Found some research i did a while back for some Uni stuff on imaginery friends....hope its helpful....
"make-believe pals are a way of exploring reality," says Marjorie Taylor, author of Imaginary Companions and the Children Who Create Them. "They help kids work through dilemmas�.
"the child is capable of abstract thought and can create make-believe worlds," explains Susan Newman., author of Parenting an Only Child: The Joys & Challenges of Raising Your One & Only. She says �40% of kids have made up a sidekick by age 7. After that, many children outgrow their "pal," or at least stop speaking about him openly. it's not just lonely, isolated kids who invent companions. While only children and firstborns with far-younger siblings are somewhat likelier to do it, "even kids from large families have them,"
Nor are children with imaginary friends destined to be loners. Quite the opposite: "Typically, they like to socialize, so when nobody's available, they make up someone," explains Tracy Gleason, professor of psychology at Wellesley College, Massachusetts.
Dr Taylor also says that �make-believe buddies are associated with positive personality traits, and that kids who have them display a better ability than their peers to see things from another person's perspective. Also studies show these kids have higher language scores, play well with other children, and ultimately have more friends�.
above quotes from
http://www.parents.com/parents/story.jhtml?*** ****=/templatedata/parents/story/data/5811.xml &catref=prt34
Jean Piaget, pioneer of cognitive psychology, relegated pretend friends to the immature stage of "magical thinking," which children needed to outgrow to achieve cognitive competence.