FORTUNE — Sometimes you have to play hardball in a business negotiation. Say your boss throws you in the ring to settle terms with a competitor or a difficult client. You’ve got to go into it like a boxer, right? Dominate, show you won’t budge.

Actually, experts say that the best negotiators get what they want without the aggression. Beyond the detrimental effects that aggression or anger can have on personal health and the health of the workplace, professional negotiators say those emotions can get in the way of getting what you want out of a discussion.

One way or another, threatening or blaming someone during a negotiation will negatively influence the outcome, says Stuart Diamond, Wharton professor and author of a book on negotiation called Getting More.

Something as subjective as a wounded ego can flub a negotiation. “People will refuse a deal even though it would have actually made them better off,” says Chris Voss, an adjunct professor at Georgetown’s McDonough business school and former kidnapping negotiator with the FBI. “If people feel like they’ve been treated unfairly, they actually get psychological satisfaction from punishing the other side.”

So it’s best not to start a discussion in a way that would provoke those people, regardless of whether or not you agree with them. In fact, the success or failure of a discussion is hardly influenced by the substance of what’s being discussed, says Diamond. Rather, the people involved — with their goals, their baggage and the way they address each other — influence a negotiation much more than the content.

Solid negotiators can control the tenor of a discussion by making sure that the other side feels heard. It’s a common misconception that you, the negotiator, are the most important person in the room, according to Diamond. Instead, negotiators should focus their energy on the person they are trying to convince. Even if their goals seem unreasonable, people will often offer up clues on how they are thinking.

“I kind of believe that really good negotiators have a firm grip on the obvious,” Diamond says. In high-pressure situations, it’s easy to focus on achieving your own mission, not picking up on hints that you can glean from the other person’s speech or behavior.

Negotiations should serve as a kind of intelligence-gathering exercise, says Voss, who picked that up during his time with the FBI. He now advises businesses on negotiation via his firm Black Swan. Even in a business discussion, much more is happening than it would seem on the surface. For example, third parties often affect discussions. In the corporate world, it’s important to know whether bosses or CEOs wield power over the people discussing terms. There are a couple of ways to key in on this; the first, of course, is to ask them. But if the information is sensitive, negotiators can pay attention to whether people speak in first person or third person. Do they react when certain subjects are mentioned? Behavioral patterns can be useful.

But being open to all of this extra information can require more than one person, says Voss. “If you give me five mediocre negotiators but they work as a team, they can perform better than one superstar,” he adds.

Negotiation isn’t about you or your emotions. It may be more difficult, but it pays to keep a cool head while playing hardball.