When it comes to package delivery, early isn’t always better.

A new study from researchers at the University of Iowa’s Tippie College of Business finds that when a package that’s ordered online is delivered earlier than the promised delivery date, the purchaser is apt to give it fewer stars when they leave an online review.

The finding surprised researchers Salar Nozari and Soogand Alavi, assistant professors of marketing. While the impact of delivery performance on how consumers rate a company’s service quality is well known, the authors didn’t expect that early or late deliveries would actually impact perceptions of the product itself. Even more interesting, they said that while we might expect that customers to ding a product in their rating for arriving late, they didn’t think they would also penalize a product for arriving early.

The two researchers analyzed online reviews of 11 million products left over the course of a year on an e-commerce platform. They found that when the delivery was made later than the platform promised, it led to a loss of .4 stars on average for products compared to on-time deliveries, presumably because the customer wasn’t happy about waiting. 

But when the delivery came before the window, the recipient also dinged the product, by an average of .2 stars. The authors said that while that might seem small on a single review, it quickly impacts the product average rating after numerous reviews and could eventually reduce sales.  

Why such counter-intuitive annoyance for an early delivery? Nozari and Alavi said it could be that the early delivery interrupted the customer’s time needed to justify purchasing the product. The researchers said that customers often use the time between purchase and acquisition to justify buying the product. If the platform says the product will be delivered Friday morning, they implicitly use the time before delivery to come to terms with their purchase, a process called cognitive dissonance theory.

But when the product comes Thursday afternoon, that early delivery stops the mental process so now they’re uneasy about the purchase and that’s reflected in the downgraded review. They said financial compensation can minimize the damage, so businesses can give a gift card or a discount on the customer’s next purchase if the delivery is late. 

Nozari and Alavi said one exception to this is food deliveries, which customers are usually more familiar with and so an early delivery doesn’t cause discomfort.