In the midst of an intense negotiation, it’s hard to know what’s motivating the person across the table — is he willing to cooperate with you to meet both your interests or does he only want to serve his? You need to build trust with your counterpart so you can align your interests and increase the likelihood that he will honor his commitments.

A powerful way to establish trust is to employ one of the mind’s most basic mechanisms for determining loyalty: the perception of similarity. If you can make someone feel a link with you, his empathy for and willingness to cooperate with you will increase.

My favorite example of this occurred outside Ypres, Belgium in 1914. The British and the Germans had been fighting a long and bloody battle, but on the eve of December 24^th, the British soldiers began to see lights and hear songs from across the field that separated their trenches from those of their foes. They soon recognized that the lights were candles and the songs were Christmas carols. What happened next was rather amazing. The men from both sides came out of their trenches and began to celebrate Christmas together. Men who had hours before been trying to kill each other were now sharing trinkets and family photos in complete trust that no violence would occur. Why? No one knows for certain, but I suspect that it was because in those moments, the men stopped viewing themselves as British and Germans, and rather saw themselves as fellow Christians. They came to perceive themselves as similar, and that meant they could trust each other.

Now you might think I’ve constructed a fanciful theory for a fluke occurrence. Fair enough. My colleague Piercarlo Valdesolo and I wondered the same, so we set out to study it. Although it made good sense that the mind would use similarity as a metric to decide to whom to be loyal, we had no hard proof. To find out if our suspicions were correct, we designed an experiment that allowed us to manipulate similarity stripped down to its most basic elements in order to see how it would affect behavior. To do this, we brought participants into the lab one at a time for what they believed was an experiment on music perception. They put on earphones and sat across from another person, who was actually an actor working with us. The task was simple; all it required was tapping the sensor in front of them with their hand to the beats they heard over their earphones. The beats were designed so that some participants could see their hands tapping in synchrony with the actor (who had his own sensors and headphones), while others would see random, unsynchronized tapping. Why the tapping? Moving in time is  to discern who’s similar. It occurs in rituals, in military drills, and in team exercises. If you’re moving in time with someone, it’s a symbol that right here, right now, the two of you are a unit.

After the tapping, we had designed a situation where the participants would see the actor get stuck while completing an onerous task from which they themselves were excused. But before they left the experiment, they were offered the opportunity to help the actor complete the onerous tasks if they so desired.

As we expected, relatively few people (18%) decided to come to the aid of the other when they hadn’t been synchronized. But if they had tapped in synchrony, the number who helped (50%) jumped dramatically. What’s more, the increase in helping was directly tied to how similar participants felt toward the actor. Surprising as it may seem, those who tapped in synch believed they shared more in common with the actor than those who didn’t, even though they had never said a word to the actor.

There’s nothing magical about hand tapping. We’ve found the same effect in several ways, including telling people they shared some esoteric (or even phony) characteristic with another. All that’s required to increase people’s willingness to support each other is any subtle marker of similarity.

Try it in your next negotiation.  Find and emphasize something – anything – that will cause your partner to see a link between the two of you, which will form a sense of affiliation. And from that sense of affiliation —  whether or not it’s objectively meaningful – comes a greater likelihood of trustworthy behavior.