Politicians: Your faces matter. After the second Republican debate in September, Donald Trump's visage dominated both social and traditional media. CNN put up a huge banner of his more extreme facial expressions. Other politicians have experienced this sort of expression scrutiny, too, getting judged by the faces they make. House Speaker John Boehner, for example, has a reputation for getting teary-eyed. After Pope Francis visited Washington, D.C., GIFs of Boehner's attempts to control his emotions went viral. "The pope brought out John Boehner's best cry face," Mashable declared.

But a facial expression doesn't have to be over-the-top to communicate a great deal, as an analysis of 2012 Republican primary candidates reveals.

The smallest muscle movements can differentiate between a smile voters find appealing and those they find off-putting, according to an article in the spring issue of the journal Politics and the Life Sciences. Voters can tell a posed smile from one that communicates true enjoyment — and, unsurprisingly, they prefer the latter.

Studying politicians' expressions goes back to at least the 1980s. Back then, facial expressions were categorized as happy/reassuring, angry/threatening or fearful/evasive, for the most part. But tiny details in these expressions have become ever-more important in a world of high-definition video and looping online GIFs. And the new study examines some of the intricacies of the "happy" face by looking at individual types of smiles.

Using the at-the-time ongoing Republican primary as a test case, University of Arkansas psychologist Patrick Stewart and his colleagues gathered video from the 2012 Conservative Political Action Conference (CPAC), where candidates had gathered to woo primary voters. They chose short video clips of each candidate smiling and analyzed them frame-by-frame to determine the anatomy of each grin. They then asked 91 participants to identify the emotions in each smile in order to determine the different smile types on display. We've broken down the smile types below; get a mirror and play along for the full effect. (Or you can watch the video clips Stewart provided.)

Type of Smile Description
Posed Lip corners pulled up and at an angle.
Enjoyment ("true smile") Lip corners up and muscles around the eyes contracted.
Amusement Lip corners up, eye muscles contracted and jaw dropped to reveal teeth.
Controlled Lip corners up, eyes contracted, jaw dropped and lip corners tightened and pulled downwards to stifle the smile somewhat.
Contempt Lip corner on only one side pulled up at an angle and tightened.

The results revealed that there were very few "true" enjoyment smiles happening at CPAC. In fact, Stewart and his team identified only one, by then-Texas governor Rick Perry. This doesn't mean the candidates weren't having fun, Stewart said — instead, they conveyed their happiness with open-mouthed amusement smiles, which were displayed by candidates Michelle Bachman, Jon Huntsman, Tim Pawlenty (twice!), Herman Cain and Newt Gingrich.

Mitt Romney, Michelle Bachmann, Rick Perry, Gary Johnson, Ron Paul and Rick Santorum all displayed controlled smiles, mostly in response to audience applause — it's likely that they wished to show pleasure but not look too smug in response to praise, the researchers wrote. Herman Cain and Mitt Romney each showed contempt smiles.

Of all of the varieties on display, participants rated posed smiles as the least happy. They singled out Newt Gingrich's posed smiles are particularly unconvincing, but believed that Jon Huntsman's were the most genuine. The amusement smiles and Rick Perry's enjoyment smile were viewed most positively, while controlled smiles were seen as less happy and more negative.

Finding contempt smiles at all was a surprise, Stewart said, because it's an odd "two-faced," sort of expression: Half disdain, half affection, usually reserved for a member of our own group who has disgusted us.

Unfortunately for candidates, attempting to control one's facial expressions usually ends with strange and stilted looks, Stewart said. But voters, on the other hand, can take home a useful message from the research.

"We have evolved to make important decisions based upon paying attention to other people's nonverbal behavior," he said. "And when we have to rely on leaders who will deal with unpredictable situations, understanding their personality through their nonverbal behavior might be a more accurate approach than choosing based upon policy positions or political party identification."