Why do hard chairs increase rigidity in negotiations?

A study by Joshua M. Ackerman (MIT), Christopher C. Nocera (Harvard), and John A. Bargh (Yale) showed that “hard objects increased rigidity in negotiations.” 

One of a series of experiments involved a simulated car price negotiation in which the subject had to make a price offer for a car, which was rejected. Then, the “buyer” had to make a second offer. The subjects were also asked to evaluate their negotiating partner. The researchers found that there was a significant difference between subjects sitting in hard and soft chairs. Those seated in hard chairs judged their negotiating partner to be less emotional. Most significantly, the “buyers” in soft chairs increased their offer by nearly 40% more than those in hard chairs. 
In short, a hard chair not changed the buyers’ perception of their negotiating partner, it made them harder bargainers.

Another experiment had subjects feel a hard block of wood or a soft blanket before rating a boss/employee interaction. The subjects who felt the hard block rated the employee as being more rigid than those who felt the blanket. 

Do these laboratory findings translate into real-world results? 
Study author Joshua Ackerman says, “I suspect that the stresses of real-world decision-making environments will act as mental distracters, making people even more susceptible to the effects of tactile cues.”