In 1986, psychology professor Linda Camras took a leave of absence from DePaul University to have a baby. For eight weeks, she recorded her daughter’s every expression and what triggered it. “I did a lot of videotaping,” she says. It was the first real-world test of a 1970s theory: that by 2 months old, babies automatically smile, knit their brows, and contort their faces to show positive versus negative emotions, like joy or fear.
The idea had made sense to Camras–until she started studying her own baby. “I found a lot of things that didn’t fit into the theory,” she says. For instance, her baby raised her eyebrows in a classic expression of surprise when playing with a familiar toy.
A baby’s facial repertoire, Camras observed, can be somewhat messy. Only with age and feedback does “it get organized so that there’s a tighter link between the facial expression and the emotion,” she says.
Michael Lewis, who studies child development at Rutgers Robert Wood Johnson Medical School, says, “Most people agree that by 6 to 9 months, there are facial expressions of emotions.” But he believes it occurs even earlier. His lab found that when infants as young as 2 months old pulled a string that cued up pictures of smiling babies and Sesame Street music, their faces showed pleasure. Later, when the string was disabled, they showed anger and sadness.
Camras, however, isn’t sure these effects occur outside laboratory settings and questions the assumption that a baby’s face, unlike an adult’s, always reveals its inner state. “If your professor slips and falls, you try not to laugh, because it’s rude,” Camras says. “As adults, we can control our facial behavior, but we assume babies don’t do that.”
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This article was originally published in the May 2015 issue of Popular Science.