A dummy may be the first thing you grab to soothe a crying baby, but a new study suggest this could stunt your child’s emotional development.

The research shows that infants learn how to interact largely through mimicry and copying adults’ or older kids’ facial expressions. Researchers found dummies actually interrupted this process in young boys as they stopped them from copying different facial expressions.

The team of psychologists found that a constant use of dummies was linked to poor results on various measures of emotional maturity.

The study is the first of its kind to associate dummies, or pacifiers, with psychological consequences.

The World Health Organisation and American Academy of Pedicatrics are already calling for limited pacifier use to promote breastfeeding and because of connections to ear infections or dental abnormalities.

The curse of the dummy?

Humans of all ages often mimic, whether we know it or not, the expressions and body language of those around them.

“By reflecting what another person is doing, you create some part of the feeling yourself,” said lead author Professor Paula Niedenthal.

“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.”

With a dummy in their mouth, babies are less able to mirror these expressions and the emotions that they represent.

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.

They also broadened the study to college-aged men who reported, by their own recollections or their parents', more pacifier use as children. These also scored lower than their peers on common tests of emotional intelligence.

The test measured 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.

Night-time dummies are said not to affect children's emotional intelligence.

“What's impressive about this is the incredible consistency across those three studies in the pattern of data,” 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.”

The reason the study focused on boys is because girls develop earlier in many ways, and it is possible that they make sufficient progress in emotional development before or despite dummy use. Niedenthal said it may be because boys are simply more vulnerable than girls and disrupting their use of facial mimicry is just more detrimental for them.

“Parents hate to have this discussion,” Niedenthal says.

“They take the results very personally. Now, these are suggestive results, and they should be taken seriously. But more work needs to be done.”

Working out why girls seem to be immune (or how they may compensate) is an important next step as is the impact of how often the dummy used.

“Probably not all pacifier use is bad at all times, so how much is bad and when?” Niedenthal said.

“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.”

But even with more research planned to further explain the new results, Niedenthal is comfortable telling parents to consider occasionally pocketing the pacifier.

Will you be?

The study was published in Basic and Applied Social Psychology.