Behavioral economics & viral marketing case studies
Charm Pricing 
Details
Charm Pricing means prices ending in 9 or 99 feel cheaper than they really are. Our brain focuses on the first digit and ignores the rest.
Most pricing decisions are intuitive, not analytical. People feel the price before they calculate it. Charm pricing exploits that quick, emotional judgment.
Think of seeing a product priced at 9.99 instead of 10.00. Even though the difference is tiny, 9.99 feels noticeably cheaper because the leading number is lower.
In marketing this bias shapes how prices are set across products, sales, and promotions. Small changes at the end of a price can create a big shift in perception.
Charm Pricing 
Guide
Charm Pricing 
Research
In 2013, Gumroad analyzed all products on its platform priced under $6. They compared items with charm prices (ending in .99) to items with round prices (like $2 or $5).
Products with charm pricing converted at 3.5%, while round-priced products converted at 2.3%. This is a ~52% higher conversion rate for prices ending in .99!
The products were otherwise similar, so the main difference was the price ending. The result shows that even a small price change that does not affect real value can strongly change how people decide to buy.
In a real retail catalog experiment, the exact same dress was sold at three prices:
Even though $34 was cheaper, the dress priced at $39 sold the most. The $44 version sold less, as expected, but the key result is that $39 outsold the cheaper $34 option.
This shows that people do not always choose the lowest price. Prices ending in 9 can increase perceived value and attractiveness, likely because buyers focus on the left digit and interpret $39 as a better deal or higher-quality option than $34.
Charm Pricing 
Examples
1. Amazon
Amazon uses .99 pricing on millions of products. Shoppers scan fast and anchor on the first digit (19, not 20). The product feels meaningfully cheaper, even though the difference is 1 cent.
Starbucks often uses prices like $4.95 instead of $5.00. The left digit (4 instead of 5) triggers the left-digit effect, so the drink feels meaningfully cheaper. At the same time, .95 feels more premium than .99, which fits Starbucks’ “affordable luxury” positioning.
Life Event Effect Details
Life Event Effect means that people are more likely to switch habits and brands when they have undergone a life event. Changes like moving, having a child, starting a new job, or a breakup make old habits break.
Think of someone who just moved to a new city. They suddenly choose new brands, new routines, and new services , not because the products changed, but because their life context did.
In marketing this bias explains why timing matters so much. Brands that show up during life changes get a rare chance to form new habits.
Life Event Effect Guide
Life Event Effect Research
About 34% of US soldiers used heroin while fighting in Vietnam, and around 20% showed signs of addiction. After a major life change (coming back home) this behavior dropped fast.
In the first year after returning to the US, only about 1% became addicted again, even though 10% tried the drug again after returning.
This shows that when life context changes, behavior can change suddenly, even without treatment.
During research, researchers ran a survey among 2,370 people. They asked two things:
Overall, 21% of people who had a recent life event had switched brands, vs 8% of regular consumers (≈2.6X higher). And in 3 categories, life-event consumers were more than 3X more likely to have switched brands.
People whose age ends in “9” (eg, 29, 39, 49) are more likely to question the meaningfulness of their lives than people at other ages.
In their study, researchers examined the ages of first-time marathon runners and found that 9-enders were overrepresented by ~48% among participants aged 25-64.
Nine-enders were also more represented on an extramarital affairs site (men with ages ending in 9 were ~18% overrepresented).
This explains the whole idea of a midlife crisis.
Life Event Effect Examples
1. Starting new job - LinkedIn Premium, Notion, Slack
A new job resets tools, routines, and identity. LinkedIn usage spikes when people start searching for a new job. LinkedIn Premium converts best when users change job titles. Notion and Slack get adopted because teams rebuild workflows from zero.
When people move, they switch internet, furniture, and home services. Comcast and AT&T aggressively target people right after an address change. IKEA wins because moving breaks old habits, and people are open to new brands.
Framing Effect Details
Framing Effect means the way information is presented changes how we feel about it, even when the facts stay the same. The frame shapes the reaction.
Think of hearing that a product is 90% effective versus hearing it has a 10% failure rate. Same numbers, but one feels safe while the other feels risky. The wording sets the mood.
In marketing, highlighting gains feels motivating, highlighting losses feels urgent, and shifting perspective can make the same offer look far more attractive.
Framing Effect Guide
Framing Effect Research
People were told a disease would kill 600 people. They had to choose between two programs.
The options were identical, only the wording changed.
1. Gain frame (positive)
Results:
2. Loss Frame (negative)
Results:
Same math. Different frame. Completely different behavior.
Framing Effect Examples
1. BetterHelp
Betterhelp therapy reframes therapy as normal self-care. The homepage frames depression as something normal. This reduces stigma and makes the purchase feel proactive, not reactive.
Liquid Death framed water as rebellious “Murder Your Thirst” beverage. They took a boring product (water) and reframed it as a punk, metal, anti-plastic energy-drink vibe.
Scarcity Details
Scarcity means we value things more when they feel limited. When supply drops, desire rises, even if nothing else changes.
Think of seeing only a few items left on a shelf. Suddenly the product feels more important, even if you didn’t want it a minute ago. The fear of losing it boosts the urge to act.
In marketing scarcity turns hesitation into action. Limited spots, low stock, and short windows make people move faster because waiting feels risky.
Scarcity Guide
ScarcityResearch
A large meta-analysis of 131 studies and 416 effects found that not all scarcity works the same.
A group of 200 female students rated how attractive cookies were when there were many of them (abundant), when there were few (scarce), and when the amount changed. When cookies became scarce, the students were told it happened either because many people wanted them or because of an accident.
As a result, the cookies were rated more desirable when they were scarce than when they were abundant.
They were also rated more valuable when they changed from abundant to scarce compared to being scarce the whole time.
Scarcity caused by high demand got the highest ratings, while “accidental scarcity” scored lower. And cookies that stayed abundant the whole time were rated higher than cookies that started scarce and later became abundant.
Scarcity Examples
When Snap released Spectacles, you could only buy them from special vending machines called Snapbots. They appeared in random places without warning, so it felt like a surprise game of first come, first served.
This unpredictable availability created strong FOM and as a result, Spectacles became a cult gadget.
MSCHF releases strange products in small, surprise drops. You never know when the next drop comes, and they never restock, so people rush to buy. This makes every product feel rare, special, and worth grabbing fast.
TBH launched only in a few high schools and only in one state at the start, nobody else could download it. This created massive FOMO in nearby schools. Because of that (and many other brilliant aspects), the app was downloaded 5M times within 2 months.
Reciprocity Details
Reciprocity means we feel the need to give something back when someone gives us something first. Even a small favor creates a quiet pressure to return it.
Think of a friend helping you move a couch. Later, when they need a small favor, you feel almost automatically obliged to say yes.
In marketing this bias drives how freebies, trials, bonuses, and helpful content convert. When brands give people value upfront, customers naturally lean toward giving something back, usually attention, trust, or a purchase.
Reciprocity Guide
Marketing rule: Give value and then ask immediately, before the emotional spike cools.
Reciprocity Research
In the famous Coca-Cola experiment (Regan, 1971), people thought they were rating paintings, sitting with another participant named Joe (actually the researcher’s assistant). Joe was made to seem either polite or rude so the researchers could see who liked him.
In one version, Joe left the room and came back with two sodas, giving one to the participant. In the other version, he came back with nothing. Later, Joe asked the participant to buy raffle tickets.
When Joe didn’t give a soda, people bought tickets only if they liked him. But when Joe did give a soda, people bought twice as many tickets, even if they didn’t like him.
A study found that the waitstaff gave diners:
The act of giving a little extra made patrons feel pleasantly obligated to give more in return.
One retail study found that when stores offered samples, about 30% more people who tried them ended up buying. Costco has even seen some products sell several times more on days when samples are available. After getting a free taste, shoppers often feel a small push to “give something back” by buying the item.
Another experiment found out that people were 45% more likely to donate their one day's salary when they received a small gift of candy while being asked for a donation.
Reciprocity Examples
1. Barbers giving you a free hot towel
Many barbers apply a free hot towel or mini neck massage at the end of a haircut. It costs almost nothing, but feels luxurious, so customers leave bigger tips or become repeat clients.
Magicians on the street often start with a free, impressive trick for one person in a group. Once people get a moment of surprise and joy for free, they feel socially pushed to stay and tip at the end.
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.
Hyperbolic Discounting Details
Hyperbolic Discounting is a fancy term for our tendency to be impatient and inconsistent over time. The closer something is in time, the more valuable it feels.
You know you should save money, but still you buy something small today because the benefit is immediate, while the long-term reward feels distant and weak. The present wins.
In marketing this bias pushes people toward instant bonuses, quick wins, and fast results. Offers that give something now beat promises of future value.
Hyperbolic Discounting Guide
Hyperbolic DiscountingResearch
In one classic study, people felt $15 today was about equal to $30 in 3 months, $60 in a year, or $100 in 3 years.
The implied annual discount rates were huge:
This shows hyperbolic (declining) discounting: extremely high rates for short waits, then much lower for longer waits.
Hyperbolic Discounting Examples
1. Klarna
Buy Now, Pay Late services like Klarna tap straight into present bias. When shoppers see they can get the item now and only pay next month (or in parts), they enjoy the reward immediately while the cost is pushed to their future self. No wonder Klarna exploded.
Duolingo is a perfect example of hyperbolic discounting in action. Learning a language takes months, so the real payoff is far in the future. To keep you hooked, Duolingo gives you small wins right away - streaks, XP, badges, chests, leaderboards, that happy owl cheering.
Cashless Effect Details
Cashless Effect means spending feels easier when no physical cash leaves your hand. Digital payments create distance between you and the pain of losing money.
Think of tapping your card or phone and barely feeling the cost, but handing over actual bills makes you pause. The physical loss hits harder than a silent digital swipe.
In marketing, the smoother and more invisible the payment, the less friction people feel and the more they’re willing to spend.
Cashless Effect Guide
Cashless Effect Research
The study asked 2 497 people to rate shopping spending on a 1–7 "pain of paying” scale (1 = no pain, 7 = it hurts to spend).
They found that people barely feel the loss of money when using digital payments.
Credit cards, surprisingly, felt more painful than cash because people worry about bills later.
Higher amounts increased pain fast: €20 (+0.22), €50 (+0.47), €100 (+0.70), €500 (+1.43).
A 2008 study called Monopoly Money (Raghubir & Srivastava) found that paying with cash hurts more because you physically see the money leaving your hand.
With cards, the “pain” is weaker, the loss feels abstract. So when people pay with credit cards, they focus more on the fun of the purchase and end up spending more and buying more freely.
Cashless Effect Examples
1. Amazon
Amazon’s 1-Click is the perfect example. By removing steps and hiding the payment friction, people buy more and check out faster. With no pause in the process, it’s easy to throw in an extra item.
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.
Motivating-Uncertainty Effect Details
Motivating-Uncertainty Effect means not knowing the exact reward can make us work harder than when the reward is guaranteed. The mystery creates excitement and keeps the brain engaged.
Think of doing extra tasks in a game just for a chance at a mystery box. You don’t know what’s inside, but the uncertainty makes the effort feel more fun and energizing than a fixed prize.
In marketing, this means surprise bonuses, mystery gifts, and unpredictable wins keep people coming back because the next reward might be even better.
The key difference from a Variable-Ratio Schedule is that motivating-uncertainty focuses on the mystery of the reward itself, while variable-ratio is about unpredictability in when the reward appears.
Motivating-Uncertainty Effect Guide
Motivating-Uncertainty Effect Research
Research that studied monetary prizes in two situations - when the prize is unknown and can vary from small to very large, and when the prize is a known amount– showed that people were much more likely to be stimulated by the unknown. The summary was that people were more motivated by tasks with an unknown reward:
People had to drink a big amount of water in 2 minutes.
Results:
That’s a 63% boost just because the reward was uncertain.
Mystery discounts beat normal discounts. Brands like GAP and Banana Republic send emails saying “Click to see your secret discount – maybe 15%, 30%, or even 50%.”
Shoppers were more likely to buy when there was a 10% chance the item was free than when everyone got a guaranteed 10% off.
Supermarkets saw the same thing. Customers preferred a “1% chance your whole cart is free” lottery over a normal 1% discount.
Motivating-Uncertainty EffectExamples
1. Loot boxes
Instead of giving players a guaranteed prize for a challenge, games often give a loot box with random items. Players end up playing more and even paying money for these boxes, chasing the excitement of a rare find.
Every time you buy certain items, you peel a sticker, and you don’t know if it’s a free fries, a drink, a rare piece, or a big prize.
Most prizes are tiny, but the uncertainty is what drives the motivation. People buy more meals because each peel feels like a small gamble with suspense, even if the expected reward is low.