The zero-price effect describes a strange asymmetry in how people value things: dropping a price to exactly $0 changes behavior far more than any other price cut of the same size. When something in a bundle becomes free, buyers stop weighing it against the paid item and instead judge the paid item in isolation, often assigning it a higher acceptable price than they would if the same items were bundled at a small discount.

The clearest demonstration comes from a 2007 study by Kristina Shampanier, Nina Mazar, and Dan Ariely, published in Marketing Science. Participants chose between a Lindt truffle priced at 26 cents and a Hershey's Kiss priced at 1 cent; most picked the truffle. When the researchers dropped both prices by the same one cent, to 25 cents and 0 cents, demand swung sharply toward the now-free Hershey's Kiss, even though the price gap between the two options never changed. Making one item free didn't just nudge preferences, it reorganized how people evaluated the whole choice.

The same mechanism shows up in product pricing. When a vendor bundles a paid core product with a free accessory, such as a plug-in, an onboarding session, or a companion app, buyers tend to stop negotiating the accessory's value and instead anchor entirely on the core product's price. A team that has been discounting a paid add-on to close deals may find that making the add-on free outright, while modestly raising the core price, closes deals just as fast: the free item removes a bargaining chip rather than adding one.

The effect has limits. It fades once the free component starts to feel essential rather than incidental, or once customers can get it elsewhere at no cost; at that point the whole offer gets judged as a commodity, and the pricing premium evaporates.