Likability is crucial to building and maintaining great relationships. Likability is crucial in influencing (in a good way) the people around you. Likability is crucial in helping people feel better about themselves.

And if those reasons aren't sufficient--if you need a bottom-line, goal-focused reason to adopt some of the ways to more likable--likable people tend to be more successful in sales, more able to enlist the help of others, more likely to be hired and promoted...likability is a huge driver of success.

Except in negotiations.

Though plenty of people believe warmth and friendliness will pay off (including Robert Cialdini, the author of the highly, um, influential book Influence: The Psychology of Persuasion), this 2017 Harvard Business School study found that "tough and firm" negotiators tend to be much more successful than "warm and friendly" negotiators.

(Keep in mind there's a big difference between "tough and firm" and "jerk.")

Here's the "warm and friendly" message researchers sent to smartphone sellers on Craigslist, an online marketplace where negotiations are commonplace and expected:

Hi there--I'm happy to see your post about the phone. This iPhone matches what I wanted to buy--you must have great taste :). Is there any chance you could sell it to me for 80 percent of the listed price? Given the prices on similar phones currently for sale, I would really appreciate it, and it would help me out a lot! I live in the area, and I can come to meet you anywhere that is convenient for you. Please let me know by tomorrow if the price is OK for you--and thank you so much for your time and consideration. Hope you have a wonderful day. 

Warm, fuzzy, personal...a solid (maybe too solid) attempt to establish rapport and maybe even a touch of empathy. 

In contrast, here's the "tough and firm" message:

I saw your post about the phone! This iPhone matches what I wanted to buy. I'm willing to pay 80 percent of the listed price. Given the prices on similar phones currently for sale, I'm firm on that price. I live in the area, and I can meet you wherever. Let me know by tomorrow if the price is OK for you or else I'll move on.

Still friendly, still a little personal...but much more to the point.

Which message performed better? 

The firm message was actively rejected--meaning responded to with a solid "no"--more often; clearly the tone was more likely to put people off.

But the tough message was less likely to be ignored than the warm message--and sellers were also more willing to accept the 80-percent offer. 

In short, while more people said "no," those who did respond tended to provide a larger discount. In aggregate, tough and firm messages generated approximately 8 percent higher savings per phone than warm and friendly messages.

Granted, 8 percent might not sound like much. Until you consider the impact of generating 8 percent higher sales revenue. Or the impact of saving 8 percent more on costs.

Or that much of your day is actually spent negotiating: with employees, with customers and vendors, with partners, with friends and family....

Use the Nine Magic Words.

The next time you're about to enter a negotiation, don't just think about what you need. Don't just think about what the other party needs. 

Think about your style. Consider taking a tougher, firmer approach than you normally do. Be nice, be friendly...but get to the point.

How? Consider weaving in this phrase: "If you can do (X), we have a deal."

Do that, and you show you're confident. You decrease the amount of haggling.

Most importantly, you show that you're ready, willing, and able to make a deal. And that you're ready, willing, and able to walk away.

Which allows you to send a very firm message.

Without being a jerk.