The rational perspective clearly states that a negotiator who does not feel or at least who does not express emotion is the best one. Putting a „poker face“ on wouldn't make him vulnerable, wouldn’t put him in danger of losing his bargaining power and it would minimize the risk of being manipulated or tricked into an unfavorable agreement. This conclusion is partially correct. True is, that people in an emotional state are influenceable by a smart and calm counterpart. Even if research does not provide empirical evidence for the entire assumption, the myth is very persistent.
Keeping emotions „closed“ is easier said than done. You probably have already experienced by your own how quickly one can be overwhelmed by his emotions, so that he reacts beyond his rationality. Prof. Susskind wrote already in 1987:
„It is important for disputants to recognize that emotions can overwhelm logic. In fact, people are sometimes trapped into acting against their own best interests, even when they recognize that they are doing so“.
In addition, the more an individual tries to block unwanted emotions or feelings („poker face“), the more accessible these emotions become. Blocking emotions can also interfere with the flow, the rhythm of human interaction. When two people negotiate with each other, they synchronize their behaviors and moods over the time. Trying to keep the „poker face“ at all means will lead to an unnatural, awkward interaction.
Positive emotion can be indeed counterproductive, when you show joy at the end of a hard negotiation. Even if you and your counterpart have been able to settle on very favorable terms for both sides, your joy about the agreement will cause your counterpart feelings of dissonance. The „poker face“ can be useful in negotiations with competitive interdependencies.
These insights are useful, when both parties are people and one or both play the role of a rational negotiator.
But what would happen, if one day we’d be forced to negotiate with purely rational negotiators? Impossible? That doesn’t exist? Well, that’s not completely absurd. Imagine you will have to close a deal with an artificial intelligence system. Which negotiation strategy would you choose? Would you trust a rational A.I.? Would you be able to create a mutual gains agreement? Or would you always feel yourself in the weaker position? All this knowledge has still to be gained through experimentation and empirical research.