The researchers, led by scientists from the University of Wisconsin, did not spend the years it would take to track a single group of kids from infancy through adulthood. Instead, they conducted three separate experiments that attempted to get at the same developmental stages.
First, they found that 6- and 7-year-old boys who had used pacifiers commonly when they were younger were less likely than other boys to mimic the smiles and frowns of faces on a video screen in front of them -- a test of kids' interpersonal empathy.
The next two studies used the age-old psychology research study group: college students. The researchers asked the students (who likely asked their parents) how often they used pacifiers when they were little. They then gave the students a test of what's called "perspective taking," which is the ability to assume someone else's point of view and is often stunted in people with autism. Finally, they also gave college students a test of emotional intelligence, which required them to make decisions that relied on understanding the feelings of others.
In both cases, heavy pacifier use was associated with poor scores.
So what's the link?
Infancy is considered a "critical period" for many human skills and capacities, including emotional and interpersonal development. That means that if we don't have the right exposure or the right experiences when we're little, we may never have them at all. And if infants have pacifiers in their mouths all the time, they are unable to mimic faces and have social interactions that rely on facial expressions - both believed to be essential building blocks of social and emotional development.
Interestingly, the effect was only found in boys, and the researchers will have to conduct further studies to determine why. One theory is that girls' parents better compensate for the pacifier by engaging with their kids more emotionally than they do with boys, though it is also possible that girls are inherently better able to cope, the researchers argue.
For parents who don't like the results - that pacifier sure does keep little Joey quiet, after all - it's easy to quibble with the methodology, which relies on the self-report of people who may not remember just how much they used a pacifier. And it is possible that the correlation the researchers discovered goes even deeper-which would be true if children genetically predisposed to autism or autistic traits exhibited behaviors, like crying, more likely to lead to a parent giving them a pacifier in the first place.
But the results are consistent across all three different studies -- something that is unlikely to happen by chance-suggesting the effect is real one way or the other, the researchers said.
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