Wouldn’t you want to know the negotiation secret to closing the deal or securing that lucrative contract? Now new research out of Harvard University suggests your best weapon may be your facial expressions.

In a new paper entitled “The Commitment Function of Angry Facial Expressions” published in Psychological Science, Harvard University psychology post-doc Lawrence Ian Reed suggests that angry facial expressions seem to boost the effectiveness of threats without actual aggression.

Reed and colleagues Peter DeScioli of Stony Brook University and Steven Pinker of Harvard University conducted an online study of over 870 participants who were told they were playing a negotiation game.

As described in an article on Science Daily, during the study, participants acting as the “proposer,” would decide how to split a sum of $1.00 with another participant, the “responder.” Each person would receive the specified sum if the responder accepted the split that was offered, but neither person would receive any money if the responder rejected the split.

Before making their offers, each proposer was shown a threat that supposedly came from the responder. In reality, the responder was played by the same female actor, who was instructed to create specific facial expressions in the video clips. One clip showed her making a neutral expression, while another showed her making an angry expression.

The clips were accompanied by a written demand for either an equal cut of 50% or a larger cut of 70%, (which would leave only 30% for the proposer).

After they saw the threat, the proposers were asked to state their offer.

The data revealed that the responder’s facial expression did have an impact on the amount offered by the proposer, but only when the responder demanded the larger share.

That is, proposers offered more money if the responder showed an angry expression compared to when they showed a neutral expression, but only when the responder demanded 70% of the take.

Facial expression had no influence on proposers’ offers when the responder demanded an equal share, presumably because the demand was already viewed as credible.

Interestingly, proposers offered greater amounts in response to angry facial expressions compared to neutral expressions even when they were told that they belonged to a “typical responder,” rather than their specific partner.

The researchers claim this works because genuine facial expressions of emotions are hard to fake. Since it’s difficult to fake your emotional expression, people unconsciously assign that more importance than what you’re actually saying. “We pay attention to what people ‘say’ with their faces more than what they say with words,” Reed says.

“The effectiveness of the threat depends on how credible it is,” he says. An angry expression makes a threat more credible because people intuitively think it’s genuine. Whoever you’re negotiating with is more likely to think you’ll follow through on taking your business elsewhere or walking away on a job offer if your words are delivered with the look you’d give a person with a cart full of groceries in the express checkout lane.