Trained mediators are usually taught to value niceness and trust-building in disputes so they can work with the negotiating parties to reach an agreement. But a new study (pdf) in Management Science suggests that in certain situations hostility from the mediator can prompt adversaries to come to an agreement. “When a mediator is hostile, those in conflict see the mediator as a common enemy,” explains organizational behavior researcher Ting Zhang at Columbia University, lead author of the study.

Mediators help settle disputes ranging from the personal to the international, from divorce settlements to peace treaties—which are not so different from each other in some ways. Both involve parties who have a lot at stake but have to be able to discuss what they each want and require if they are to come to an agreement.

Zhang and her co-authors posed various conflict scenarios to participants who were playing the role of either the mediator or one of two parties negotiating a dispute. Next the role-players were asked to reach an agreement by communicating via text-based chat rooms. In some of the experiments the participant interacted with a computerized mediator and computerized opponent but was led to believe they were real people. In other sessions all three parts were played by human beings. Zhang and colleagues found through a series of these experiments involving hundreds of people that negotiating parties came to more agreements when the mediator—human or computerized—was hostile, apparently because they perceived the mediator as a common enemy.

The work was inspired by classic sibling arguments, Zhang says. “Young siblings fighting with one another often start getting along better when their parents intervene in a stern way. That led us to wonder whether negative experiences, such as exposure to people with abrasive, rude or mean personalities, could bring two people in conflict together.”

In personal—or professional—disputes the negotiating parties have genuine anger or frustration at the opposing side. But anger can also be used as a tactic to force opposing parties to take the mediator seriously, explains negotiation and conflict resolution researcher Ray Friedman at Vanderbilt University, who was not involved in the study. “The key is that the anger not be an ongoing pattern by the mediator. It has to be used carefully to signal when a certain issue is really critical.”

Although mediators usually aim to be calm and detached, negotiators in real-world disputes are almost always emotionally invested and often upset. This condition is difficult to reproduce in laboratory settings and is a limitation of Zhang’s study and others like it, observes Jeanne Brett, director of the Dispute Resolution Research Center at Northwestern University who did not take part in the study. “When people go into a dispute they’re angry because either their claim has been rejected or they’re rejecting a claim,” she says. “And in all simulations of dispute resolution, my own research included, we violate that.”

Contrived scenarios by definition involve parties who are not previously acquainted and are not actually angry with each other. Brett says she and other researchers try to adjust for this by getting participants angry at the start of the experiment—something that was not done in Zhang’s study. Another factor to consider is that people in real-world disputes have a genuine stake in the outcome and need to minimize their costs, Brett explains, adding that this consideration is not a factor in simulated situations.

But Zhang’s study does help tease apart the question of mean versus nice mediators, says Daniel Ames, who studies social judgment and behavior at Columbia and was also not part of the work. He says such research builds on a foundation of knowledge about solving disputes by finding common ground, which can hold a lot of value for mediators.

Making yourself the common enemy might be worthwhile in some scenarios, Ames notes. “But this work also shows that the mean mediators themselves are not beloved by the disputants when the dust settles.” If the mediator has an ongoing relationship with the two parties—as can be the case in many complex legal disputes—playing the bad cop may not be the way to go.