Scarcity
Limited-quantity pressure: '3 seats left', 'almost sold out', a capped cohort. Supply is the constraint, not the clock.
See it
What it is
Scarcity is supply pressure: there is a limited amount of this, so waiting risks missing out entirely. '4 seats left in the cohort', '2 rooms at this price', a 500-piece run, a drop that never restocks. Cialdini filed it as one of the six persuasion principles, and it is the quantity-shaped cousin of urgency, which pressures on time.
It lands hardest where the limit is obviously real: physical inventory, coaching or agency capacity, event seats, a cohort size, a founding-customer tier. On a page, that means an honest counter next to the buy button, a struck-out sold-out variant, or a waitlist once the thing is gone. Selling out and saying so also doubles as social proof.
Gotcha: infinitely reproducible digital goods have no natural scarcity, so an invented '7 licenses left' banner reads as theater and burns trust. If you want the effect on software, tie it to a constraint that actually exists (onboarding slots you can staff, a price tier you will genuinely close, a beta cap) and then honor it. Booking.com-style pressure counters are also under regulator scrutiny when the numbers are not real.
Ask AI for it
Add a scarcity cue to this product page, reading remaining inventory or capacity from [data source or API field]. Show the remaining count beside the buy button as plain text plus a thin depletion bar, and switch the label to 'Sold out' with a waitlist form at zero. Show no scarcity cue at all when that value is unavailable, stale, or not a genuine constraint, and never hard-code or estimate the number. One cue per page, no flashing, no fake '12 people are viewing this' line.