Imagine you're watching a movie. Ryan Gosling is having a drink with a male friend at a bar. They're talking about, say, sports. 

Suddenly, a beautiful woman steps up beside him to order a drink. Ryan can't help but notice her. He exchanges a meaningful glance with his friend. He turns her way... 

At this point, would you be surprised if Ryan spoke to her in a lower-pitched voice than he did when talking with his friend? Of course not. That's what guys in movies do when they meet women they're attracted to.

Turns out that's what most guys do.

And so do most women.

"Both sexes used a lower-pitched voice and showed a higher level of physiological arousal when speaking to the more attractive, opposite-sex target. Furthermore, independent raters evaluated the voice samples directed toward the attractive target (versus the unattractive target) as sounding more pleasant when the two voice samples from the same person presented had a reasonably perceptually noticeable difference in pitch.

"These findings may have implications for the role voice plays in mate selection and attraction."

Or, in non researcher speak, when we think another person is attractive... we instinctively speak with a lower-pitched voice. 

As Dr. Susan Hughes, one of the authors of the study, says

"These findings may have implications for the important role voice plays in mate selection and attraction.

"If people can perceive changes in others' voices when speaking to attractive individuals, this perception may be adaptive for identifying interested potential mates, detecting partner interest in others, and possible detection of partner infidelity."

All of which is fine -- but not particularly helpful in a professional setting.

But then again...

A week ago I was with a friend who was buying a car. He looked at one model, test-drove it, talked with the salesperson, seemed skeptical... and then there was this moment that I realized he was going to buy the car. Of course he hadn't said so. He was still playing the negotiation game with the salesperson; he was still deep in "I could walk away at any moment" mode.

So how did I know? I realized his voice had changed. His voice had gotten deeper. He was speaking a little more slowly. His interest in the car had turned to serious attraction. Ten minutes later he said, "Let's do it."

The same thing happened a few days ago. The director of a conference called to gauge my interest in speaking (and her interest in having me speak.) I had just said, "In addition to a keynote... research says 60 percent of people dream of writing a book, and if you'd like me to do an informal breakout session on my experiences in writing a book, getting a publishing deal, etc, I would be happy to..." and her voice changed.

"You know, that's a really good idea," she said, her voice pitched lower than it had been. "That might really add to the scope of sessions we're holding..."

And five minutes later she said, "Let's do it."

So while gauging another person's voice tone may not be an infallible guide to their interest in what you're proposing, or requesting, or offering... paying attention to any changes may help you stop selling -- and start closing.