To find out when in life the behavior develops, Helt read a story to 120 healthy kids, ages one through six. The kids were grouped by age, so that all of the one-year-olds heard one reading, all the two-year-olds heard another, and so on. There were 20 kids in each age group.
During each 10-minute story, Helt intentionally yawned every 90 seconds. A camera recorded whether the kids were watching her and if so, whether they yawned, too. She and colleagues repeated the experiment with 28 autistic kids, ages 6 to 15. Some of the children were further along the autistic spectrum than others.
Given four opportunities to catch a yawn, the researchers report today in the journal Child Development, none of the healthy one-year-olds did. Only one of the two-year-olds yawned back, and two of the three-year-olds caught a yawn.
There was a dramatic leap in the group of four-year-olds, where yawning spread to 9 out of 20 kids. That rate held steady for the older groups.
And it matches experiments in grown-ups, which find that between 40 and 60 percent of healthy adults yawn after seeing someone yawn, thinking about yawning or even reading the word "yawn."