It’s one of parenting’s most common -- and cliché -- threats: if you keep an expression long enough, your face will stay that way.

A new study published in Basic and Applied Social Psychology doesn’t exactly prove the warning true -- but it provides an interesting twist on the association between a little one's range of expressions and his later emotional intelligence. The study suggests that pacifiers physically block baby boys’ (and notably not girls') ability to copy expressions they see on the adults around them – thereby getting in the way of their emotional development.

Intrigued by the effect of Botox on adults -- “Botox users experience a narrower range of emotions and often have trouble identifying the emotions behind expressions on other faces,” a press release for the study explains -- Paula Niedenthal of the University of Wisconsin-Madison started “thinking about critical periods of emotional development, like infancy,” she says.

“[T]he way we communicate with infants at first is by using the tone of our voice and our facial expressions,” she explains in the release. Those expressions are not just used to get adult feelings across; children can observe their parents’ smiling or showing concern and learn to copy. That is, unless something gets in the way. As Niedenthal puts it: "What if you always had something in your mouth that prevented you from mimicking and resonating with the facial expression of somebody?"

The findings suggest that the problem is specific to boys, but it’s unclear why. Niedenthal offers one hypothesis: “It could be that parents are inadvertently compensating for girls using the pacifier, because they want their girls to be emotionally sophisticated. ... 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."

Researchers measured emotional reactions and development in young children (aged 6-7) and college students. Boys in the younger group were measured based on their reactions to “faces peering out from a video” -- with those who had used pacifiers more heavily being found “less likely to mimic the emotional expressions.” Emotional intelligence and perspective-taking tests applied to college students also showed that “heavier pacifier use went hand-in-hand with lower scores.”

Since the study measures babies’ response to signals sent by adults during waking hours -- or “learning time” -- Niedenthal says it seems that putting a baby down at night with a binky doesn't make a difference.

While many parents have other specific concerns about pacis (will they contribute to nipple-confusion? how long is it okay for kids to use them? how do I help my toddler break up with her binky!?), the American Academy of Pediatrics states straightforwardly that “Pacifiers do not cause any medical or psychological problems" (study authors say this is the first time evidence has been found linking pacifier use with psychological development). The Mayo Clinic also says that pacifiers can be “the key to contentment between feedings,” but notes that they shouldn’t be used immediately with babies who are breast-fed, and adds:

To reduce the risk of SIDS, the American Academy of Pediatrics recommends offering a pacifier at naptime or bedtime until age 1. However, the risks of pacifier use begin to outweigh the benefits as your baby gets older. While most kids stop using pacifiers on their own between ages 2 and 4, others need help breaking the habit.

With regard to the new research, Jon Bardin writes in the Los Angeles Times that "it's easy to quibble with the methodology, which relies on the self-report of people who may not remember just how much they used a pacifier."

Joseph Campos of UC Berkeley also points out in The Christian Science Monitor that expressions aren’t solely registered with the mouth -- noting that babies and adults can still communicate feelings with other parts of their faces.

So, if your baby boy really loves his paci -- and you don't want to ask him to give it up -- consider taking a lesson from "Up All Night's" Mr. Bob and playing a few extra rounds of very animated peek-a-boo.

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