Researchers have found that when the ability to make facial expressions has been hindered, either by mechanical means (wearing a hockey mouth guard, or biting on a chopstick or chewing gum) or by temporary paralysis (Botox injections), individuals are less able to interpret other people’s facial expressions and experience related emotions. In fMRI scans, people who have had Botox injections have less activation in areas of the brain used to interpret and modulate emotional states. Other studies have shown a decrease in the intensity of emotional experience following Botox injections, which is why some have suggested it as a treatment for depression. The trouble is that you might end up losing the ability to feel very happy just as you lose the ability to feel very sad.

People who have had a stroke or who have other medical conditions that impair facial expressiveness, such as Parkinson’s disease, may have similar trouble recognizing and internalizing emotions. So, too, do people who grew up in households where their parents or caregivers were emotionally flat or often angry. The thinking is that, as children, they didn’t have the opportunity to mirror a range of emotions and thereby develop the facility to recognize, express and feel them. There is also some indication that people high in psychopathic or callous behavioral traits demonstrate less spontaneous mimicking behavior, which may be why they are not moved by the fearful or angry faces of their victims.

“Muscle movements in the face sustain interactions between people, and if you take that out, you’re working with a blank slate,” said Jeffrey Cohn, a professor of psychology and psychiatry at the University of Pittsburgh, who studies the link between the lack of facial expressiveness and depression. “That’s not an effective way of maintaining rapport or establishing connection.”

Interestingly, researchers have also found that people injected with Botox in the crucial expressive muscles around the eyes and forehead also had greater difficulty and were slower at interpreting and understanding emotions expressed in written statements like “You spring up the stairs to your lover’s apartment,” or “Reeling from the fight with that stubborn bigot, you slam the car door.” Another study showed that people had greater difficulty remembering emotional words when their faces were immobilized by wearing a face-hardening algae mask.

“I think we might be grossly underestimating just how powerful our facial expressions are,” said David Havas, associate professor of psychology at the University of Wisconsin-Whitewater who studies facial mimicry. “We have to recognize how informationally rich facial feedback is and when we block it, we are cutting off a major channel about our own emotions and information about social emotions.”

While primates also engage in facial mimicry, evolution gave humans the advantage of more refined musculature, especially around the eyes, and we have thinner and smoother skin, making it easier to see the tiniest twitches. The result is humans have a greater variety of facial expressions — thousands, in fact — that we detect with amazing speed and precision. This ability to read one another and empathize has arguably been fundamental to all human achievement.

“Facial mimicry is a very ancient mechanism of connecting and not something you want disrupted by Botox or other procedures,” said Frans de Waal, a primatologist and ethologist at Emory University. “Today I think primates are sometimes more perceptive than humans at reading facial expressions and body language because that’s all they have to go by, whereas we are always waiting for the words — and words can be highly misleading.”