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Scarcity bias
When something is perceived as limited, its subjective value increases, even if its usefulness remains the same.
Only show scarcity when it is true, and say what the limit is.
What seems limited seems more valuable, even when it is identical. Stephen WorchelRan the biscuit experiment: the same biscuit is worth more when the jar empties in front of whoever is watching.One reference in this work:1975Effects of supply and demand on ratings of object valueSee in the bibliography → and colleagues showed this in 1975 with biscuits in glass jars, and the most interesting part of the study is the second one: when a second experimenter came into the room and swapped the jar of ten biscuits for the one with two, the rating rose more than when the jar had been nearly empty from the start, and rose even more when the stated reason was that other people had eaten them, and not that he had picked up the wrong jar. Scarcity produced by other people’s demand is worth more than scarcity by chance. That is why the places-left counter works so well, and why it is the easiest thing to fake in an interface.
What the brain does with it
The biological mechanism behind the effect, and how much of it was actually measured.
The effect is behavioural and replicated in varied contexts: the same item is rated as more valuable when described as rare. The mechanism is unknown. There are hypotheses linking scarcity to value and urgency systems, but none has been tested in a way that would authorise an assertion, and attributing the effect to a neurotransmitter, common in neuromarketing material, is precisely the kind of explanation that sounds scientific without supporting anything. What has a measured basis is valuation itself, linked to the ventromedial prefrontal cortexIt makes the global judgement, the final opinion on an entire experience, not on each moment of it. It is because of it that the person sums up a ten-screen flow in a single sentence.See it in Emotion →See in the glossary → in a survey of 206 imaging studies on choice (BartraOne reference in this work:2013The valuation system: a coordinate-based meta-analysis of BOLD fMRI experiments examining…See in the bibliography →, McGuire and Kable, 2013). Nobody measured the effect of rarity there, and the link to this bias is mine.
The body this law presumes
Who it was measured on, and what changes when the body on the other side is another.
The bias says that what is scarce seems more valuable. Whoever already lives in real scarcity decides with less margin for error, because the hasty decision costs more for those who have less. From there to supposing that this person also reacts more strongly to the commercial trigger is a leap nobody has measured, and the literature on living with little measures something else, the load that lack takes up in the head. In practice, I treat the same countdown as persuasion for one audience and as pressure for another.
How design translates it
What to do with it on a screen, without turning a finding into a rule.
How to measure this in your product
The honest use is boring to describe because it is just telling the truth with emphasis. Ten places on the course, three copies in stock, the deadline the law imposes: show the number and show where it comes from. Whoever saw “2 seats left at this price”, came back the next day and found the price higher learned that that site means what it says. That learning is what scarcity buys, and it only holds up as long as the number is real.
Onde vira manipulação
The same lever, pointed at the person on the other side.
The dishonest use is inventing urgency to force an impulsive decision. I only use scarcity when I can show where the number comes from, and I say what the deadline is and what the limit is.
Where it breaks
Where the law does not hold, holds less, or holds in reverse.
Invented scarcity breaks the first time the person comes back and the counter has reset, and what is lost along with the trigger is trust in everything else on the screen. And there is a limit before that one: scarcity weighs differently on those who already live in it. For someone whose account is in the red, urgency is not a stimulus, it is one more thing occupying their head.
Um caso
A real product where this showed up, with what happened and where to check it.
The last one in stock, and what it doesn’t say
Two hotel screens show the same room at the same nightly rate. One says “2 rooms left at this price”, the other says nothing, and the real number is the same on both. The first converts more. If the figure is true, it merely reported something that matters to the decision. If it was fabricated, it charged an anxiety that did not need to exist. From the screen, the two are identical, and what separates one from the other is not on the screen, it is in the origin of the number. Can anyone show where it came from? The same question applies to the counter that resets when the page reloads, to the “17 people viewing right now” and to the last unit that never runs out.
Worchel; Lee; Adewole, 1975
Experimente
A piece to check in your own body what the text has just claimed.
Where the number comes from
A hotel screen like so many others. Click Reload a few times and notice what changes and what does not. Then reveal where the number comes from.
Double room, sea view
R$ 480 a night
19 people viewing now
Only 2 rooms left at this price
pessoasVendo = 9 + Math.floor(Math.random() * 23)
Primeiro carregamento.
Neither number comes from real data: one is drawn at random on every load and the other is fixed. A real counter comes from a system that knows how many people are on the page and how many rooms are left, and so it changes in a way that makes sense and does not restart when you reload. The screen is the same in both cases. What separates the honest screen from the dishonest one is someone being able to show where the number came from.
Neighbouring concepts
Structures linked by a bridge of my own
Sibling concepts
Fontes
Enunciado citado de Worchel; Lee; Adewole, 1975.
- WORCHEL, S.; LEE, J.; ADEWOLE, A. Effects of supply and demand on ratings of object value. Journal of Personality and Social Psychology, v. 32, n. 5, p. 906-914, 1975. DOI
Leitura complementar: The Scarcity Principle in UX NN/g em inglês