A study led by psychologists at the University of Wisconsin-Madison associates pacifiers with psychological and emotional consequences for boys.
Three studies looked into pacifier use in babies and what that meant for children as they grew into adults.
Humans of all ages, often unwittingly or otherwise, mimic the expressions and body language of people who are around them.
The studies looked at facial and emotional responses of boys and girls ages 6 and 7 years old as well as college-aged men.
The research found that pacifiers may stall the emotional development of baby boys and that frequent pacifier use during the day may disrupt emotional development.
Researchers said that's because it limits opportunities to mimic facial expressions.
"The longer boys used pacifiers, the lower their sort of emotional intelligence in that particular study was, so the less mimicry they showed over time, the lower their scores on perspective taking aspects of empathy," said lead author of the study, Paula Niedenthal.
The study found that pacifier use didn't affect baby girls' emotional progress due to their emotional development being stimulated in other ways.
Researchers said the next step is to study why girls react differently.