How much the children looked at mouths followed a similar pattern. And although each toddler watched the videos without other children present, identical twins often moved their eyes at nearly the same moment — as close as 16.7 milliseconds apart — and in the same direction.

“It’s a really remarkable set of findings in that it really shows that genetic factors are driving differences in the way that toddlers are looking at faces,” said Brad Duchaine, a professor of psychological and brain sciences at Dartmouth, who was not involved in the study. “This suggests that genetic differences drive this important aspect of the way that we interact with others.”

Dr. Jones, whose co-authors include Dr. John Constantino, director of child and adolescent psychiatry at Washington University School of Medicine in St. Louis, and Ami Klin, director of the Marcus Autism Center at Children’s Healthcare of Atlanta, said: “When we started to get the results back, I thought that I had the wrong data because the match between identical twins was so strong. I thought I might have mistakenly matched data from the same twin.”

With the children with autism, the researchers found that, compared with typically developing toddlers, they spent significantly less time looking at faces and more time looking at objects. That difference was especially pronounced with the day care videos, scenes presenting many more things to look at than the close-up videos of women talking to the camera, Dr. Jones said. When watching the day care videos, toddlers with autism looked at faces half as often as typical children did, and at objects almost twice as much.

The difference was so consistent that researchers could identify most children with autism just by looking at the eye-tracking results, Dr. Jones said. That result reinforced previous research in which Dr. Jones, Dr. Klin and colleagues showed that babies from 2 months to 6 months old who looked less at people’s eyes in videos were more likely to be given an autisim diagnosis at age 3 and that eye-tracking could provide an early behavioral indicator of autism.

Experts said that because the study shows that a social behavior that is significantly different in children with autism is strongly influenced by genetics, it might help scientists home in on specific genes to better understand autism or at least a key autism characteristic.

Image The study being performed. “It’s a really remarkable set of findings in that it really shows that genetic factors are driving differences in the way that toddlers are looking at faces,” said Brad Duchaine, a professor of psychological and brain sciences at Dartmouth. Credit... Robert J. Boston

Even when identical twins watched completely different videos, their results matched. “How much Twin 1 looked at the eyes in a video that Twin 2 didn’t get to see predicted how much Twin 2 would look at the eyes in a different video,” Dr. Jones said.