However, a baby with a pacifier in their mouth 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 paralyse 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. Professor Paula Niedenthal, who led the study, said: "By reflecting what another person is doing, you create some part of the feeling yourself.
"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. "We can talk to infants, but at least initially they aren't going to understand what the words mean.
"So the way we communicate with infants at first is by using the tone of our voice and our facial expressions. That work got us thinking about critical periods of emotional development, like infancy.
"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 recollected more pacifier use as kids scored lower than their peers on common tests measuring emotional intelligence.
"What's impressive about this is the incredible consistency across those three studies in the pattern of data," Prof Niedenthal said.
"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 Prof 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 continued.
"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," Prof Niedenthal said.
"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 is an important next step, said the study team. Prof Niedenthal said: "Probably not all pacifier use is bad at all times. We already know from this work that night time 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."
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