Lead author Paula Niedenthal, a psychology professor at the University of Wisconsin-Madison, said a baby with a pacifier in its mouth is less able to mirror expressions -- a child's first communication.
The researchers found 6- and 7-year-old boys who had spent more time with pacifiers in their mouths when younger were less likely to mimic the emotional expressions of faces peering out from a video.
In addition, college-age men who reported -- by their own recollections or their parents' -- more pacifier use as kids scored lower than their peers on common tests of perspective-taking, a component of empathy, Niedenthal said.
The study, published in the journal Basic and Applied Social Psychology, found college students who had been heavier users of pacifiers when young scored lower on a standard emotional intelligence test measuring the way they make decisions based on assessing moods of others than those who had not used pacifiers as much.
Niedenthal said the pacifier effect was not noted in girls. She said it is possible they make sufficient progress in emotional development before or despite pacifier use.
"It could be that parents are inadvertently compensating for girls using the pacifier, because they want their girls to be emotionally sophisticated. Because that's a girly thing," Niedenthal said. "Since girls are not expected to be unemotional, they're stimulated in other ways. But because boys are desired to be unemotional, when you plug them up with a pacifier, you don't do anything to compensate and help them learn about emotions."