When the women looked at happy babies with a pacifier or with a white square obscuring their mouth, they showed less activity in their Zygomaticus—the muscle responsible for drawing the mouth into a smile—and they also believed the babies’ happiness was less intense. When the women looked at pictures of babies who were sad or angry, facial obstruction by a pacifier or a square again caused the women to rate the babies’ feelings as less intense. Regardless of the babies’ emotion, the womens’ reciprocal facial expressions were muted when the babies’ faces were partially covered.

Throughout the experiment, it didn’t make a difference whether a pacifier or a white square was used to hide the baby’s mouth; adults’ emotional responses were similarly dampened either way. This suggests that the women’s responses to the babies with pacifiers were less intense not because of negative stereotypes about infants with pacifiers—the researchers hypothesize that adults may believe that only difficult or whiny babies are given pacifiers—but because the baby’s mouth was covered.

The implications for babies’ emotional development could be significant. “The relationship between facial activity and the corresponding experience of emotion is particularly important for preverbal infants, because they rely on facial expressions of caretakers for behavioral regulation and learning,” write the authors. “Perceivers may find interactions with infants using a pacifier less enjoyable and less stimulating. … Smiles are powerful social rewards … and sadness, despite its negative valence, elicits talk, play, and supportive behavior of adults.” Adults—whether caretakers or not—could feel less inclined to fully engage with infants using pacifiers, and the infants themselves may benefit less from the interaction if they’re unable to mimic the adults’ emotions.