“If you’re being deluged by constant communication, the pressure to answer immediately is quite high,” she added. “So if you’re in the middle of a thought, forget it.”

Michael Hausauer, a psychotherapist in Oakland, Calif., said teenagers had a “terrific interest in knowing what’s going on in the lives of their peers, coupled with a terrific anxiety about being out of the loop.” For that reason, he said, the rapid rise in texting has potential for great benefit and great harm.

“Texting can be an enormous tool,” he said. “It offers companionship and the promise of connectedness. At the same time, texting can make a youngster feel frightened and overly exposed.”

Texting may also be taking a toll on teenagers’ thumbs. Annie Wagner, 15, a ninth-grade honor student in Bethesda, Md., used to text on her tiny LG phone as fast as she typed on a regular keyboard. A few months ago, she noticed a painful cramping in her thumbs. (Lately, she has been using the iPhone she got for her 15th birthday, and she says texting is slower and less painful.)

Peter W. Johnson, an associate professor of environmental and occupational health sciences at the University of Washington, said it was too early to tell whether this kind of stress is damaging. But he added,

“Based on our experiences with computer users, we know intensive repetitive use of the upper extremities can lead to musculoskeletal disorders, so we have some reason to be concerned that too much texting could lead to temporary or permanent damage to the thumbs.”

Annie said that although her school, like most, forbids cellphone use in class, with the LG phone she could text by putting it under her coat or desk.