As if parents didn’t already have enough to worry about: Turns out something as simple as giving your baby boy a pacifier could affect his emotional development.

A group of researchers led by University of Wisconsin emotion and cognition expert Paula Niedenthal say that the pacifier hinders a young boy’s ability to mimic the facial expressions of those around him.

Mimicry, researchers believe, is key to human development.

"By reflecting what another person is doing, you create some part of the feeling yourself,” Niedenthal said in a statement Tuesday. "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."

This is especially important for nonverbal babies, who can only have some inkling of what is going on around them by observing someone’s voice tone and facial expressions, according to Niedenthal. She and her colleagues performed three experiments examining how a child’s pacifier use affected his or her emotional development.

In one experiment, they recruited 106 French elementary schoolers, quizzed their parents on their past pacifier use and thumb-sucking behavior, then showed them movies of morphing facial expressions. Observers watched the children to see if the children were mimicking the faces in the movie. Boys that spent more time with pacifiers were less likely to mirror the expressions shown in the movie.

Another experiment recruited 167 university students in the U.S. The researchers asked the participants how long they estimated they’d used a pacifier for during their child and had them complete questionnaires, deciding whether statements like “I try to look at everybody’s side of a disagreement before I make a decision” described them well or not at all. Pacifier use was correlated with lower signs of “perspective taking,” one component of empathy.

124 American college students and 304 French university students participated in the third study, similar to the second, but this time with questions on emotional intelligence like whether or not they agreed with the statement “I’m able to cope well in new environments.” Longer pacifier use in boys, again, was associated with lower emotional intelligence.

The team says that work still needs to be done to confirm the association and to investigate how often and how long pacifier use has to occur to become problematic. There’s also the possibility that parents will mimic infants’ facial expressions less if they’re using a pacifier, leading to both failed mimicry on the part of the baby and those around them.

But why is the pacifier primarily affecting boys?

Baby girls might not be as affected by pacifier use, because they tend to exhibit signs of emotional development –- eye contact, facial expression and mimicry -– at an earlier age than boys. Plus, girls are generally expected to be better at reading other people’s emotions and responding to them, according to the researchers.

“While caretakers of girls may compensate for the deleterious effects of pacifier use, caretakers of boys may not, and this could leave boys more vulnerable to the consequences of disrupted facial mimicry,” they wrote.

Niedenthal and her colleagues aren’t recommending that parents immediately chuck the binky, though.

“Pacifiers should have a lesser effect on facial mimicry if children only use them at night while sleeping or even during the day outside of home (i.e., when they do not interact with their primary caregiver and may use the pacifier to remain quiet in a group setting, for instance, while listening to a story),” the researchers wrote.

SOURCE: Niedenthal et al. “Negative Relations Between Pacifier Use and Emotional Competence.” Basic and Applied Social Psychology in press.