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.

Also on HuffPost:

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  • Their Baby Cuteness Doesn't Predict Adult Attractiveness

    A study published in the journal <em>Infant Behavior & Development</em> revealed that the standard "<a href="http://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&ved=0CCQQtwIwAA&url=http%3A%2F%2Fwww.youtube.com%2Fwatch?v%3DXTV8bOv3Jhs&ei=0uLBToKrMuPu0gHkmNH0BA&usg=AFQjCNFtutJJhlTFZJ2fm-cmsDo46XMpzw" target="_hplink">You Must Have Been a Beautiful Baby</a>" has little to do with reality. When 253 college students were asked to rank photos of the same individuals as infants and young adults (without being told who was who), there was <a href="http://bodyodd.msnbc.msn.com/_news/2011/08/31/7542626-must-have-been-a-beautiful-baby-maybe-not" target="_hplink">no relationship between how cute the students found the babies and how attractive they found the grown-ups</a>.

  • They're Good At Sharing

    No, really, it's true. It doesn't matter how many times you've heard the shout "Mine!" -- research shows babies can sense fairness at 15 months. During one study at the <a href="http://www.washington.edu/news/articles/babies-show-sense-of-fairness-altruism-as-early-as-15-months-1" target="_hplink">University of Washington</a>, 47 babies observed videos of an experimenter distributing milk and crackers to two people. When one recipient received more food than the other, the babies paid more attention. That means they had expected a fair distribution. The researchers also found that babies who did notice unfairness were more likely to share their own toys.

  • They Read Minds

    OK, so they're not exactly psychic. But a <a href="http://www.sciencedaily.com/releases/2011/11/111101130204.htm" target="_hplink">recent study</a> from the University of Missouri found that babies just 10 months old are starting to follow the thought processes of others. Yuyan Luo, an associate professor of developmental psychology who conducted the study, tells The Huffington Post, "Babies, like adults, when they see something for the first time -- when something is surprising -- they look for a long time. It shows [they recognize] something is inconsistent." It's called the "violation of expectation," she explained. When babies are surprised by something or notice something unexpected has happened, they tend to gaze at that thing longer. In Luo's research, babies watched actors consistently choose object A (such as a block or a cylinder) over object B. When an actor then switched to object B, the babies stared for about five to six seconds longer, meaning they recognized the change in preference.

  • They're A Little Bit Racist

    Don't judge a book by its cover. Treat all people the same. We're all equals. These are sentiments parents strive to teach their kids from a very young age. And they should. Starting, like, immediately. Researchers at the University of Sheffield in the United Kingdom found that babies at three months <a href="http://www.world-science.net/exclusives/060212_racefrm2.htm" target="_hplink">begin showing a preference for the faces of people of their own race</a>. But not all hope for equality is lost. The same study showed that babies who are exposed to people of all different races are less likely to develop bias at such an early age.

  • The Rhythm Is Gonna Get Them

    Researchers from Brigham Young University found that five-month-old babies can <a href=" http://news.byu.edu/archive08-oct-babymusic.aspx" target="_hplink">identify an upbeat song as being different from a series of sad, slow songs</a>. In other words, they are happy. They know it. They will clap their hands. Or stare longer, as the case may be. The experimenters showed babies an emotionless face while music played. When they played a new sad song, the babies looked away. When the music pepped up, the babies stared for three to four seconds longer.

  • They Can Tell The Good Guy From The Bad Guy

    Babies have a sense of morality at six months old, <a href="http://www.dailymail.co.uk/news/article-1275574/Babies-know-difference-good-evil-months-study-reveals.html" target="_hplink">say Yale researchers</a>. During the Yale study, babies watched a puppet show in which a wooden shape with eyes tried to climb a hill over and over again. Sometimes a second puppet helped him up the hill, and other times a third puppet pushed him down. After watching the act several times, the babies were presented with both puppets. They showed a clear preference for the good characters over the bad ones by reaching to play with the good puppet.

  • They Can Read Lips ... Kind Of

    Dr. Janet Werker of the University of British Columbia, who studies how babies perceive language, found that if a mother spoke two languages while pregnant, her infant could <a href="http://www.livescience.com/13016-bilingual-babies-brain-language-learning.html" target="_hplink">recognize the difference</a> between the two. And they don't even have to be spoken out loud. Werker's research found that infants four to six months old can <a href="http://www.nytimes.com/2011/10/11/health/views/11klass.html" target="_hplink">visually discriminate two languages</a> when watching muted videos of someone speaking both.