Their opinion seems to be that poltergeists are the psychic discharges of a young person between the ages of ten and twenty. If there is a young person in your house who is very frustrated, and suddenly pots are flying through the air at your head, books are falling off the bookshelf, or your chair is spontaneously catching on fire, you may want to consider calling a psychiatrist as well as a psychic. It could be a ghost, or it could be the psychic energy of the psychically-charged young person. Ghosts don't always take up residence. Sometimes they just visit to check things out and see how things are going.
Ghosts are just dead people, after all.
They might be orbs now but at one time, they had hopes and dreams and a personality wrapped up in flesh and bone. The flesh and bone may be gone, but the personality is still there. Either they never left, or they have a preference for a particular location. They might stay in a place because of the memories contained there, or because of the view, or because they really like the family living in the house. Or, as you might expect, ghosts linger at a site because they were there first.