Robust social psychology research indicates that people lie - and lie often. One prominent study found that people tell, on average, one or two lies every day. Negotiators are no exception. Judging from studies done in 1999 and 2005, roughly half of those making deals will lie when they have a motive and the opportunity to do so. Typically they see it as a way to gain the upper hand (although it can actually cause backlash and prevent the kind of creative problem solving that leads to win-win deals). Deception is thus one of the intangibles that negotiators have to prepare for and take steps to prevent.
"How to Negotiate with a Liar" is from the July-August 2016 issue of Harvard Business Review.