The paper Examining the Efficacy of Time Scarcity Marketing Promotions in Online Retail from the Journal of Marketing Research has some great research (as covered on the Nudge Podcast).

Even the abstract is informative, if a bit unwieldy in that classically academic way:

Time scarcity promotions (e.g., “40% off for a limited time”) are mainstays of online retail marketing. Although positive effects of time scarcity promotions on consumer interest have been evidenced in the brick-and-mortar world, should retailers expect similarly robust effects online? The present research suggests the answer may be no. First, the authors report meta-analytic and experimental results suggesting that previously identified positive effects of time scarcity promotions observed offline may not emerge in online shopping contexts. Then, consistent with the prediction that online time scarcity promotions activate more persuasion knowledge than identical control promotions, the authors detail findings suggesting that providing retailer-exogeneous justifications for online time scarcity promotions’ time restriction (e.g., consumers’ birthdays, seasonal changes) can increase the potential of observing positive effects on consumer interest online. Further, results suggest that the positive effects of including exogenous time justification may be more likely when less time remains until the online promotion’s expiration. However, results stop short of suggesting that online time scarcity promotions will consistently yield superior outcomes compared with identical online control promotions. Therefore, the authors highlight the continued need for careful managerial use as well as further research examining the optimal translation of offline tactics to online retail.

I will now try to translate this into actual human speak, line by line…

Limited time promos / flash sales are popular with ecommerce.

These types of offers have proven success in brick-and-mortar / offline retail, but does that transfer online?

Spoiler alert: probably not.

This is based on a review of pervious research.

Providing a reason for the promo’s limited run can boost the offer’s performance online (e.g., holidays, celebrations, seasons, etc).

This can be boosted further by shortening the offer’s time frame (e.g. 24 hours only).

But these steps won’t make them super promos when compared to other offer types you might run.

We recommend you use these carefully. And more research is needed on the performance of offline tactics for ecomm.

A few guesses as to why flash sales and limited time promos aren’t as effective online:

• Offline limited time promos have the added constraint of physical presence / travel. There may be 3 days left on a sale, but can you get back to this store in the next 3 days? Our brains might even process the sale as being positive ROI on our time investment that got us into that store (unsubstantiated hypothesis).
• Snaps and Stories may have altered the timeline of the internet to 24 hours. Anything longer than one solar day is no longer urgent.
• The internet is an infinite shelf of content, decisions are made in terms of “should I do this now?” 72 hours left in a sale means it doesn’t need to be shopped RIGHT NOW, and is then likely to be forgotten before the expiration date.
• Grounding the promo in something outside the feed or brand’s business goals can stand out as a hook. “25% for 24 hours” doesn’t have a hook (discounts and promos are ubiquitous now, they aren’t hooks in and of themselves). “25% off on your birthday” might trigger “I should treat myself.' “Spring Sale - 25% off” might trigger “I could use some pieces to freshen things up.”