Pacifiers may stunt the emotional development of baby boys by robbing them of the opportunity to try on facial expressions during infancy.

Three experiments by a team of researchers led by psychologists from the University of Wisconsin–Madison tie heavy pacifier use as a young child to poor results on various measures of emotional maturity.

The study, published today by the journal Basic and Applied Social Psychology, is the first to associate pacifiers with psychological consequences. The World Health Organization and American Academy of Pediatrics already call for limiting pacifier use to promote breast-feeding and because of connections to ear infections or dental abnormalities.

Humans of all ages often mimic — unwittingly or otherwise — the expressions and body language of the people around them.

“By reflecting what another person is doing, you create some part of the feeling yourself,” says Paula Niedenthal, UW–Madison psychology professor and lead author of the study. “That’s one of the ways we understand what someone is feeling — especially if they seem angry, but they’re saying they’re not; or they’re smiling, but the context isn’t right for happiness.”

Mimicry can be an important learning tool for babies.

“We can talk to infants, but at least initially they aren’t going to understand what the words mean,” Niedenthal says. “So the way we communicate with infants at first is by using the tone of our voice and our facial expressions.”

With a pacifier in their mouth, a baby is less able to mirror those expressions and the emotions they represent.

The effect is similar to that seen in studies of patients receiving injections of Botox to paralyze facial muscles and reduce wrinkles. Botox users experience a narrower range of emotions and often have trouble identifying the emotions behind expressions on other faces.

“That work got us thinking about critical periods of emotional development, like infancy,” says Niedenthal, whose work is supported by the French Agence Nationale de la Recherche. “What if you always had something in your mouth that prevented you from mimicking and resonating with the facial expression of somebody?”

The researchers found six- and seven-year-old boys who spent more time with pacifiers in their mouths as young children were less likely to mimic the emotional expressions of faces peering out from a video.

College-aged men who reported (by their own recollections or their parents’) more pacifier use as kids scored lower than their peers on common tests of perspective-taking, a component of empathy.

A group of college students took a standard test of emotional intelligence measuring the way they make decisions based on assessing the moods of other people. Among the men in the group, heavier pacifier use went hand-in-hand with lower scores.

“What’s impressive about this is the incredible consistency across those three studies in the pattern of data,” Niedenthal says. “There’s no effect of pacifier use on these outcomes for girls, and there’s a detriment for boys with length of pacifier use even outside of any anxiety or attachment issues that may affect emotional development.”

Girls develop earlier in many ways, according to Niedenthal, and it is possible that they make sufficient progress in emotional development before or despite pacifier use. It may be that boys are simply more vulnerable than girls, and disrupting their use of facial mimicry is just more detrimental for them.

“It could be that parents are inadvertently compensating for girls using the pacifier, because they want their girls to be emotionally sophisticated. Because that’s a girly thing,” Niedenthal says. “Since girls are not expected to be unemotional, they’re stimulated in other ways. But because boys are desired to be unemotional, when you plug them up with a pacifier, you don’t do anything to compensate and help them learn about emotions.”

Suggesting such a simple and common act has lasting and serious consequences is far from popular.

“Parents hate to have this discussion,” Niedenthal says. “They take the results very personally. Now, these are suggestive results, and they should be taken seriously. But more work needs to be done.”

Sussing out just why girls seem to be immune (or how they may compensate) is an important next step, as is an investigation of what Niedenthal calls “dose response.”

“Probably not all pacifier use is bad at all times, so how much is bad and when?” she asks. “We already know from this work that nighttime pacifier use doesn’t make a difference, presumably because that isn’t a time when babies are observing and mimicking our facial expressions anyway. It’s not learning time.”

But even with more research planned to further explain the new results, Niedenthal is comfortable telling parents to consider occasionally pocketing the pacifier.

“I’d just be aware of inhibiting any of the body’s emotional representational systems,” Niedenthal says. “Since a baby is not yet verbal — and so much is regulated by facial expression — at least you want parents to be aware of that using something like a pacifier limits their baby’s ability to understand and explore emotions. And boys appear to suffer from that limitation.”