In an article published in the journal Social Psychological and Personality Science, University of California at San Diego behavioral scientist Ayelet Gneezy and University of Chicago business professor Nicholas Epley tracked people’s responses to three types of promises: broken ones, kept ones, and then ones that were fulfilled beyond expectations.

And while it’s true that everyone gets upset when a promise is broken (I’m looking at you, housing-contractors-who-claim-bathroom-renovations-will-be-done-in-a-week), it turns out that overdelivering on something won’t make anyone significantly more impressed by your awesomeness. “Going above and beyond a promise didn’t seem to be valued at all,” says Epley.

Epley and Gneezy conducted several studies, ranging from a simple survey of people’s satisfaction after a promise was exceeded to actually promising their subjects something and then seeing what would happen when they broke, met, or outshined it. It turned out that there was almost no change in people’s levels of satisfaction when they were given more than what they were promised. Epley finds this particularly interesting in light of all the promises that companies make to their customers. “If you deliver books for Amazon.com and you promise four-day delivery, getting it to people in three days isn’t that beneficial to you,” says Epley. In other words, this explains why I’m only mildly pleased when my plane flight is a few minutes early but I’m furious when it’s delayed.

The reason for this, Epley says, is that promises work a bit like verbal contracts. If I promise you something and you accept that promise, you assume I’ll do it, nothing more, nothing less. Interestingly, Epley and Gneezy found that when there was no such contract—when someone merely hoped that something might happen, but wasn’t promised anything—exceeding that expectation made them much, much happier. But when the hope was simply met, people weren’t as pleased as when it was promised.

“It really makes you think about how you spread your effort and how to use your resources wisely,” Epley says. If you can guarantee an outcome, you’ll make your customers (or bosses) happiest when you promise it. But if you’re not sure you can do it—or if you think you can do it even better—you might not want to promise anything and surprise them instead.