell it for, and asked another group how much they’d pay for the same mug.
Sellers asked for $7.12, while buyers were only willing to pay $2.87. Sellers were willing to pay almost 2.5X more for the mug! Same mug, same quality, but owning it made people value it more.
Endowment Effect Examples
1. Amazon Prime
That's how Amazon’s 30-day free Prime trial works. Once you’ve experienced free shipping on every order and started relying on it, you feel like Prime membership is something you have, and letting it lapse feels like losing a benefit you own
IQOS used the endowment effect with free 30-day trials.
You don’t just hear about the product – you own it for a month. You get used to less smell, the “healthier” feeling, the new routine. After 30 days, giving it back feels like a loss, so you’re more likely to keep (and buy) it.
They even swapped tests for your pack of cigarettes or lighter, nudging you to “trade in” the old habit for the new one.
Anchoring Bias Details
Anchoring Bias means our brains grab the first piece of information they see and use it as a reference for everything that comes next. It shapes what we think is fair or normal.
Think of seeing a jacket priced at $800 before another one for $300. The second one suddenly feels like a great deal, even if it’s still overpriced. The first number set the anchor.
The first price, comparison, or offer you show becomes the baseline in your customer’s head. A higher original price makes discounts feel bigger. Premium options make mid-tier ones look reasonable.
Anchoring Bias Guide
Anchoring Bias Research
In a PeopleScience experiment, participants wrote down their last two phone digits, tasted a red wine, and wrote how much they'd pay. Those with numbers ending in 76–100 were willing to pay 3x more than those with 0–25 ($36 vs. $12).
Even after revealing the trick, they tried again with white wine and the last three digits. The results were pretty much the same: the high-digit group offered $38, the low-digit group $16. The random phone numbers acted as anchors, dragging their willingness to pay up or down without them realizing it.
Tversky and Kahneman ran a classic 1974 experiment to show anchoring in action. Participants spun a wheel that was secretly rigged to land on either 10 or 65. Then they were asked two questions:
People who saw “10” guessed about 25%. People who saw “65” guessed about 45%. The random number pulled their final estimate in its direction, even though it had nothing to do with the question
Anchoring Bias Examples
1. Rolls-Royce at Aircraft Shows
Rolls-Royce quit regular car shows and started showing their cars at aircraft shows instead. Next to a $10M private jet, a $400k car suddenly feels like a bargain.
Williams-Sonoma couldn't sell their $279 breadmaker. So they added a fancier $429 model. They barely sold any of the expensive ones, but sales of the $279 model doubled. People suddenly had something to compare it to.
KFC Australia ran a promo saying “Just 4 packs per customer!” for their fries. That limit acted as an anchor, pushing customers to buy more than they planned.
Campbell’s added a sign saying “Limit 12 per person.” Without the limit, people bought about 3 cans. With the limit, people bought about 7.
When Snickers changed their sign from "Buy them for your freezer" to "Buy 18 for your freezer," customers bought 38% more, jumping from around 3 bars to 6 bars on average.