Children under the age of four and kids with autism appear to be immune from contagious yawning, the phenomenon where one person's yawn triggers a chain reaction of open mouths, a study published Wednesday says.
Researchers from the University of Connecticut observed 120 typically developing one- to six-year-olds and 30 six- to 15-year-olds with autism spectrum disorders for the study, which was published in the journal Child Development.
They found that although babies yawn spontaneously even before they leave the womb, most children show no signs of succumbing to contagious yawning until they reach the age of four.
Kids with autism were less likely to yawn contagiously than typically developing children with the same mental and chronological ages, and the more severe a child's autism, the less likely he or she was to yawn contagiously, the study also found.
"Given that contagious yawning may be a sign of empathy, this study suggests that empathy and the mimicry that may underlie it develop slowly over the first few years of life, and that children with autism spectrum disorders may miss subtle cues that tie them emotionally to others," the researchers said.
Among adults, meanwhile, contagious yawning is almost epidemic: half of grown-ups do it, the study says.