But perhaps most surprising is how positively he writes about imaginary friends. In the past, child psychologists have backed the theory that children want playmates, and if one isn't available they make one up.
According to this view, if a child has an imaginary friend, it's a sure sign they're feeling lonely. But parents' great fear is that fantasy playmates are a sign their child is vulnerable to a mental illness.
Harrowing research published as recently as 1997 could heighten this fear.
Dorothy Otnow, a psychiatrist at the New York University School of Medicine, conducted in-depth interviews with 12 murderers who claimed to kill while in the grip of a personality other than their own. The startling finding was that 10 of the 12 appeared to have had long-standing imaginary companions during childhood. But the study involved only a small number of murderers and, if the number of children judged to have imaginary friends in Professor Seiffge-Krenke's research is anything to go by, it can't be that ominous a sign.
Another more recent school of psychology argues that children create companions as a positive way of dealing with stress. Jean Piaget, perhaps the most renowned child psychologist, regarded his own three-year-old's imaginary friend as a healthy sign of a rich imagination. He saw that the friend frequently consoled his daughter when she was unhappy, and so subscribed to the view that pretend playmates serve a protective function in stressful situations. Professor Seiffge-Krenke's work supports this, as well as confirming that children with pretend playmates tend to be imaginative and have superior language and social skills.
One surprise was that imaginary friends are likely to be female, regardless of the child's sex. This could be because of the established psychological finding that people are more likely to disclose personal information to women.
A new theory as to why imaginary friends disappear during adolescence is based on the recent discovery that the nerves connecting the brain's right and left hemispheres complete their development during the teenage years.
Until then, the relative isolation of the two sides could mean it's easier for us to display two differing sides of our characters; that children have two minds that slowly come together. This could also make it easier to create imaginary friends who appear so different from ourselves.
Psychoanalysts are interested in the personality of the imaginary friend; they argue that as the fanciful figure is a product of your own mind, he or she reveals a lot about you. For example, your imaginary friend might be more competent, better-looking or wittier than you, so subtly expressing your secret longings.
Intriguingly, Susan Harter and Christine Chao, child psychologists at the University of Denver, recently reported that boys tend to create companions who are more competent than themselves, while girls invent companions who are less able. The psychologists speculate that the "super-competent" male imaginary friends reflect a masculine aspiration to be more like the "all-powerful" male ideal in our society. For girls, a less competent imaginary friend allows them to have someone to look after, so fulfilling a stereotyped feminine role.
Whatever the theories, all this research converges on the suggestion that imaginary friends reveal something about you, but exactly what depends a lot on the kind of friend you created and why.
Professor Raj Persaud is a consultant psychiatrist at the Maudsley hospital and senior lecturer at the Institute of Psychiatry in London. Email: email@example.com