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.
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.
Centre-Stage Effect Details
Centre-Stage Effect means we naturally prefer the option placed in the middle. Our eyes go there first, and our brain treats it as the safest, most balanced choice.
Think of picking a snack from a shelf or choosing a seat. The middle spot feels more comfortable and more “right,” even when all options are the same. The position alone pulls you in.
In marketing this effect shapes how people choose plans, products, and bundles. Putting the preferred option in the center increases its chances of being picked because it feels like the default.
Centre-Stage Effect Guide
Centre-Stage Effect Research
In this study, people had to choose from options that were literally identical (identical products on store shelves). Even with no differences at all, the middle positions dominated:
Christenfeld also found similar patterns in everyday behaviour, like people using toilet paper from middle dispensers more often than from the ends.
Across several experiments, people strongly preferred the middle option when choosing between similar items.
Centre-Stage Effect Examples
1. Netflix
When Netflix shows recommended titles in a row, the platform often places its top push (the show they want you to watch) dead center.
On category pages, Sephora frequently positions high-margin or trending products in the middle column of the 3-column product grid.
Law Of Proximity Details
Law of Proximity means we see things that are close together as belonging together. Our brains group nearby elements automatically, even if they’re not actually connected.
Think of looking at a row of icons on your screen. If two icons sit close to each other, you assume they’re related or part of the same category. Distance changes the meaning.
In marketing and design this law shapes how people read layouts, menus, and messages. Putting elements close together makes them feel linked, while spacing them out separates ideas and reduces confusion.
Law Of Proximity Guide
Law Of Proximity Research
A study showed that people who strongly react to the Law of Proximity in vision tasks are also more likely to show the attraction effect when choosing products.
In 2 experiments (100+ participants), people who grouped nearby shapes more strongly were also more likely to choose the option that sat “closest” to similar alternatives in a choice set, meaning proximity in layout makes one option feel naturally more attractive. This means product cards, pricing plans, or features placed close together can steer users toward the grouped option.
Law Of Proximity Examples
1. Trello
Trello keeps related things close together. Cards stay in one list, and lists stay together on a board. Because they sit side by side, your brain reads them as one unit. This makes the whole project feel easier to understand and manage.
HubSpot’s CRM uses proximity by keeping every key customer detail (notes, emails, deals, tasks) tightly grouped in one clean panel, so your brain instantly reads it as one story instead of scattered data.
Coca-Cola used the Law of Proximity to kill Crystal Pepsi. They launched Tab Clear, a cheap look-alike positioned as a diet soda, and placed it right next to Crystal Pepsi so shoppers would mentally link the two.
Because diet sodas were seen as weak and inferior, Crystal Pepsi instantly lost its “healthy mainstream” positioning, and the whole clear-cola idea collapsed.
Within 18 months, both products were dead, and Coca-Cola successfully destroyed the entire category with a deliberate sabotage strategy.
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.
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.
Weber’s Law Details
Weber’s Law means we notice changes only when the difference is big enough compared to what we already sense. Small changes get ignored unless they cross a certain threshold.
Think of turning up the volume by one tiny step when the music is already loud. You barely notice it. But that same small jump feels big when the room is quiet. The starting point decides what feels different.
In marketing this law explains why tiny price changes, small discounts, or small design tweaks often go unnoticed. People only react when the change is large enough to stand out from what they’re used to.
Weber’s Law Guide
Weber’s LawResearch
Weber found that people notice weight changes only when the difference is big enough compared to the original weight.
In his classic test, blindfolded participants held two equal weights, then one hand received a slightly heavier “test” weight. He discovered that the just-noticeable difference (JND) was roughly 1/30 to 1/50 of the starting weight (≈ 2–3% difference needed). So if someone held 100 g, they needed about 103 g to feel a difference; with 200 g, they needed around 206 g.
This proportional gap (not an absolute one) is the basis of Weber’s Law, proven repeatedly in later psychophysics studies.
A recent pricing study (2025) tested Weber’s Law with furniture buyers and found mixed support. Over 45% of consumers said they’d notice even small price changes in furniture, showing that many are sensitive even to slight uptick.
However, past research by marketing scholars indicates that in general, consumers often don’t react to price changes below about 10%.
Another study revealed consumers are 4X more sensitive to price increases than to package size reductions. For instance, one retailer noted that a 1% price increase cut sales by ~1.2%, whereas a 1% package size cut had a much smaller effect.
These findings confirm we’re quicker to spot a higher price tag than a lighter box of cereal. In practice, businesses often adopt a rule of thumb (e.g. “keep changes under 5%”) to stay within Weber’s threshold and avoid customer backlash.
Weber’s Law Examples
1. Dairy Milk reduced its bar weight
In 2012, Dairy milk standard bar chocolate was reduced from 49 grams to 45 grams and maintained 59 p as price. Consumers didn't even notice.
If the weight was reduced to 39 grams (you need 8-10% change to feel the difference) consumer would find the difference.
Tropicana Orange Juice experienced a major rebranding disaster in 2009 because they didn’t follow Weber’s Law. Instead of gradually changing their logo over time (like Google did), Tropicana altered nearly every aspect of their classic design overnight. They invested $35M and lost $20M due to their logo being unrecognizable.
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.