Smile, they say, and the world smiles with you.

But, there's more than one kind of smile, and I’d like to talk today about a study suggesting that nothing stresses people out like being smiled at the wrong way.

In fact, it turns out there's an immense power that can be leveraged behind subtle changes in your facial expressions.

First, we've all had the experience of being on the receiving end of a "fake smile," and perhaps becoming agitated by it. Think of a negative customer service experience, for example, in which the employee's facial expression conveys a sentiment that is the exact opposite of their behavior.

This study goes beyond that common experience, by measuring stress hormones in research participants whose reactions to various types of smiles were measured and judged.

It starts with the presumption that broadly speaking, there are three major types of smiles:

• Dominance smiles, which are intended to convey status,

• Affiliation smiles, which are intended to convey that you're unthreatening, and communicate bonds between people, and

• Reward smiles, which are described as "the sort of beaming, toothy smile you'd give someone to let them know they're making you happy."

I enjoy imagining the controlled environment in which researchers conducted this study. They recruited 90 male college students and told them they'd have to give short, impromptu speeches about personal topics, and that they'd be speaking over a webcam to another student.

"After responding to each of the three topics, participants saw videos of their evaluator's facial expressions, which they believed represented spontaneous reactions," according to the report of the study, published in the journal Scientific Reports.

However, the students on the other end of the webcam were actually working with the researchers—and "the videos were, in fact, prerecorded."

Thus, the researchers provided a reaction—which seemed to the speakers to be live—that consisted of either a dominant smile, an affirmative smile, or a reward smile.

The whole time, the researchers were measuring the speaking students' heartrates, and periodically asking them for saliva samples, so that they could calculate the level of cortisol, a stress-related hormone.

"If they received dominance smiles, which they would interpret as negative and critical, they felt more stress, and their cortisol went up and stayed up longer after their speech," said Paula Niedenthal, a professor of psychology at the University of Wisconsin, Madison who was co-author of the study, along with her graduate student Jared Martin.

It was a little bit harder to distinguish between the reactions to affiliation and reward smiles, Niedenthal said. Both resulted in reduced levels of cortisol in the speaking subjects—perhaps not an unexpected result since both of these types of smiles communicate fundamentally positive messages.

So, what to do with this information, and this unexpected power?

I think ultimately it comes down to simply being aware of the nonverbal cues you're providing during interactions.

Avoid those fake or dominant smiles, unless you intentionally want to provoke stress in whomever you're dealing with (maybe in a competitive or negotiation context, for example).

Just don't expect the other person to smile back.

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Thanks for reading. Photo: Pixabay. I wrote about part of this before for Inc.com. Want to see all my mistakes? Click here.