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Parents who don鈥檛 want their baby boys to grow up emotionally stunted may want to pocket their pacifiers during the daytime.

A new study from the University of Wisconsin-Madison suggests frequent pacifier use during the day may disrupt the emotional development of baby boys because it limits their opportunity to mimic the facial expressions of others 鈥� a tool that may help them better understand emotions and learn empathy.

The study assumes babies can鈥檛 smile, pout or furrow a brow with a pacifier in their mouths, and many parents may beg to differ with that.

Girls appear to make sufficient progress emotionally, despite pacifier use, suggests the research published in Tuesday鈥檚 issue of the journal Basic and Applied Social Psychology.

Humans of all ages read each other鈥檚 emotions partly by mimicking their facial expressions, which helps them process what the person is thinking and feeling by creating some part of the feeling for themselves, says the study鈥檚 lead author, Paula Niedenthal, a University of Wisconsin psychology professor.

A baby with a pacifier in his mouth is less able to mirror expressions and the emotions they represent, she said.

Niedenthal and her team of researchers conducted three studies to test the relationship between pacifier use and emotional information processing, both in the United States and in France.

In the first study, researchers found 6- and 7-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 in a video they were shown. In the next study, college-aged men who reported (by their own recollections or their parents鈥�) more pacifier use as kids scored lower than their peers on common tests of perspective-taking, one of many components of empathy.

The third study involved a group of college students who took a standard emotional intelligence test measuring the way they make decisions based on assessing the moods of other people. The men in the group who had heavier pacifier use as babies scored lower.

鈥淲hat鈥檚 impressive about this is the incredible consistency across those three studies in the pattern of data,鈥� Niedenthal said. 鈥淭here鈥檚 no effect of pacifier use on these outcomes for girls, and there鈥檚 a detriment for boys with length of pacifier use even outside of any anxiety or attachment issues that may affect emotional development.鈥�

Why the gender difference?

鈥淚t could be that parents are inadvertently compensating for girls using the pacifier because they want their girls to be emotionally sophisticated,鈥� Niedenthal said. 鈥淭hat鈥檚 a girlie thing.鈥�

Suggesting that a pacifier can have lifelong consequences is far from popular among parents, Niedenthal acknowledged.

鈥淭hey take the results very personally,鈥� she said.

The study鈥檚 results are suggestive, and should be taken seriously, Niedenthal said. But she acknowledged further research is needed.

鈥淚t鈥檚 fascinating and challenging to many assumptions made about emotional development,鈥� said Joseph Campos, a professor of psychology at the University of California, Berkeley. 鈥淚t makes students of emotional development take notice. But as professor, Niedenthal acknowledges, there is much more work to be done.鈥�

Mothers report babies can show oral expressions of emotion, even with a pacifier in their mouth, Campos said. And there are other facial components of emotion, including eyebrows, cheeks and wrinkles around the eyes, not affected by a pacifier, he said.

Not all researchers agree there鈥檚 a connection between the ability to mimic facial expressions and the ability to perceive others鈥� emotions.

And then there鈥檚 the gender question.

Figuring out why girls seem to be immune 鈥� or how they compensate 鈥� is an important next step for researchers, Niedenthal said.

Ruby Natale, assistant professor of clinical pediatrics at the University of Miami School of Medicine, co-wrote the book 鈥淧acifiers Anonymous: How to Kick the Pacifier or Thumb Sucking Habit.鈥�

The book notes that roughly 75 percent to 90 percent of all infants suck a pacifier or thumb. In the past, thumb-sucking was the No. 1 choice. But the latest estimates of pacifier use in the U.S. puts it at about 74 percent. One study found 20 percent of children sucked pacifiers beyond age 3. They鈥檙e affectionately referred to as 鈥減addicts鈥� (pacifier addicts).

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Natale, who has a 5-year-old daughter, became interested in the effects of pacifier use when she had concerns about its effect on breast-feeding. Both the World Health Organization and American Academy of Pediatrics recommend limiting pacifier use to promote breast-feeding and because of its connection to ear infections and dental abnormalities.

鈥淭here鈥檚 research that pacifier use can cause speech delays because they don鈥檛 mimic the parents鈥� speech,鈥� Natale said in an interview Monday. 鈥淚f a child can鈥檛 mimic the parents鈥� emotional expressions, it takes it one step further.鈥�

She agreed that further research is needed.