Pacifiers may stunt a child's emotional development - but only if the child is a boy, according to a new study out of the University of Wisconsin-Madison.

Putting a pacifier in an infant's mouth has an effect similar to that of Botox, in that it limits the child's ability to make facial expressions, according to the study. And mimicking the reactions of those around them is one of the ways babies learn to understand feelings and develop empathy.

"By reflecting what another person is doing, you create some part of the feeling yourself," said the study's lead author, Prof. Paula Niedenthal.

Through a series of tests with children aged 6 and 7, as well as with college-age men, the study found boys who'd had "heavier pacifier use" were less able to assess people's moods.

Girls, however, showed no similar effect, the researchers found. Either pacifiers don't interfere with their emotional development or parents compensate, inadvertently, because girls are expected to be emotional.

"It may be that boys are simply more vulnerable than girls, and disrupting their use of facial mimicry is just more detrimental for them," the authors said.

"Since a baby is not yet verbal -- and so much is regulated by facial expression -- you want parents to be aware that using something like a pacifier limits their baby's ability to understand and explore emotions. And boys appear to suffer from that limitation."