Be it an invisible girl who lives under the bed, a 7-inch-tall elephant who wears shorts or a green dog, imaginary friends are common childhood companions even into elementary school, researchers from the University of Washington and University of Oregon discovered.

By age 7, more than two-thirds of children reported inventing an imaginary friend at some point in their lives, according to the study in the current issue of the journal Developmental Psychology.

"The main message to parents is how common imaginary friends are, even with children older than previously studied," said UW psychology professor Stephanie Carlson. "Parents shouldn't be concerned if elementary-school-age children still engage in this form of play."

University of Oregon professor Marjorie Taylor wanted to find out how children ditched their imaginary friends as they grew up. To her surprise, imaginary friends don't peak at age 3 or 4 as previously believed; in fact, 7-year-olds were just as likely to play with an imaginary companion. Young children who dropped imaginary friends often created new ones, and some kids who didn't have them in preschool made them up later.

"It's not a phenomenon that goes away, like people thought," said Taylor, author of "Imaginary Companions and the Children Who Create Them" (Oxford University Press, 1999). Indeed, she suspects imaginary companions simply change form. Teens often address their diaries to an imaginary friend; adult writers complain their fictional characters take over novels.

Children with imaginary friends typically had two to three (one kept track of 13 entities) with fairly frequent turnover. Faux friends sometimes meet dramatic ends, such as being run over, but "mostly they just fade away," Taylor said.

Two-thirds of elementary-school children's imaginary friends were invisible (mostly humans, but many animals, too), while preschoolers favored props such as toys or stuffed animals. This went beyond one-time pretend play; to qualify, the child had to endow a friend with psychological qualities, à la Winnie the Pooh or Hobbes of "Calvin and Hobbes."

Brier resident Telly Presho shares her home with two kids and three imaginary friends. They weren't part of the study, but she's watched her 5-year-old son, Russell, set up a chess board to play his imaginary friend, Madine. "Sometimes Madine wins, sometimes he wins," she said. Apparently they also converse: "All of a sudden he'll laugh and say, 'You're so funny, Madine.' "

Her daughter Zoe, 4, has two imaginary friends. Kyla (sometimes her sister and sometimes a friend) is a jet-setter who has lived in New York, Paris and Australia. Then her iron bedpost is Sally, whom Zoe talks to and dresses in different outfits. The clothes fit over Sally's "head," the ball on top of the post.

Imaginary pals are not always pleasant companions. They're often scapegoats for a child's misdeeds; others complained about imaginary friends who bothered them while they were trying to read, for example. "It's something that comes out of their own mind that's perceived as outside their mental control," Carlson said.

Parents can glimpse into a child's mind by monitoring what's happening with an imaginary friend, Taylor said. For example, an imaginary friend might be scared of a big dog next door, or sick if Grandma has cancer. "It's a way for children to mull things over in the context of pretend play."

While kids do use mock friends as a coping device, "the major reason is it's a fun way to pass the time," she said.

For parents, it can get annoying to wait for invisible friends to, say, tie their shoes. "Imaginary companions tend to be slow getting out the door," Carlson laughed.

Presho plays along with her kids' imaginations but drew the line when imaginary Kyla started telling Zoe it was OK to do things she wasn't supposed to. "I told her that wasn't Kyla's decision to make," she explained.

Even when friends live vivid, detailed lives, kids know they're not really real and can still distinguish between fact and fantasy. After a long line of serious questioning, researchers even had kids pull them aside to ask, "You know this is just pretend, right?"

Despite previous concerns that only lonely kids create faux pals, research found preschoolers with imaginary friends tend to demonstrate higher verbal skills and have more real friends.

Researchers don't know if these kids are more popular because they practice social skills with imaginary friends or if already social kids are more likely to invent pretend companions. "It could be that when more sociable kids don't have a real friend, they just make one up," Carlson said.

Preschool children with imaginary friends also showed a keener ability to understand others' perspectives; by elementary school, these children were better able to read emotions.

The differences lessen by school age; since so many kids have had imaginary friends, it crosses all personality styles.

Studies suggest parental disapproval can't keep kids from creating pretend friends (they just keep them quiet; nearly a quarter of children had companions parents didn't know about); neither can parents encourage their development.

"We do get some conscientious parents who read about the positive benefits and try to get their child to have one," Carlson said. "But it doesn't take."