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.