Politicians often try to avoid saying what they really think. But their faces and body language broadcast much more than they might realize. The last several months of US presidential debates have been no exception. If you muted the volume and watched what the candidates were saying you might have been surprised by how much you can learn just by looking.

Social communication is almost entirely emotional. Former House speaker John Boehner famously teared up over matters that didn’t move others in the least. His mutable affect, quickly shifting from positive to negative and back again, was beyond his control for the same reason almost everyone finds it hard to maintain a poker face. Emotion wants to express itself.

The emotional brain learns to surmise what lies behind facial expressions, decipher body language, and interpret tone.

We start learning to read others from the day we are born. Newborns mirror back smiles. Infants imitate gestures. Quite soon they start figuring out others’ intentions. The emotional brain learns to surmise what lies behind facial expressions, decipher body language, and interpret tone. We read between the lines. We hear what isn’t said.

And this should spell trouble for politicians. As I’ve written previously in my Psychology Today column, it can be unsettling to observe someone who seems incapable of smiling naturally. Texas senator Ted Cruz is one of the best examples of this: his body language, whether purposeful or inadvertent, comes across as inauthentic at best. At worst, his downturned face and eyebrows give some voters the creeps.

Quite often we pay attention to the wrong thing. We fail to question a split between body language and spoken words when each is sending a different message. Our passions are swayed by clever lines and emotionally moving rhetoric. And we fail to distinguish the charisma and presentation skills needed on the campaign trial from the far different assets actually needed to lead and govern. We judge good talkers to be good leaders, and smarter than quiet types. Yet, as Susan Cain showed in her 2013 book Quiet, successful leaders actually tend to be introverts such as Bill Gates, Charles Schwab, Warren Buffett, and Andy Grove .

Before we had language, our Stone Age ancestors had to read faces quickly to tell friend from foe.

Reading others was once a matter of life and death. Before we had language, our Stone Age ancestors had to read faces quickly to tell friend from foe. Whom should you trust, and from whom should you run away? We live in a far different world today but still possess the same brain as our ancient predecessors. After thousands of years of practice, many researchers think we’ve gotten pretty good at it. But today we seem to give so little thought to the fact that we constantly broadcast our attitudes and feelings. Politicians are all about persuading voters and winning them over—an emotional decision on our part—so every politician tries to manipulate us to some degree. Our challenge is to detect insincerity.

When, in the film Avatar, the planet’s inhabitants say, “I see you,” they are referring to this acute perspicacity for reading others and discerning states of mind, a skill that was outsourced long ago to the unconscious where it became automatic. Nowhere is emotion’s play on the face more evident that in the authentic “Duchenne smile” which Cruz has so much trouble with. This particular expression was named after the 19th century French neurologist who experimented with electrical stimulation of the facial muscles and noted a stereotyped expression across individuals: The corners of the mouth turn up, and the eyes narrow and form crow’s–feet.

We can purposely contract the zygomatic muscles that raise the mouth in order to flash a calculated Miss America smile, but we cannot willfully contract the muscles that encircle the eye sockets. The eyes give away one’s game. Mother may have needled you to put on a happy face, but you can’t if it isn’t sincere. A 10-month–old will give a tentative grin to a stranger, but a genuine Duchenne smile to its mother.

Reuters/Carlos Barria (L), Scott Audette (C), Richard Carson (R) Happy campers.

We have dozens of facial muscles, but we need only one pair to open and close the eyes, and another to do the same with the mouth. If the remaining are superfluous then evolution should have jettisoned them long ago. But it didn’t, indicating their important biological purpose in social signaling and non–verbal communication.

Studies have shown that looks sway us more than words or policies, findings sure to upset those who consider themselves rational-minded. We judge a round, youthful face that has arched eyebrows and prominent cheekbones as the most trustworthy and genial. Rubio’s face is textbook perfect—his stroke of luck is to have an inherent positive bias among voters. Such a face has the greatest sex appeal, too, a factor in democratic elections since their inception. We rate faces more highly the more symmetrical they are, even with photos viewed upside down. Established research also that people weigh their social decisions in a split–second. Give them more time, and they simply confirm their first impression.

Because candidates are extensively coached and drilled by image consultants and media handlers, what we see are studied performances, not real personalities; all the more reason to pay attention apart from words. One way to heighten the difference so it’s easier to see is to watch speakers with the sound muted. Another is to listen carefully with eyes averted. Separate words from action, and see if your opinion changes.